$$\begin{aligned} \theta\_P(t) &= \theta\_f \exp\left[-k\_d \left(t - t\_f\right)\right] \quad \text{(a)}\\ \theta\_P(t) &= \frac{\theta\_f}{1 + k\_d \theta\_f \left(t - t\_f\right)} \quad \text{(b)}\end{aligned} \tag{19}$$

0.000 0.005 0.010 0.015

**Figure 5.** Molecular desorption (a) and dissociative desorption (b), data and fit. Demetriou C., Nestoros M. and Chris‐

Time (sec)

25ppb H2 50ppb H2

100ppb H2

0 2000 4000 6000 8000

8 (b)

Photothermal Techniques in Material Characterization

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349

150ppb H2

**PH2 (Pa)**

**Figure 6.** Response (lower curve) and recovery (upper) times versus hydrogen partial pressure. Demetriou C., Nestoros

As shown in Figure 6, the response and the recovery times increase by introducing larger concentrations of hydrogen gas into the cell. As mentioned before, the increase of tc for higher hydrogen partial pressure contradicts the results that occur by studying the time progress of the desorption process. The apparent contradiction of recovery time increase for higher partial pressures of hydrogen may be due to hydrogen bonding between adjacent adsorbate protons or even molecules (i.e. before dissociation occurs on the Pd surface). This would effectively account for the dependence of the rate constant on pressure and its decrease with increasing coverage. Moreover, it should be pointed out that the response and the recovery times are determined by the experimental conditions. Previous experimental work has shown that the sensor's response time is decreased if the hydrogen gas flow rate into the cell is increased. In addition, it has been proved that by introducing into the cell an inert gas, in order to remove hydrogen, the recovery time decreases drastically. In the experiment presented [18] in this chapter nitrogen gas was introduced into the measurement cell. The presence of nitrogen gas

0

800

1600

**Time (sec)**

M. and Christofides C. Appl. Phys. A 2008; 92 (3), 651-658

2400

3200

4000

0 2000 4000 6000 8000

8 (a)

150ppb H2

tofides C. Appl. Phys. A 2008; 92 (3), 651-658

PMTR Amplitude

 (mV)

> 25ppb H2 50ppb H2

100ppb H2

Response Time Recovery Time

In Figure 5 (a) and (b) the experimental data are fitted using the above equations with the theory of molecular and dissociative desorption respectively. Note that by fitting the equation of molecular desorption to the experimental data one gets much better results. Table 4 presents the resulting values of the rate constant of desorption kd for the different hydrogen concen‐ trations. As shown in Table 4, the rate constant kd is of the same order of magnitude and varies slightly with hydrogen concentration. This contradicts the fact that the recovery time of the PMTR signal is larger for higher hydrogen concentrations, as will be shown later. Using the data of Figure 3(a), the response time ts and the recovery time tc of the hydrogen sensor were determined. The response time is defined as the time needed for the PMTR signal to reach saturation, i.e. for the system to reach equilibrium. The time needed for the system to recover from saturation to its initial state is defined as the recovery time.

**Table 4.** Desorption rate constants

**Figure 5.** Molecular desorption (a) and dissociative desorption (b), data and fit. Demetriou C., Nestoros M. and Chris‐ tofides C. Appl. Phys. A 2008; 92 (3), 651-658

of the palladium surface coverage θf

348 Materials Science - Advanced Topics

=∆ζf

, at time tf, presented in Tables 2 and 3 are larger for higher

.

(19)

hydrogen concentrations. The values of θf were calculated using the following relationship θf

**P (ppb) θf (%) Κdiss(Pa-1/2) σ(Κdiss) (Pa-1/2) θs (%)** 61.77 50.016 ±23.150 71.44 70.33 41.628 ±5.256 74.64 76.15 34.778 ±1.038 77.67 90.62 879.380 ±4170.413 99.08

A similar process was followed in order to study the desorption process (nitrogen enters the chamber). The equations were solved once more letting the partial pressure to be zero. The solutionsformolecularanddissociativeadsorptionaregivenbyeqs.(19a)and(19b),respectively.

( )

In Figure 5 (a) and (b) the experimental data are fitted using the above equations with the theory of molecular and dissociative desorption respectively. Note that by fitting the equation of molecular desorption to the experimental data one gets much better results. Table 4 presents the resulting values of the rate constant of desorption kd for the different hydrogen concen‐ trations. As shown in Table 4, the rate constant kd is of the same order of magnitude and varies slightly with hydrogen concentration. This contradicts the fact that the recovery time of the PMTR signal is larger for higher hydrogen concentrations, as will be shown later. Using the data of Figure 3(a), the response time ts and the recovery time tc of the hydrogen sensor were determined. The response time is defined as the time needed for the PMTR signal to reach saturation, i.e. for the system to reach equilibrium. The time needed for the system to recover

exp (a)

( b)

/α.The quantity *α* was obtained by fitting experimental data to theory and Δξf

( ) ( )

= -- é ù ë û

q

*t ktt*

*f*

*k tt*

*df f*

**P (ppb) kd (s-1) σkd (s-1)** 2.592×10-3 ±0.012×10-3 1.887×10-3 ±0.007×10-3 1.475×10-3 ±0.002×10-3 1.463×10-3 ±0.002×10-3

*P f df*

q

+ -

( )

from saturation to its initial state is defined as the recovery time.

**Table 4.** Desorption rate constants

*t*

*P*

q

q

**Table 3.** Calculations of Kdiss and θs using the values of ka and kd from Table 1

1

=

 q

**Figure 6.** Response (lower curve) and recovery (upper) times versus hydrogen partial pressure. Demetriou C., Nestoros M. and Christofides C. Appl. Phys. A 2008; 92 (3), 651-658

As shown in Figure 6, the response and the recovery times increase by introducing larger concentrations of hydrogen gas into the cell. As mentioned before, the increase of tc for higher hydrogen partial pressure contradicts the results that occur by studying the time progress of the desorption process. The apparent contradiction of recovery time increase for higher partial pressures of hydrogen may be due to hydrogen bonding between adjacent adsorbate protons or even molecules (i.e. before dissociation occurs on the Pd surface). This would effectively account for the dependence of the rate constant on pressure and its decrease with increasing coverage. Moreover, it should be pointed out that the response and the recovery times are determined by the experimental conditions. Previous experimental work has shown that the sensor's response time is decreased if the hydrogen gas flow rate into the cell is increased. In addition, it has been proved that by introducing into the cell an inert gas, in order to remove hydrogen, the recovery time decreases drastically. In the experiment presented [18] in this chapter nitrogen gas was introduced into the measurement cell. The presence of nitrogen gas in the cell helps the hydrogen desorption process in the following mechanism: At saturation point, the hydrogen concentration in gas phase (above the palladium film) is in balance with the adsorbed hydrogen concentration. When nitrogen is introduced into the cell, it drifts the hydrogen molecules which are in gas phase, thus disturbing the system's equilibrium. In order for the system establish its balance again, hydrogen is desorbed from the palladium film and returns to gas phase. For bigger gas flow rates in the chamber the recovery of the signal from the saturation point to the initial value occurs faster. The measurements presented in Figure 3(a) were carried out under constant flow rate of gases of 150 mL/min, hence the comparison between the resulted values of ts and tc is possible. It should be noted that the sorption rate constants ka and kd, have an exponential dependence on the absolute temperature T of the form K~exp(-EA/R T) where EA is the activation energy of adsorption (EA ads) or desorption (EA des). Therefore, at higher temperatures the adsorption and desorption rate constants are increased resulting to the reduction of the response and the recovery time. It is also important to note that the Pd-H (hydride) phase transition does not occur in this system. According to [24] for a 6.5 nm palladium film, which is close to the thickness of our film (6 nm) no transition has been observed for hydrogen concentrations lower to 1%. Thus, for hydrogen concentrations of a few ppb no phase transition can occur. The possibility of a Pd-H (hydride) phase transition is considered to be very small, since the hydrogen concentrations used are extremely low and the sample thickness is very small. Some previous work done [25,26] from other researchers investigating the above phase transition does not present data for palladium films with thickness below 6 nm.

Much work has been done concerning the modelling and measurement of the mechanical, optical and electrical properties of the composites while less work has been done concern‐ ing the evaluation of their thermal properties. Nevertheless the knowledge of thermal properties (thermal effusivity and conductivity) is of great interest in applications (biologi‐ cal implants, electronic packaging) where thermal management is important orin thermoelec‐ tric power applications. The polymers have low values of thermal conductivity (a representative value is 0.2 W/m K) which depends on the degree of crystallinity of the polymer among other factors. Since carbon nanotubes (CNT's) single wall (SWCNT's) or multi-wall (MWCNT's) have very high thermal conductivity (2800-6000 W/m K) it is expected that their introduction to the polymer will greatly enhance the effective thermal conductivity. Al‐ though there is a large scatter in the experimental results [31] (some of them are contradicto‐ ry) the polymer-CNT composites still present quite low values of thermal conductivity. This is attributed to the phonon mismatch at the polymer/CNT interface resulting in high inter‐ face thermal resistance. The work [32] described in this paragraph employs a non-contact and non-destructive technique, photothermal radiometry (PTR) to study heat transport in a novel type of polymer loaded with SWCNT's at various concentrations. A two layer photothermal model is used to analyze thermal transport in the composite film which is deposited on quartz substrate. Furthermore, optical measurements are used to extract the absorption coefficient of the thin films at the excitation beam wavelength and hence eliminate one of the unknown parameters in the theoretical model. A simultaneous fitting of the experimental signal amplitude and phase is compared to the calculated signal in order to extract the thermal

Photothermal Techniques in Material Characterization

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351

A series of well-defined diblock copolymers consisting of 2-(N-carbazolyl)ethyl methacry‐ late (CbzEMA) and 2,2,3,3,4,4,4-heptafluorobutyl methacrylate (HFBMA) (CbzEMAx-*b*-HFBMAy) was synthesized by Reversible Addition-Fragmentation chain Transfer (RAFT) controlled radical polymerization [33]. The RAFT process allows for the preparation of functional polymeric materials characterized by pre-determined molecular weights, nar‐ row molecular weight distributions and pre-defined architectures [34,35]. The polymer films were generated by spin coating on 0.9 cm x 0.9 cm quartz substrates of a dilute THF solution of CbzEMA52*-b-*HFBMA69 and SWCNTs at room temperature. The single wall carbon nanotubes (SWCNT's) used, where 4-5nm (diameter) x 500-1500nm (length) bundles, with 80-90 % carbonaceous purity from Sigma-Aldrich.The first series of samples consisted of three samples (A1,A2,A3) with varying thicknesses and constant % weight of SWCNT's. The thickness of the A series samples measured by a mechanical profilometer varied in a big extend across the surface of the sample. Later on a second series of samples (B0,B1,B2,B3) with different % weight of SWCNT's was fabricated. The spin coater angular velocity was increased at 100 rev/s in order to achieve better thickness homogeneity across the sample, something which was verified by the profilometer measurements. The thickness (with a tolerance of 10 nm) and % weight of SWCNT's are shown in Table 5. The PTR experimen‐ tal setup was described earlier. The excitation beam (512 nm) with incident power of 45 mW

properties of the thin composite films.

*4.2.2. Materials*

#### **4.2. Photothermal radiometry applied on polymer-nanotube blends**

#### *4.2.1. Introduction*

Polymer-based materials with electro-active functionalities have attracted significant attention due to their potential technological applications in many fields ranging from optoelectronics (solar cells, LED's, sensors, thin film transistors), to informatics (optical data storage) and biomaterials (artificial muscles). Carbazole-containing polymers present a lot of interest due to their potential applications as photoconductors, solar cells, gas sensors and photorefractive or charge transporting materials [27-28]. Carbazolyl groups easily form relatively stable radical cations (holes), present comparatively high charge carrier mobility and exhibit high thermal and photochemical stability. Furthermore polymers with fluorinated groups present either in the main backbone or in the side chain, exhibit some outstanding properties, including high thermal stability, chemical inertness, low dielectric constant and dissipation factors, low water absorptivity, and good resistance to surface properties [29-30]. In addition the ability of these materials to stabilize carbon nanotubes in organic solvents via non-covalent (physical) adsorption, renders them promising for use in carbon nanotube (CNT)-based nanotechnology. In general the introduction of carbon nanotubes (CNT's) enhances the mechanical stability of polymers and modifies the thermal and optoelectronic properties since CNT's act as electron acceptors. Although the mechanical properties of composites improve with the addition of CNT's, the electronic behavior is often optimum at low concentrations of nanotubes usually close to the percolation threshold.

Much work has been done concerning the modelling and measurement of the mechanical, optical and electrical properties of the composites while less work has been done concern‐ ing the evaluation of their thermal properties. Nevertheless the knowledge of thermal properties (thermal effusivity and conductivity) is of great interest in applications (biologi‐ cal implants, electronic packaging) where thermal management is important orin thermoelec‐ tric power applications. The polymers have low values of thermal conductivity (a representative value is 0.2 W/m K) which depends on the degree of crystallinity of the polymer among other factors. Since carbon nanotubes (CNT's) single wall (SWCNT's) or multi-wall (MWCNT's) have very high thermal conductivity (2800-6000 W/m K) it is expected that their introduction to the polymer will greatly enhance the effective thermal conductivity. Al‐ though there is a large scatter in the experimental results [31] (some of them are contradicto‐ ry) the polymer-CNT composites still present quite low values of thermal conductivity. This is attributed to the phonon mismatch at the polymer/CNT interface resulting in high inter‐ face thermal resistance. The work [32] described in this paragraph employs a non-contact and non-destructive technique, photothermal radiometry (PTR) to study heat transport in a novel type of polymer loaded with SWCNT's at various concentrations. A two layer photothermal model is used to analyze thermal transport in the composite film which is deposited on quartz substrate. Furthermore, optical measurements are used to extract the absorption coefficient of the thin films at the excitation beam wavelength and hence eliminate one of the unknown parameters in the theoretical model. A simultaneous fitting of the experimental signal amplitude and phase is compared to the calculated signal in order to extract the thermal properties of the thin composite films.

#### *4.2.2. Materials*

in the cell helps the hydrogen desorption process in the following mechanism: At saturation point, the hydrogen concentration in gas phase (above the palladium film) is in balance with the adsorbed hydrogen concentration. When nitrogen is introduced into the cell, it drifts the hydrogen molecules which are in gas phase, thus disturbing the system's equilibrium. In order for the system establish its balance again, hydrogen is desorbed from the palladium film and returns to gas phase. For bigger gas flow rates in the chamber the recovery of the signal from the saturation point to the initial value occurs faster. The measurements presented in Figure 3(a) were carried out under constant flow rate of gases of 150 mL/min, hence the comparison between the resulted values of ts and tc is possible. It should be noted that the sorption rate constants ka and kd, have an exponential dependence on the absolute temperature T of the form

Therefore, at higher temperatures the adsorption and desorption rate constants are increased resulting to the reduction of the response and the recovery time. It is also important to note that the Pd-H (hydride) phase transition does not occur in this system. According to [24] for a 6.5 nm palladium film, which is close to the thickness of our film (6 nm) no transition has been observed for hydrogen concentrations lower to 1%. Thus, for hydrogen concentrations of a few ppb no phase transition can occur. The possibility of a Pd-H (hydride) phase transition is considered to be very small, since the hydrogen concentrations used are extremely low and the sample thickness is very small. Some previous work done [25,26] from other researchers investigating the above phase transition does not present data for palladium films with

Polymer-based materials with electro-active functionalities have attracted significant attention due to their potential technological applications in many fields ranging from optoelectronics (solar cells, LED's, sensors, thin film transistors), to informatics (optical data storage) and biomaterials (artificial muscles). Carbazole-containing polymers present a lot of interest due to their potential applications as photoconductors, solar cells, gas sensors and photorefractive or charge transporting materials [27-28]. Carbazolyl groups easily form relatively stable radical cations (holes), present comparatively high charge carrier mobility and exhibit high thermal and photochemical stability. Furthermore polymers with fluorinated groups present either in the main backbone or in the side chain, exhibit some outstanding properties, including high thermal stability, chemical inertness, low dielectric constant and dissipation factors, low water absorptivity, and good resistance to surface properties [29-30]. In addition the ability of these materials to stabilize carbon nanotubes in organic solvents via non-covalent (physical) adsorption, renders them promising for use in carbon nanotube (CNT)-based nanotechnology. In general the introduction of carbon nanotubes (CNT's) enhances the mechanical stability of polymers and modifies the thermal and optoelectronic properties since CNT's act as electron acceptors. Although the mechanical properties of composites improve with the addition of CNT's, the electronic behavior is often optimum at low concentrations of nanotubes usually

ads) or desorption (EA

des).

K~exp(-EA/R T) where EA is the activation energy of adsorption (EA

**4.2. Photothermal radiometry applied on polymer-nanotube blends**

thickness below 6 nm.

350 Materials Science - Advanced Topics

*4.2.1. Introduction*

close to the percolation threshold.

A series of well-defined diblock copolymers consisting of 2-(N-carbazolyl)ethyl methacry‐ late (CbzEMA) and 2,2,3,3,4,4,4-heptafluorobutyl methacrylate (HFBMA) (CbzEMAx-*b*-HFBMAy) was synthesized by Reversible Addition-Fragmentation chain Transfer (RAFT) controlled radical polymerization [33]. The RAFT process allows for the preparation of functional polymeric materials characterized by pre-determined molecular weights, nar‐ row molecular weight distributions and pre-defined architectures [34,35]. The polymer films were generated by spin coating on 0.9 cm x 0.9 cm quartz substrates of a dilute THF solution of CbzEMA52*-b-*HFBMA69 and SWCNTs at room temperature. The single wall carbon nanotubes (SWCNT's) used, where 4-5nm (diameter) x 500-1500nm (length) bundles, with 80-90 % carbonaceous purity from Sigma-Aldrich.The first series of samples consisted of three samples (A1,A2,A3) with varying thicknesses and constant % weight of SWCNT's. The thickness of the A series samples measured by a mechanical profilometer varied in a big extend across the surface of the sample. Later on a second series of samples (B0,B1,B2,B3) with different % weight of SWCNT's was fabricated. The spin coater angular velocity was increased at 100 rev/s in order to achieve better thickness homogeneity across the sample, something which was verified by the profilometer measurements. The thickness (with a tolerance of 10 nm) and % weight of SWCNT's are shown in Table 5. The PTR experimen‐ tal setup was described earlier. The excitation beam (512 nm) with incident power of 45 mW and dimensions of approximately 2mm illuminated almost at normal incidence the center of the sample surface (composite film). As the modulation frequency was varied in the 102 -105 Hz the signal amplitude and phase was recorded. The reproducibility of the PTR signal measurements was verified within less than 1%. In order to calculate the instrumental transfer function with which we have to correct the experimental data a frequency scan of a thick and opaque sample (steel disc) was performed.

0 [ ]0

*f f z*

*d dz*

k

k

() ()

( )0

*f s s s*

T = =T =

T -- =

103 104 105

f (Hz)

wb

*d dz*

k

sample over the infrared detection range (2-12 μm).

10-9

0

b

w

amplitude and phase.

Ds=1.4x10-6 m2

10-8

10-7

*zld*

= +

( )( )0

 k

*f s s f*

T T - -- =

*d d dz dz zl zl*

= T -- =

*z l z l*

= =

The modulated blackbody radiation emission **SPTR** of the sample is proportional to the weighted depth integral of the temperature field in the sample and it can be separated in two terms, the contribution of the composite film and the contribution of the substrate, as presented in eq. (23) [17] below. The weighting is related to the infrared absorption coefficient **βir** of the

> -90 -85 -80 -75 -70 -65 -60 -55 -50

PTR phase (O

**Figure 7.** Dependence of photothermal signal amplitude (a) and phase (b) on the thermal conductivity (ĸf) of the film

[ ( , ) exp( ) ( , ) exp( ) ]

A series of simulations was performed in order to examine the influence of the film properties (absorption coefficient α, thickness, thermal conductivity and diffusivity) on the signal

The values of the optical and thermal properties of the quartz substrate used in the simulations and calculations used for fitting the theoretical results to the experimental data are:

the quartz substrate were taken from literature while the absorption coefficient at the excitation beam was calculated from the transmission measurements. The IR absorption coefficient of the quartz substrate was taken to be practically infinite. This simplification was backed from the experiment since the PTR signal was dropping from a high value (mV) to practically noise

 b

*l*

+ µ × T × - × × + × T × - × × ò ò (23)

( ) , , , ,

*f s*

*S z z dz z z dz*

*PTR ir f ir f ir s ir s*

*l l d*

/s, ĸs=1.5x10-6 W/(m K), αs=30 m-1, βirs=1x107

)

(a)

103 104 105

f (Hz)

wb

m-1. The thermal and properties of

 0.75 1.5 3.5 6 12 25

Photothermal Techniques in Material Characterization

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(b)

(22)

353

**Table 5.** Sample characteristics

#### *4.2.3. Two layer photothermal model and simulations*

A two layer model was developed in order to simulate the ac temperature field in the samples.

$$\frac{d^2 \Delta T\_f(z, oo)}{dz^2} - \sigma^2 \, \_{T, f} \cdot \Delta T\_f(z, oo) = -\frac{a\_f \cdot I\_0 \cdot \{1 - R\} \cdot e^{-a\_f \cdot \tau}}{\kappa\_f} \text{for } 0 \le z \le l \tag{20}$$

$$\frac{d^2 \Delta T\_s(z, o)}{dz^2} - \sigma^2\_{T, s} \cdot \Delta T\_s(z, o) = -\frac{a\_s \cdot I\_0 \cdot \left(1 - R\_2\right) \cdot e^{-a\_s \cdot \left(z - d\right)}}{\kappa\_s} \text{ for } l \le z \le d + l \tag{21}$$

The illuminated surface of the polymer-nanotube blend is at z=0 while the interface between the blend and the substrate at z=l. The rear surface of the quartz glass is at z=l+d. The ac temperature ∆T is a function of depth z, measured from the surface of the sample and the angular modulation frequency ω=2πf. The subscripts s and f stand for the film and substrate respectively. Using the boundary conditions below, concerning the continuity of heat flux and temperature at the interfaces, the solutions for ∆T(z) in the film and substrate can be evaluated at different values of the modulation frequency.

$$\begin{aligned} -\left[ -\kappa\_f \frac{d\Gamma\_f}{dz} \Big|\_{z=0} \right] &= 0\\ -\left( -\kappa\_s \frac{d\Gamma\_s}{dz} \Big|\_{z=l} \right) - \left( -\kappa\_f \frac{d\Gamma\_f}{dz} \Big|\_{z=l} \right) &= 0\\ \Upsilon\_f(z=l) &= \Upsilon\_s(z=l) \\ -\left( -\kappa\_s \frac{d\Gamma\_s}{dz} \right) \Big|\_{z=l} &= 0 \end{aligned} \tag{22}$$

The modulated blackbody radiation emission **SPTR** of the sample is proportional to the weighted depth integral of the temperature field in the sample and it can be separated in two terms, the contribution of the composite film and the contribution of the substrate, as presented in eq. (23) [17] below. The weighting is related to the infrared absorption coefficient **βir** of the sample over the infrared detection range (2-12 μm).

and dimensions of approximately 2mm illuminated almost at normal incidence the center of the sample surface (composite film). As the modulation frequency was varied in the 102

Hz the signal amplitude and phase was recorded. The reproducibility of the PTR signal measurements was verified within less than 1%. In order to calculate the instrumental transfer function with which we have to correct the experimental data a frequency scan of a thick

**sample film thickness SWCNT % weight**

A1 4,62 μm 4.11

A2 6,16 μm 4.11

A3 8 μm 4.11

B0 35 nm 0

B1 30 nm 1.64

B2 40 nm 4.76

B3 62 nm 9.09

A two layer model was developed in order to simulate the ac temperature field in the samples.

*f*

a

*z*

*f*

a

*T z z l*

*s*

k

The illuminated surface of the polymer-nanotube blend is at z=0 while the interface between the blend and the substrate at z=l. The rear surface of the quartz glass is at z=l+d. The ac temperature ∆T is a function of depth z, measured from the surface of the sample and the angular modulation frequency ω=2πf. The subscripts s and f stand for the film and substrate respectively. Using the boundary conditions below, concerning the continuity of heat flux and temperature at the interfaces, the solutions for ∆T(z) in the film and substrate can be evaluated

*T z lzdl*

k

, 1 , for0

 a

( ) , 1 , for

and opaque sample (steel disc) was performed.

*4.2.3. Two layer photothermal model and simulations*

2 ,

s

*Ts s*

at different values of the modulation frequency.

w

2 ,

s

*dz*

*dz*

w

( ) ( ) ( ) 2 2 0

*dTz I Re*

 w

2 0 2

 a

*s s s*

*z d d Tz I Re*

*f f Tf f*

( ) ( ) ( ) 2

 w

**Table 5.** Sample characteristics

352 Materials Science - Advanced Topics

**Figure 7.** Dependence of photothermal signal amplitude (a) and phase (b) on the thermal conductivity (ĸf) of the film

$$\mathcal{S}\_{\boldsymbol{\rho}\_{\rm FR}(o)} \propto \| \boldsymbol{\mathcal{B}}\_{\boldsymbol{\dot{\nu}},f} \cdot \Big[ \boldsymbol{\Lambda} \Gamma\_f(\boldsymbol{\dot{z}},o) \cdot \exp(-\boldsymbol{\beta}\_{\boldsymbol{\dot{\nu}},f} \cdot \boldsymbol{z}) \cdot d\boldsymbol{z} + \boldsymbol{\beta}\_{\boldsymbol{\dot{\nu}},s} \cdot \int\_{\boldsymbol{l}} \boldsymbol{\Lambda} \Gamma\_s(\boldsymbol{z},o) \cdot \exp(-\boldsymbol{\beta}\_{\boldsymbol{\dot{\nu}},s} \cdot \boldsymbol{z}) \cdot d\boldsymbol{z} \Big] \tag{23}$$

A series of simulations was performed in order to examine the influence of the film properties (absorption coefficient α, thickness, thermal conductivity and diffusivity) on the signal amplitude and phase.

The values of the optical and thermal properties of the quartz substrate used in the simulations and calculations used for fitting the theoretical results to the experimental data are: Ds=1.4x10-6 m2 /s, ĸs=1.5x10-6 W/(m K), αs=30 m-1, βirs=1x107 m-1. The thermal and properties of the quartz substrate were taken from literature while the absorption coefficient at the excitation beam was calculated from the transmission measurements. The IR absorption coefficient of the quartz substrate was taken to be practically infinite. This simplification was backed from the experiment since the PTR signal was dropping from a high value (mV) to practically noise (Df) of the film

level when the sample orientation was reversed (beam entering the sample from the quartz substrate and then meeting the thin film). This means that the quartz substrate is opaque to the IR emission of the polymer-nanotube blend film. In addition the substrate was assumed to be transparent at the excitation beam wavelength 512 nm). Concerning the thermal prop‐ erties of the composite film typical values for polymers were used for the simulations, as well as for starting values of the fitting process. The simulations show that the contribution of the quartz substrate to the overall PTR signal (second term of eq.(23)) can be ignored since the quartz substrate has a very low absorption coefficient at the excitation wavelength. Neverthe‐ less the substrate presence in the theoretical model is necessary since it defines the correct boundary conditions at the interface. The effect of thermal conductivity on the PTR signal as function of modulation frequency is shown in Figure 7. An increase of the thermal conductivity of the film allows the generated heat to diffuse away more efficiently and hence the PTR signal amplitude decreases as the temperature field in the film weakens. The signal phase in the low frequency regime reduces with the increase of thermal conductivity, leading to the formation of an extremum at a frequency defined by the thermal diffusivity of the film. The values of the optical and thermal properties of the quartz substrate used in the simulations and calculations used for fitting the theoretical results to the experimental data are: Ds=1.4x10-6 m2 /s, s=1.5x10-6 W/(m K), s=30 m-1,irs=1x107 m-1. The thermal and properties of the quartz substrate were taken from literature while the absorption coefficient at the excitation beam was calculated from the transmission measurements. The IR absorption coefficient of the quartz substrate was taken to be practically infinite. This simplification was backed from the experiment since the PTR signal was dropping from a high value (mV) to practically noise level when the sample orientation was reversed (beam entering the sample from the quartz substrate and then meeting the thin film). This means that the quartz substrate is opaque to the IR emission of the polymer-nanotube blend film. In addition the Figure 8. Dependence of photothermal signal amplitude (a) and phase (b) on the thermal diffusivity f(Hz)

significantly in the UV range. The transmission deep in the 2650 nm-2900 nm is associated with the OH- bonds. The optical absorption coefficient, presented in Figure 9(b), at the excitation beam wavelength (512 nm) was calculated from the transmission and reflection spectra of the samples. Since the thickness tolerance was 10 nm the samples were assumed to have an average thickness of 42 nm. The PTR signal amplitude and phase, for A series samples, as a function

**Figure 9.** Transmission measurements (a) and calculated value of absorption coefficient (b) at excitation wavelength

5,0x105

1,0x106

af (m-1

)

1,5x106

2,0x106

(a)

signal amplitude differentiates well for the three samples. Since the % weight of SWCNTS's is the same for the three samples one can assume that the optical and thermal properties of the films should be very similar, ignoring non-homogeneities due to the fabrication process, hence the signal differences should be attributed to the composite film thickness variation. More

60

80

100

PTR phase(o

The phase channel of the signal (Figure 10(b)) can differentiate between the three samples up

film thickness is increasing. The decrease of the signal amplitude has to do with the fact that the contribution of the film to the signal depends on the IR emission of the film, which is proportional to the integral of the temperature field over the thickness of the sample multiplied by the factor exp (-βir.z) which results in a lower signal for thicker films. The extraction of the

)

120

140

Hz) range the

(b)

103 104 105

f (Hz)

Hz) range and is increasing and changing slope as the composite

 A1 A2 A3

0 1 2 3 4 5 6 7 8 9 10

(b)

http://dx.doi.org/10.5772/56039

355

Photothermal Techniques in Material Characterization

% weight SWCNT

of modulation frequency are shown in Figure 10. In the low frequency (<103

500 1000 1500 2000 2500 3000 40

l (nm)

 Quartz substrate B0 (0% weight of SWCNT's) B1 (1.64 % weight of SWCNT's) B2 (4.76 % weight of SWCNT's) B3 (9.09 % weight of SWCNT's)

as a function of SWCNT concentration.

Transmission (%)

103 104 105

**Figure 10.** Experimental PTR signal amplitude (a) and phase (b) for the A series of samples

f (Hz)

 A2 A3

10-2 A1

10-4

to a higher frequency (104

10-3

specifically the signal amplitude decreases as the composite film thickness increases.

(a)

thermal properties of the composite film typical values for polymers were used for the simulations, as well as for starting values of the fitting process. The simulations show that the contribution of the **Figure 8.** Dependence of photothermal signal amplitude (a) and phase (b) on the thermal diffusivity (Df) of the film

substrate was assumed to be transparent at the excitation beam wavelength 512 nm). Concerning the

quartz substrate to the overall PTR signal (second term of eq.(23)) can be ignored since the quartz substrate has a very low absorption coefficient at the excitation wavelength. Nevertheless the substrate presence in the theoretical model is necessary since it defines the correct boundary conditions at the interface. The effect of thermal conductivity on the PTR signal as function of modulation frequency is shown in Figure 7. An increase of the thermal conductivity of the film allows the generated heat to diffuse away more efficiently and hence the PTR signal amplitude decreases as the temperature field in the film weakens. The signal phase in the low frequency regime reduces with the increase of thermal conductivity, leading to the formation of an extremum at a frequency defined by the thermal diffusivity of the film. An increase in thermal diffusivity results in an increase of the thermal diffusion length causing the shift of the phase extremum to higher modulation frequencies as seen in Figure 8, as well as an An increase in thermal diffusivity results in an increase of the thermal diffusion length causing the shift of the phase extremum to higher modulation frequencies as seen in Figure 8, as well as an overall increase of the signal phase. The signal amplitude increases with increasing thermal diffusivity although at a first sight one would expect the opposite since high values of thermal diffusivity would mean that heat is carried out faster from the region generated. One should not forget though that the modulated IR emission of the sample is proportional to the integral of the temperature field over the thickness of the sample. Since a higher diffusivity results in a longer thermal diffusion length a thicker part of the sample will contribute to the IR emission leading to stronger signal amplitude.

14 A series of transmission and reflection measurements was performed in the 200 nm -3000 nm in order to help evaluate the absorption coefficient of the samples and reveal its dependence on the SWCNT content. Although the thicknesses of the samples are quite different the optical transmission measurements presented in Figure 9(a) indicate a decrease of transmission with the increase of concentration of SWCNT's. In addition it is clear that the polymers absorb

level when the sample orientation was reversed (beam entering the sample from the quartz substrate and then meeting the thin film). This means that the quartz substrate is opaque to the IR emission of the polymer-nanotube blend film. In addition the substrate was assumed to be transparent at the excitation beam wavelength 512 nm). Concerning the thermal prop‐ erties of the composite film typical values for polymers were used for the simulations, as well as for starting values of the fitting process. The simulations show that the contribution of the quartz substrate to the overall PTR signal (second term of eq.(23)) can be ignored since the quartz substrate has a very low absorption coefficient at the excitation wavelength. Neverthe‐ less the substrate presence in the theoretical model is necessary since it defines the correct boundary conditions at the interface. The effect of thermal conductivity on the PTR signal as function of modulation frequency is shown in Figure 7. An increase of the thermal conductivity of the film allows the generated heat to diffuse away more efficiently and hence the PTR signal amplitude decreases as the temperature field in the film weakens. The signal phase in the low frequency regime reduces with the increase of thermal conductivity, leading to the formation

The values of the optical and thermal properties of the quartz substrate used in the simulations and calculations used for fitting the theoretical results to the experimental data are: Ds=1.4x10-6 m2

were taken from literature while the absorption coefficient at the excitation beam was calculated from the transmission measurements. The IR absorption coefficient of the quartz substrate was taken to be practically infinite. This simplification was backed from the experiment since the PTR signal was dropping from a high value (mV) to practically noise level when the sample orientation was reversed (beam entering the sample from the quartz substrate and then meeting the thin film). This means that the quartz substrate is opaque to the IR emission of the polymer-nanotube blend film. In addition the

Figure 8. Dependence of photothermal signal amplitude (a) and phase (b) on the thermal diffusivity

f(Hz)

substrate was assumed to be transparent at the excitation beam wavelength 512 nm). Concerning the thermal properties of the composite film typical values for polymers were used for the simulations, as well as for starting values of the fitting process. The simulations show that the contribution of the quartz substrate to the overall PTR signal (second term of eq.(23)) can be ignored since the quartz substrate has a very low absorption coefficient at the excitation wavelength. Nevertheless the substrate presence in the theoretical model is necessary since it defines the correct boundary conditions at the interface. The effect of thermal conductivity on the PTR signal as function of modulation frequency is shown in Figure 7. An increase of the thermal conductivity of the film allows the generated heat to diffuse away more efficiently and hence the PTR signal amplitude decreases as the temperature field in the film weakens. The signal phase in the low frequency regime reduces with the increase of thermal conductivity, leading to the formation of an extremum at a frequency defined

An increase in thermal diffusivity results in an increase of the thermal diffusion length causing the shift of the phase extremum to higher modulation frequencies as seen in Figure 8, as well as an overall increase of the signal phase. The signal amplitude increases with increasing thermal diffusivity although at a first sight one would expect the opposite since high values of thermal diffusivity would mean that heat is carried out faster from the region generated. One should not forget though that the modulated IR emission of the sample is proportional to the integral of the temperature field over the thickness of the sample. Since a higher diffusivity results in a longer thermal diffusion length a thicker part of the sample will contribute to the

**Figure 8.** Dependence of photothermal signal amplitude (a) and phase (b) on the thermal diffusivity (Df) of the film

f(Hz) f(Hz)

PTR phase (o

)

m-1. The thermal and properties of the quartz substrate

103 104 105

 0.08x10-6 (m2 /s)

 0.18x10-6 0.28x10-6 0.48x10-6 0.68x10-6 0.88x10-6 /s,

of an extremum at a frequency defined by the thermal diffusivity of the film.

(a)

103 104 105

s=1.5x10-6 W/(m K), s=30 m-1,irs=1x107

(Df) of the film

354 Materials Science - Advanced Topics

10-9

10-8

10-7

by the thermal diffusivity of the film.

IR emission leading to stronger signal amplitude.

14

A series of transmission and reflection measurements was performed in the 200 nm -3000 nm in order to help evaluate the absorption coefficient of the samples and reveal its dependence on the SWCNT content. Although the thicknesses of the samples are quite different the optical transmission measurements presented in Figure 9(a) indicate a decrease of transmission with the increase of concentration of SWCNT's. In addition it is clear that the polymers absorb

An increase in thermal diffusivity results in an increase of the thermal diffusion length causing the shift of the phase extremum to higher modulation frequencies as seen in Figure 8, as well as an

**Figure 9.** Transmission measurements (a) and calculated value of absorption coefficient (b) at excitation wavelength as a function of SWCNT concentration.

significantly in the UV range. The transmission deep in the 2650 nm-2900 nm is associated with the OH- bonds. The optical absorption coefficient, presented in Figure 9(b), at the excitation beam wavelength (512 nm) was calculated from the transmission and reflection spectra of the samples. Since the thickness tolerance was 10 nm the samples were assumed to have an average thickness of 42 nm. The PTR signal amplitude and phase, for A series samples, as a function of modulation frequency are shown in Figure 10. In the low frequency (<103 Hz) range the signal amplitude differentiates well for the three samples. Since the % weight of SWCNTS's is the same for the three samples one can assume that the optical and thermal properties of the films should be very similar, ignoring non-homogeneities due to the fabrication process, hence the signal differences should be attributed to the composite film thickness variation. More specifically the signal amplitude decreases as the composite film thickness increases.

**Figure 10.** Experimental PTR signal amplitude (a) and phase (b) for the A series of samples

The phase channel of the signal (Figure 10(b)) can differentiate between the three samples up to a higher frequency (104 Hz) range and is increasing and changing slope as the composite film thickness is increasing. The decrease of the signal amplitude has to do with the fact that the contribution of the film to the signal depends on the IR emission of the film, which is proportional to the integral of the temperature field over the thickness of the sample multiplied by the factor exp (-βir.z) which results in a lower signal for thicker films. The extraction of the optical and thermal properties for the composite films was accomplished by comparing (fitting) the theoretical signal to the experimental data, with the aid of a MATLAB code.

absorption coefficient and the thermophysical parameters (thermal conductivity and diffu‐ sivity) due to the variation of the concentration of the CNT's. Nevertheless the fact that the relative uncertainty in the film thickness is quite high creates an additional obstacle for the

**Figure 12.** Theoretical signal amplitude (a) and phase (b) fit (solid lines) to the experimental signal amplitude and

PTR phase (o

**Figure 14.** Theoretical signal amplitude (a) and phase (b) fit (solid lines) to the experimental signal amplitude and

103 104 105

**Figure 13.** Experimental PTR signal amplitude (a) and phase (b) for the B series of samples

f (Hz)

 B1 B2 B3

)

(a)

B1 B2 B3

103 104 105

Photothermal Techniques in Material Characterization

http://dx.doi.org/10.5772/56039

357

(b)

f (Hz)

fitting process.

phase for sample A

10-4

10-3

phase for sample B1

10-2

**Figure 11.** Theoretical signal amplitude (a) and phase (b) fit (solid lines) to the experimental signal amplitude and phase for sample A1

**Table 6.** Extracted thermal and optical parameters for the A series samples

Both channels of experimental information i.e signal amplitude and signal phase were used in order to achieve higher accuracy. The fitting results for samples A1, A2 are shown in Figures 11 and 12 respectively while the extracted values of the thermal and optical properties of the samples are tabulated in Table 6. The theoretical curves fit quite well the experimental data, except at the edges of the frequency spectrum. This discrepancy could be due to the simplified theoretical model which ignores multiple reflections in the thin film. According to Arjona et.al [36] this is more significant for phase channel in the higher modulation frequencies while for the amplitude channel is more significant for the lower modulation frequencies. The calculated thermal parameters are within the range of values reported in the literature [30] for polymer carbon-nanotube composites. The second series (B series) of samples consists of very thin composite layers (nm range) and is more interesting since in this group we have a variation of the concentration of SWCNT's. The increase of the experimental signal amplitude among the B series samples as observed in Figure 13 could be attributed to the increased optical absorption coefficient and thermal conductivity as a function of the % weight of SWCNT's which increases from B1 to B3. The samples of B series have different concentration of SWCNTS's ranging from 0-9 % weight. Due to the increase of the angular speed of the spin coater the thicknesses of the films were now in the nm range and are actually very similar within the experimental tolerance which is 10 nm. The observed differences have then to be attributed to the change of the optical absorption coefficient and the thermophysical parameters (thermal conductivity and diffu‐ sivity) due to the variation of the concentration of the CNT's. Nevertheless the fact that the relative uncertainty in the film thickness is quite high creates an additional obstacle for the fitting process.

optical and thermal properties for the composite films was accomplished by comparing (fitting) the theoretical signal to the experimental data, with the aid of a MATLAB code.

**Figure 11.** Theoretical signal amplitude (a) and phase (b) fit (solid lines) to the experimental signal amplitude and

D (m2/s) 0.19x10-6 0.33x10-6 0.59x10-6 ĸ (W/m K) 2.8 1.45 0.98 βir (m-1) 4.4x105 4.7x105 4.7x105 α (m-1) 1.4x106 1.4x106 1.4x106

Both channels of experimental information i.e signal amplitude and signal phase were used in order to achieve higher accuracy. The fitting results for samples A1, A2 are shown in Figures 11 and 12 respectively while the extracted values of the thermal and optical properties of the samples are tabulated in Table 6. The theoretical curves fit quite well the experimental data, except at the edges of the frequency spectrum. This discrepancy could be due to the simplified theoretical model which ignores multiple reflections in the thin film. According to Arjona et.al [36] this is more significant for phase channel in the higher modulation frequencies while for the amplitude channel is more significant for the lower modulation frequencies. The calculated thermal parameters are within the range of values reported in the literature [30] for polymer carbon-nanotube composites. The second series (B series) of samples consists of very thin composite layers (nm range) and is more interesting since in this group we have a variation of the concentration of SWCNT's. The increase of the experimental signal amplitude among the B series samples as observed in Figure 13 could be attributed to the increased optical absorption coefficient and thermal conductivity as a function of the % weight of SWCNT's which increases from B1 to B3. The samples of B series have different concentration of SWCNTS's ranging from 0-9 % weight. Due to the increase of the angular speed of the spin coater the thicknesses of the films were now in the nm range and are actually very similar within the experimental tolerance which is 10 nm. The observed differences have then to be attributed to the change of the optical

**Table 6.** Extracted thermal and optical parameters for the A series samples

phase for sample A1

356 Materials Science - Advanced Topics

(a) (b)

**A1 A2 A3**

**Figure 12.** Theoretical signal amplitude (a) and phase (b) fit (solid lines) to the experimental signal amplitude and phase for sample A

**Figure 13.** Experimental PTR signal amplitude (a) and phase (b) for the B series of samples

**Figure 14.** Theoretical signal amplitude (a) and phase (b) fit (solid lines) to the experimental signal amplitude and phase for sample B1

In general an increase in thermal conductivity which is expected with the addition of CNT's should be accompanied with a reduction of the signal amplitude in contrast with the experi‐ mental results shown in Figure 13. Nevertheless the simulations indicate that for thinner (~20 nm-B series) the amplitude variation with the increase of thermal conductivity is much smaller as compared to the case of thicker films (~μm range-A series). In addition the simulations show that the signal amplitude is affected more by the optical absorption coefficient. This means that the small decrease in amplitude due to the increase of thermal conductivity is masked by a much bigger amplitude increase due to the increase of the absorption coefficient at the excitation wavelength. In the case of sample B1 the fitted values were ĸf =2.0 W/mK and Df =1.06x10-7 m2 /s.The fitting for samples B1and B2 was not attempted due to the high uncer‐ tainty in the film thickness which was about the same size as the measured value of the thickness.

Cyprus where the photothermal experiments took place. I would like to express my thanks to Dr. T. Krasia and Ms. I. Savva from the Department of Mechanical and Manufacturing Engineering, University of Cyprus for the fabrication of the polymer-nanotube blend samples as well as to Dr. N. C. Papanicolaou from the Department of Computer Science, University of Nicosia for his contribution in the modelling and simulations of the photothermal response of

Photothermal Techniques in Material Characterization

http://dx.doi.org/10.5772/56039

359

Department of Electrical & Computer Engineering, University of Nicosia, Nicosia, Cyprus

[1] Mandelis A. Diffusion-Wave Fields, Mathematical Methods and Green Functions.

[2] Fournier D., Boccara C., Skumanich A. and Amer N.M. Photothermal investigation of transport in semiconductors: Theory and experiment. Journal of Applied Physics

[3] Ikari T., Salnick A. and Mandelis A. Theoretical and experimental aspects of threedimensional infrared photothermal radiometry of semiconductors. Journal of Ap‐

[4] Rosengwaig A., Opsal J., Smith W. L. and Willenborg D. L. Detection of thermal waves through optical reflectance. Applied Physics Letters 1985;46(11) http://

[5] Wurm S., Alpern P., Savignac D and Kakoschke R. Modulated optical reflectance measurements on amorphous silicon layers and detection of residual defects. Ap‐

[6] Christofides C. Photomodulated Thermoreflectance of Implanted Wafers. In Christo‐ fides C., Ghibaudo G. (eds.) Effect of Disorder and Defects in Ion-Implanted Semi‐ conductors: Optical and Photothermal Characterization, Semiconductors and

[7] Fotsing J. L. N., Hoffmeyer M., Chotikaprakhan S., Dietzel D., Pelzl J., Bein B. K., Cer‐ queira F., Macedo F., and Ferreira J. A. Laser modulated optical reflectance of thin semiconductor films on glass. Review of Scientific Instruments 2003;74(1) 873-876.

Semimetals, Vol. 46. New York: Academic Press; 1997. p 115-147.

the polymer-carbon nanotube blends.

New York: Springer; 2001.

dx.doi.org/10.1063/1.95794.

plied Physics 1999; 85(10), 7392 – 7397.

plied Physics A 1988; 47(2) 147-155.

1986; 59(3) 787-795.

**Author details**

Marios Nestoros

**References**

## **5. Conclusion**

In this chapter we introduced the physics of thermal and electronic waves that underpin photothermal phenomena and described two basic photothermal techniques, namely Photo-Modulated Thermoreflectance (PMTR) and Photothermal Radiometry (PTR). The abilities of these experimental techniques were explored in the study of thin metal film-gas interaction kinetics and the optical and thermal characterization of composite materials.

In the first application it was verified that that hydrogen adsorbs to palladium dissociatively while desorption is characterized by a molecular reaction. As a consequence of the high sensitivity of the PMTR sensor, the determination of the rate constants of adsorption ka and desorption kd for very small H2 concentrations was possible. In addition the analysis of the time evolution of the PMTR signal showed that the values of kd were several orders of magnitude smaller than the ones resulting for the rate constant of adsorption ka. Thus, in order to study the adsorption process, the competitive mechanism of desorption should not be taken into account. For the adsorption mechanism it was observed that the resulting values of the adsorption rate constant ka decrease with the increase of H2 concentration. These results also justify the increase of the sensor's response time for larger H2 concentrations.

In the second application it was shown that Photothermal Radiometry (PTR) is able to characterize composite materials like polymer-carbon nanotube blends. The photothermal signal (amplitude and phase) is very sensitive to the change of SWCNT's concentration as well as to the polymer film thickness. With the aid of a two layer photothermal model it was possible to extract thermal and optical properties of the composite materials.

## **Acknowledgements**

I gratefully acknowledge the contribution and collaboration of Prof. C. Christofides and Prof. A. Othonos, directors of the Photonics and Optoelectronics Laboratory at the University of Cyprus where the photothermal experiments took place. I would like to express my thanks to Dr. T. Krasia and Ms. I. Savva from the Department of Mechanical and Manufacturing Engineering, University of Cyprus for the fabrication of the polymer-nanotube blend samples as well as to Dr. N. C. Papanicolaou from the Department of Computer Science, University of Nicosia for his contribution in the modelling and simulations of the photothermal response of the polymer-carbon nanotube blends.

## **Author details**

In general an increase in thermal conductivity which is expected with the addition of CNT's should be accompanied with a reduction of the signal amplitude in contrast with the experi‐ mental results shown in Figure 13. Nevertheless the simulations indicate that for thinner (~20 nm-B series) the amplitude variation with the increase of thermal conductivity is much smaller as compared to the case of thicker films (~μm range-A series). In addition the simulations show that the signal amplitude is affected more by the optical absorption coefficient. This means that the small decrease in amplitude due to the increase of thermal conductivity is masked by a much bigger amplitude increase due to the increase of the absorption coefficient at the

tainty in the film thickness which was about the same size as the measured value of the

In this chapter we introduced the physics of thermal and electronic waves that underpin photothermal phenomena and described two basic photothermal techniques, namely Photo-Modulated Thermoreflectance (PMTR) and Photothermal Radiometry (PTR). The abilities of these experimental techniques were explored in the study of thin metal film-gas interaction

In the first application it was verified that that hydrogen adsorbs to palladium dissociatively while desorption is characterized by a molecular reaction. As a consequence of the high sensitivity of the PMTR sensor, the determination of the rate constants of adsorption ka and desorption kd for very small H2 concentrations was possible. In addition the analysis of the time evolution of the PMTR signal showed that the values of kd were several orders of magnitude smaller than the ones resulting for the rate constant of adsorption ka. Thus, in order to study the adsorption process, the competitive mechanism of desorption should not be taken into account. For the adsorption mechanism it was observed that the resulting values of the adsorption rate constant ka decrease with the increase of H2 concentration. These results also

In the second application it was shown that Photothermal Radiometry (PTR) is able to characterize composite materials like polymer-carbon nanotube blends. The photothermal signal (amplitude and phase) is very sensitive to the change of SWCNT's concentration as well as to the polymer film thickness. With the aid of a two layer photothermal model it was possible

I gratefully acknowledge the contribution and collaboration of Prof. C. Christofides and Prof. A. Othonos, directors of the Photonics and Optoelectronics Laboratory at the University of

/s.The fitting for samples B1and B2 was not attempted due to the high uncer‐

=2.0 W/mK and

excitation wavelength. In the case of sample B1 the fitted values were ĸf

kinetics and the optical and thermal characterization of composite materials.

justify the increase of the sensor's response time for larger H2 concentrations.

to extract thermal and optical properties of the composite materials.

Df

=1.06x10-7 m2

358 Materials Science - Advanced Topics

**5. Conclusion**

**Acknowledgements**

thickness.

#### Marios Nestoros

Department of Electrical & Computer Engineering, University of Nicosia, Nicosia, Cyprus

## **References**

[8] Lavega-Sanchez A., Salazar A., Ocariz A., Pottier L., Gomez E., Villar L.M, Macho E., Thermal diffusivity measurements in porous ceramics by photothermal methods. Applied Physics A 1997; 65 (1) 15-22.

[20] Kalli K., Othonos A. and Christofides C. Characterization of reflectivity inversion, αand β-phase transitions and nanostructure formation in hydrogen activated thin Pd films on silicon based substrates. Journal of Applied Physics 2002; 91(6) 3829 – 3840.

Photothermal Techniques in Material Characterization

http://dx.doi.org/10.5772/56039

361

[21] Mandelis A. and Christofides C. Physics, Chemistry and Technology of Solid State Gas Sensor Devices, Vol. 125 in Chemical Analysis. New York: Wiley Interscience;

[22] Lundström I. Why bother about gas-sensitive field-effect devices? Sensors and Ac‐

[23] Lundström I., Shivaraman M.S, and Svensson C. Chemical reactions on palladium surfaces studied with Pd-MOS structures. Surface Science 1977;64 497-519.

[24] Wang C., Mandelis A. and Garcia J.A. Pd/PVDF Thin Film Hydrogen Sensor System based on Photopyroelectric Purely-Thermal-Wave Interference. Sensors Actuators B

[25] Christofides C., and Mandelis, A. Operating characteristics and comparison of photo‐ pyroelectric and piezoelectric sensors for trace hydrogen gas detection. I. Develop‐ ment of a new photopyroelectric sensor. Journal of Applied Physics (1989); 66(9)

[26] Christofides C., Mandelis A., Rawski J. and Rehm S. Photopyroelectric detection of hydrogen/oxygen mixtures. Review of Scientific Instruments 1993; 64(12) 3563- 3571.

[27] Gerad M., Chaubey A. and Malhotra B. Application of conducting polymers to bio‐

[28] Brar A. S. and Gandhi S. Microstructure determination of 9-ethyl-3-hydroxymethyl‐ carbazolyl acrylate/methacrylonitrile using two-dimensional NMR spectroscopy.

[29] Zheng-hong L. and H. Teng-yun H. Synthesis and characterization of poly(dimethyl‐ siloxane)-block-poly(2,2,3,3,4,4,4-heptafluorobutyl methacrylate) diblock copolymers with low surface energy prepared by atom transfer radical polymerization. Reactive

[30] Wang QY., Zhang QH., Zhan XL. and Chen FQ., Low Surface Energy Polymer with

[31] Han Z., Fina A. Thermal conductivity of carbon nanotubes and their polymer nano‐

[32] Nestoros M., Savva I., Krasia T., Papanicolaou N. C., Othonos A. and Christofides C. Thermal and optical characterization of methacrylate-based polymers with embed‐ ded carbon nanotubes, 9th International Conference on Nanosciences & Nanotech‐

[33] Savva I., Demetriou M., Othonos. A, Turcu R., Popa A., Macavei S. and Krasia-Chris‐ toforou T. Well-defined fluoro- and carbazole-containing diblock copolymers: syn‐

Fluorinated Side Group. Progress in Chemistry 2009; 21(10) 2183-2187.

composites: A review. Progress in Polymer Science 2011; 36(7) 914-944.

nologies, 3-6 July, 2012, Thessaloniki, Book of Abstracts p. 134.

sensors Biosensors & Bioelectronics 2002; 17(5) 345–359.

Journal of Molecular Structure, 2007; 832(1-3) 26–37.

& Functional Polymers, 2008, 68(5) 931–942.

1993.

tuators A 1996; 56(1-2),75-82.

1999; 60 228 – 237.

3986-3992.

[20] Kalli K., Othonos A. and Christofides C. Characterization of reflectivity inversion, αand β-phase transitions and nanostructure formation in hydrogen activated thin Pd films on silicon based substrates. Journal of Applied Physics 2002; 91(6) 3829 – 3840.

[8] Lavega-Sanchez A., Salazar A., Ocariz A., Pottier L., Gomez E., Villar L.M, Macho E., Thermal diffusivity measurements in porous ceramics by photothermal methods.

[9] Schmidt A. J., Cheaito R., and Chiesa M. Characterization of thin metal films via fre‐ quency-domain thermoreflectance. Journal of Applied Physics 2010; 107(2), 024908.

[10] Ikari T., Roger J. P, and Fournier D. Photothermal microscopy of silicon epitaxial lay‐ er on silicon substrate with depletion region at the interface. Review of Scientific In‐

[11] Sheard S., Somekh M., and Hiller T. Non-contacting determination of carrier lifetime and surface recombination velocity using photothermal radiometry. Material Science

[12] Othonos A., Christofides C. and Mandelis A. Photothermal radiometric investigation of implanted silicon: The influence of dose and thermal annealing. Applied Physics

[13] Ravi J., Lu Y., Longuemart S., Paoloni S., Pfeiffer H.,Thoen J., and Glorieux C. Opto‐ thermal depth profiling by neural network infrared radiometry signal recognition.

[14] Christofides C., Nestoros M. and Othonos. A Photothermal Radiometric Study of Im‐ planted Semiconductors. In: Mandelis A. and Hess P. (eds.) Progress in Photothermal and Photoacoustic Science and Technology, Volume IV: Semiconductors and Elec‐

[15] Depriester M., Hus P., Delenclos S., and Sahraoui H. A. Study of thermal parameters' temperature dependence in solids using photothermal radiometry. Review of Scien‐

[16] Macedo F., Vaz1 F., Torrelli M., Faria R. T., Cavaleiro A., Barradas N. P., Alves E., Junge K. H. and Bein B. K. TiO2 coatings with Au nanoparticles analysed by photo‐ thermal IR radiometry. Journal of Physics D: Applied Physics 2012; doi:

[17] Hellen A., Matvienko A., Mandelis A., Finer Y., and Amaechi B. T. Optothermophys‐ ical properties of demineralized human dental enamel determined using photother‐ mally generated diffuse photon density and thermal-wave fields. Applied Optics

[18] Demetriou C., Nestoros M. and Christofides C. Photothermal hydrogen sensor: the technique, experimental process, and physicochemical analysis. Applied Physics A

[19] Schlapbach L., Züttel A., Gröning P., Gröning O. and Aebi P. Hydrogen for novel

materials and devices. Applied Physics A 2001;72(2), 245-253.

Journal of Applied Physics 2005; 97(1) http://dx.doi.org/10.1063/1.1821635

Applied Physics A 1997; 65 (1) 15-22.

360 Materials Science - Advanced Topics

struments 2003;74(1) 553-556.

Letters 1996; 69(6) 821-823.

and Engineering: B 1990; 5(2) 101-105.

tronic Materials. Bellingham; SPIE; 2000. p110-142.

tific Instruments 2007; 78(3), 109-113.

10.1088/0022-3727/45/10/105301

2010;49, (36) 6938-6951.

2008; 92 (3), 651-658.

thesis, characterization and immobilization onto Au-coated silicon surfaces. RSC Advances 2012; 2(10) 8741-8751.

**Section 3**

**Soldering**

**Section 3**

## **Soldering**

thesis, characterization and immobilization onto Au-coated silicon surfaces. RSC

[34] Sun G., Cheng C. and Wooley K.L. Reversible Addition-Fragmentation Chain Trans‐ fer (RAFT) Polymerization of 4-Vinylbenzaldehyde. Macromolecules 2007; 40(4)

[35] Konkolewicz D., Hawkett B. S., Weale A. G., Perrier S. Raft Polymerization Kinetics: How long are the Cross-Terminating Oligomers? Journal of Polymer Science: Part A:

[36] Arjona-Zambrano M. A., Miranda-Ordonez J., Esquivel-Medina R. A, Peñuñuri F., Martinez P. and Gil-Alvarado J. J. Effect of the multiple reflections of a light beam on the thermal wave field of a sample of finite thickness. Journal of Applied Physics

Advances 2012; 2(10) 8741-8751.

Polymer Chemistry 2009; 47(14) 3455-3466.

2012; 111(9) http://dx.doi.org/10.1063/1.4717115

793-795.

362 Materials Science - Advanced Topics

**Chapter 15**

**Laser Soldering**

Ezeonu Stella Ogochukwu

http://dx.doi.org/10.5772/51527

**1. Introduction**

the reflow process [2].

**1.1. Laser soldering**

Additional information is available at the end of the chapter

The trends toward miniaturization of electronic devices and the use of expensive, temperature –sensitive components used in the telecom equipment industry have led to the demand for new, highly controllable selective laser soldering technology [1, 2]. Secondly, modern highdensity electronic and electro-optic subassemblies usually include thermally-sensitive components as well as complex three-dimensional (3-D) circuit geometries that cannot be soldered using conventional wave or reflow soldering techniques [6]. The laser is especially used for soldering of the temperature-sensitive assemblies and boards and it is also used for joining of assemblies with high thermal capacity that could not be conventionally soldered in

Laser soldering is a technique where a precisely focused laser beam provides controlled heating of the solder alloy leading to a fast and non-destructive of an electrical joint. The process uses a controlled laser beam to transfer energy to a soldering location where the absorbed energy heats the solder until it reaches its melting temperature leading to the

Laser soldering uses solder, which in liquid state wet the materials to be joined and provide mechanically and electrically stable connections when solidified. The ease of controlling the shape and location of the heating area leads to reliable solder joints with minimal component heating and is well suitable for high density packaging. Since it is only the solder joint region is heated during this type of soldering process without damaging the electronic components or the board, laser soldering process is of more benefit to interconnections using solders with a wide range of melting temperatures when compared with convectional infrared reflow process [4].The energy for melting of solder is applied by mean of laser beam. The use of laser

> © 2013 Ogochukwu; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

© 2013 Ogochukwu; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

soldering of the contact and this completely eliminates any mechanical contact [3].

## **Chapter 15**

## **Laser Soldering**

Ezeonu Stella Ogochukwu

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51527

## **1. Introduction**

The trends toward miniaturization of electronic devices and the use of expensive, temperature –sensitive components used in the telecom equipment industry have led to the demand for new, highly controllable selective laser soldering technology [1, 2]. Secondly, modern highdensity electronic and electro-optic subassemblies usually include thermally-sensitive components as well as complex three-dimensional (3-D) circuit geometries that cannot be soldered using conventional wave or reflow soldering techniques [6]. The laser is especially used for soldering of the temperature-sensitive assemblies and boards and it is also used for joining of assemblies with high thermal capacity that could not be conventionally soldered in the reflow process [2].

### **1.1. Laser soldering**

Laser soldering is a technique where a precisely focused laser beam provides controlled heating of the solder alloy leading to a fast and non-destructive of an electrical joint. The process uses a controlled laser beam to transfer energy to a soldering location where the absorbed energy heats the solder until it reaches its melting temperature leading to the soldering of the contact and this completely eliminates any mechanical contact [3].

Laser soldering uses solder, which in liquid state wet the materials to be joined and provide mechanically and electrically stable connections when solidified. The ease of controlling the shape and location of the heating area leads to reliable solder joints with minimal component heating and is well suitable for high density packaging. Since it is only the solder joint region is heated during this type of soldering process without damaging the electronic components or the board, laser soldering process is of more benefit to interconnections using solders with a wide range of melting temperatures when compared with convectional infrared reflow process [4].The energy for melting of solder is applied by mean of laser beam. The use of laser

© 2013 Ogochukwu; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Ogochukwu; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

technology provides precise heating, preventing the high sensitive components from being subjected to thermal stresses while allowing soldering of such sensitive components at high temperatures [2, 3]. Using focusing optics allows soldering in small spaces and also of fine pitch components and the availability of motorized optics optimizes the focal point for each joint. [3].

fiber optic cable. The required temperature at the solder joint is generated by absorbing of the heat irradiation. The application of energy can be controlled precisely. This procedure is suitable both for reflow soldering with soldering paste and soldering with solder wire.

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The output of the laser is delivered by the fiber optic module, consisting of input coupling optics mounted on an adjustable mount, a connectorized armor jacketed fiber cable, and the output coupling optics, which collimates and focuses the beam toward the target region.

Due to the precision and controllability offered by laser, the devices are normally mated to automated precision X-Y positioning tables. The devices are placed into soldering position by a servo-operated XY table. This X-Y table is used to accurately position and move the workpiece under a fixed laser beam and this actually provides precise, contactless soldering. A peripheral component interconnect-bus card in the computer controls the X-Y stage [14]. During solder‐ ing, the movement of the laser spots on a given surface mount device (SMD) is taken care of by a computer controlled positioning mirrors, called galvanometers and these galvanometers are used to control the path of the laser. These galvanometers are programmed for each

The solder material needed can be provided by wire feeding system or can be applied by solder

The high level of automation of the laser soldering system results in a very repetitive process. For the process stability to be increased a closed loop temperature control of the solder joint is necessary and this can be done by using a pyrometer. This pyrometer serves as a reliable quality control during the soldering of miniaturized components in temperature sensitive surroundings [15]. The pyrometer continually monitors the surface temperature every 10ms during the optimal control process and this ensures a repeatable and reliable process [16]. This pyrometer is integrated in the processing head and aligned in the optical path of the laser beam. The CCD camera could be integrated to directly view the laser spot using accessories that allow co-axial real time viewing of the laser spot. When there is a deviation in the temperature (either high or low) of the emitted heat waves, a feedback control circuit would help in adapting the

In contrast to other conventional soldering techniques, laser soldering offers a lot of advan‐

**•** It reduces intermetallic compound formation, due to rapid joint formation which results

**•** It involves a fine grain size (due to rapid cooling), resulting in better fatigue properties.

component type, and paths are stored in computer interface.

laser output in less than few seconds.

**•** Contactless, locally limited application of energy

**•** Temporally and spatially well-controlled energy input,

tages. They include [6, 17]:

**•** Low thermal stress

high quality joint.

**•** It is also has low maintenance **•** Very flexible and easily adapted

paste.

Laser soldering has been applied in the production of electronic assemblies and in the soldering of electronics components to circuit boards [5-8]. The biomedical sector is also experimenting with the laser soldering as a means of joining tissues without sutures. Its use in this sector is driven by the desire for minimally invasive surgical joining proce‐ dure in which the surrounding tissues is unaffected [9]. A lot of work has been carried out on its biomedical applications [10-13].

### **1.2. The laser soldering system**

The laser soldering system consist of laser generator, fiber optic module, focusing optics, computer vision module with camera, illuminator and the motion module, the X-Y positioning stage with the servo control system shown in figure 1

**Figure 1.** The block diagram of a laser soldering system

The laser beam is generated from the laser diode and modulated. With the optical system the laser beam is focused accurately on the solder joint since the laser beam is guided by a flexible fiber optic cable. The required temperature at the solder joint is generated by absorbing of the heat irradiation. The application of energy can be controlled precisely. This procedure is suitable both for reflow soldering with soldering paste and soldering with solder wire.

The output of the laser is delivered by the fiber optic module, consisting of input coupling optics mounted on an adjustable mount, a connectorized armor jacketed fiber cable, and the output coupling optics, which collimates and focuses the beam toward the target region.

Due to the precision and controllability offered by laser, the devices are normally mated to automated precision X-Y positioning tables. The devices are placed into soldering position by a servo-operated XY table. This X-Y table is used to accurately position and move the workpiece under a fixed laser beam and this actually provides precise, contactless soldering. A peripheral component interconnect-bus card in the computer controls the X-Y stage [14]. During solder‐ ing, the movement of the laser spots on a given surface mount device (SMD) is taken care of by a computer controlled positioning mirrors, called galvanometers and these galvanometers are used to control the path of the laser. These galvanometers are programmed for each component type, and paths are stored in computer interface.

The solder material needed can be provided by wire feeding system or can be applied by solder paste.

The high level of automation of the laser soldering system results in a very repetitive process. For the process stability to be increased a closed loop temperature control of the solder joint is necessary and this can be done by using a pyrometer. This pyrometer serves as a reliable quality control during the soldering of miniaturized components in temperature sensitive surroundings [15]. The pyrometer continually monitors the surface temperature every 10ms during the optimal control process and this ensures a repeatable and reliable process [16]. This pyrometer is integrated in the processing head and aligned in the optical path of the laser beam.

The CCD camera could be integrated to directly view the laser spot using accessories that allow co-axial real time viewing of the laser spot. When there is a deviation in the temperature (either high or low) of the emitted heat waves, a feedback control circuit would help in adapting the laser output in less than few seconds.

In contrast to other conventional soldering techniques, laser soldering offers a lot of advan‐ tages. They include [6, 17]:

technology provides precise heating, preventing the high sensitive components from being subjected to thermal stresses while allowing soldering of such sensitive components at high temperatures [2, 3]. Using focusing optics allows soldering in small spaces and also of fine pitch components and the availability of motorized optics optimizes the focal point for each

Laser soldering has been applied in the production of electronic assemblies and in the soldering of electronics components to circuit boards [5-8]. The biomedical sector is also experimenting with the laser soldering as a means of joining tissues without sutures. Its use in this sector is driven by the desire for minimally invasive surgical joining proce‐ dure in which the surrounding tissues is unaffected [9]. A lot of work has been carried

The laser soldering system consist of laser generator, fiber optic module, focusing optics, computer vision module with camera, illuminator and the motion module, the X-Y positioning

The laser beam is generated from the laser diode and modulated. With the optical system the laser beam is focused accurately on the solder joint since the laser beam is guided by a flexible

joint. [3].

366 Materials Science - Advanced Topics

out on its biomedical applications [10-13].

**Figure 1.** The block diagram of a laser soldering system

stage with the servo control system shown in figure 1

**1.2. The laser soldering system**

The limitation of laser soldering lies with its extreme non-equilibrium nature. For a given laser soldering tasks, every joint have its own individual thermal mass and reflectivity and therefore demands a precisely defined laser impulse. Any slight deviation such as bent lead or a slight change in the amount of solder paste may lead to open or destroyed joint [18]. Other limitations of laser soldering include; limited ability to solder large areas, high relative price of the laser and additional cost for safety requirements.

*Single spot soldering*: This method of soldering is particularly applicable for multi-dimensional configurations, tight packages and heat-sensitive components or substrates. In this process, a single contact is quickly and precisely soldered by the laser spot, offering high flexibility, quality and reproducibility. A point laser beam controlled by software moves to each pro‐ grammed point. Here the solder points are heated with individually matched beam diameter

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*Simultaneous soldering*: This concept allows fast soldering of individual components or complete connection arrays. Several contact points are radiated and soldered simultaneously and no relative movement between the laser beam and the workpiece is necessary. Here line-

*Mask soldering*: Mask soldering is applicable for very sensitive substrates or for the soldering of fine contacts and structure. In this process, a mask is placed between the collimated laser beam which is mostly line shaped and the target. The mask covers all parts which should not be exposed to the laser radiation. The laser beam only passes through the openings in the mask only where a contact to be soldered lies underneath which ensures locally confined heating. The areas between the contact points are protected and no damage is done to the substrate. It

Three main types of lasers have been found suitable for soldering process. They are carbon dioxide laser (gas laser), Nd:YAG laser (a solid state laser) and the semiconductor Laser (diode

Carbon dioxide laser is a gas laser and it has a wavelength of 10.6μm in the far infrared region [21 - 23]. It is reflected from metal surfaces but is strongly absorbed by flux and the heated flux then in turn transfers heat to the solder and metal in the joint [22]. The active medium in a carbon dioxide laser is a mixture of carbon dioxide, nitrogen and helium. Carbon dioxide gas is the lasing gas while nitrogen molecules help excite the CO2 molecules and increase the efficiency of the light generation processes. The helium plays a dual role in assisting heat transfer from excited nitrogen molecules to the co2 molecules and also helps to maintain the population inversion by getting co2 molecules to drop from the lower laser level to the ground level [24]. The carbon dioxide laser has an efficiency of up to 20% and is cooled by pumping

The active medium requires external pump source in order for lasing to occur. The active medium contains carbon dioxide, nitrogen and helium. The choice of pumping depends on the type of laser medium and since the active medium is in gas state, the best excitation is by

shaped laser beams are used. it allows short cycle times with high output volume.

and laser power profile.

is fast and precise.

**2.1. Carbondioxide laser**

the gas through the heat exchanger.

*2.1.1. The pump source*

laser).

**2. Types of lasers used in soldering**

#### **1.3. Process parameters**

Process parameters such as laser power, process time, and geometry of the laser beam can easily be programmed, permitting for consistent soldering results. According to [3], some variables such as the pad geometry or the size of the crown which can affect the formation of the solder joint interfere with the laser soldering process. Of importance is the ratio between the crown size and through hole. If the ratio is not optimal- if the hole is too big or too small with regard to the pin diameter, problems may occur in the reflow of tin on components sides. If the pin is either too long or too short, problems of poor adherence may occur both on the pin and pad.

In addition to physical and geometrical variables of the printed circuits and components, it is essential to take into consideration the soldering parameters applied during the process. The thermal profile control, assigned to each joint, permits the optimization of these soldering parameters particularly where the pads are connected to ground planes, the circuit has a number of layers or when the components to be soldered have a large thermal mass [3].

Pyrometer used as a temperature measuring tool continually monitors the surface temperature of the joint being soldered every 10ms during the process for optimal process control [16]. Moreover, the use of pyrometer speeds up the soldering program generation process.

#### **1.4. Types of laser soldering processes**

There are three different laser soldering processes: single spot, simultaneous and Mask soldering [19, 20]:

*Single spot soldering*: This method of soldering is particularly applicable for multi-dimensional configurations, tight packages and heat-sensitive components or substrates. In this process, a single contact is quickly and precisely soldered by the laser spot, offering high flexibility, quality and reproducibility. A point laser beam controlled by software moves to each pro‐ grammed point. Here the solder points are heated with individually matched beam diameter and laser power profile.

*Simultaneous soldering*: This concept allows fast soldering of individual components or complete connection arrays. Several contact points are radiated and soldered simultaneously and no relative movement between the laser beam and the workpiece is necessary. Here lineshaped laser beams are used. it allows short cycle times with high output volume.

*Mask soldering*: Mask soldering is applicable for very sensitive substrates or for the soldering of fine contacts and structure. In this process, a mask is placed between the collimated laser beam which is mostly line shaped and the target. The mask covers all parts which should not be exposed to the laser radiation. The laser beam only passes through the openings in the mask only where a contact to be soldered lies underneath which ensures locally confined heating. The areas between the contact points are protected and no damage is done to the substrate. It is fast and precise.

## **2. Types of lasers used in soldering**

Three main types of lasers have been found suitable for soldering process. They are carbon dioxide laser (gas laser), Nd:YAG laser (a solid state laser) and the semiconductor Laser (diode laser).

## **2.1. Carbondioxide laser**

The limitation of laser soldering lies with its extreme non-equilibrium nature. For a given laser soldering tasks, every joint have its own individual thermal mass and reflectivity and therefore demands a precisely defined laser impulse. Any slight deviation such as bent lead or a slight change in the amount of solder paste may lead to open or destroyed joint [18]. Other limitations of laser soldering include; limited ability to solder large areas, high relative price of the laser

Process parameters such as laser power, process time, and geometry of the laser beam can easily be programmed, permitting for consistent soldering results. According to [3], some variables such as the pad geometry or the size of the crown which can affect the formation of the solder joint interfere with the laser soldering process. Of importance is the ratio between the crown size and through hole. If the ratio is not optimal- if the hole is too big or too small with regard to the pin diameter, problems may occur in the reflow of tin on components sides. If the pin is either too long or too short, problems of poor adherence may occur both on the

In addition to physical and geometrical variables of the printed circuits and components, it is essential to take into consideration the soldering parameters applied during the process. The thermal profile control, assigned to each joint, permits the optimization of these soldering parameters particularly where the pads are connected to ground planes, the circuit has a number of layers or when the components to be soldered have a large thermal mass [3].

Pyrometer used as a temperature measuring tool continually monitors the surface temperature of the joint being soldered every 10ms during the process for optimal process control [16].

There are three different laser soldering processes: single spot, simultaneous and Mask

Moreover, the use of pyrometer speeds up the soldering program generation process.

and additional cost for safety requirements.

**1.3. Process parameters**

368 Materials Science - Advanced Topics

pin and pad.

**Figure 2.** the soldering profile

soldering [19, 20]:

**1.4. Types of laser soldering processes**

Carbon dioxide laser is a gas laser and it has a wavelength of 10.6μm in the far infrared region [21 - 23]. It is reflected from metal surfaces but is strongly absorbed by flux and the heated flux then in turn transfers heat to the solder and metal in the joint [22]. The active medium in a carbon dioxide laser is a mixture of carbon dioxide, nitrogen and helium. Carbon dioxide gas is the lasing gas while nitrogen molecules help excite the CO2 molecules and increase the efficiency of the light generation processes. The helium plays a dual role in assisting heat transfer from excited nitrogen molecules to the co2 molecules and also helps to maintain the population inversion by getting co2 molecules to drop from the lower laser level to the ground level [24]. The carbon dioxide laser has an efficiency of up to 20% and is cooled by pumping the gas through the heat exchanger.

### *2.1.1. The pump source*

The active medium requires external pump source in order for lasing to occur. The active medium contains carbon dioxide, nitrogen and helium. The choice of pumping depends on the type of laser medium and since the active medium is in gas state, the best excitation is by electrical discharge of the gas. An electric discharge passing through the laser gas excites the co2 laser. The energy of the accelerated electrons are transferred by collisions to the nitrogen molecules and then to co2 molecules. Thus the co2 molecules are raised to the excited state

with an arc lamp or series of flash lamp pulses. Flash lamp pump is common for pulsed solid state lasers while continuous –wave solid state lasers can be pumped with an electric arc lamp. Energy pumping selectively energizes the Nd ions that subsequently lead to a cascading effect and stimulated emission of light. This light is bright enough to sustain population inversion in some solid state laser materials that are capable of continuous laser operation. The maximum average power from an Nd:YAG laser can exceed 1000W, although most operate at a much

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The most commonly used resonator design is composed of two spherical or flat mirrors facing each other and the beam propagation properties are determined by the curvature of the reflective mirrors as well as the distance these mirrors are apart [27]. The two ends of the medium which is a solid crystal is silver coated, one slightly and the other heavily and so acts as a resonant cavity. As the laser absorbs the pump energy, the laser rod heats up. If the frequency of the pump energy exceeds the thermal relaxation time of the crystal, the temper‐ ature of the crystal increases. This induces temperature gradients in the laser rod crystal that give rise to thermal lensing, whereby the crystal acts as a lens to diffract the laser-which reduces

**2.** There is uneven energy intensity due to a decrease in the overall amount of energy

Nd:YAG laser emits at a wavelength of 1.06μm at near infrared and fortunately the near infrared spectrum is less reflective on metallic surfaces and less absorbent on organic materials thereby making Nd:YAG a more desirable candidate for laser soldering when compared to co2 laser [21]. The shorter emission also enables flexible and cheaper beam shaping and

This is just like a diode, i.e. a PN-junction which is operated in forward bias mode. They are sometimes called a semiconductor diode offering wavelength ranging from 790nm to 980nm. A population inversion is generated in the active region by injecting electrons. By recombining electrons and holes in the active region laser operation occurs above a characteristic threshold current density. The wavelength of the laser radiation can be tuned within certain limits by varying the Al-doping of the AlxGa1-xAs semiconductor, and so influencing the width of the bandgap [28]. They are mainly based on gallium arsenide (GaAs) and gallium aluminum

In this diode, electrons are injected and they combined with holes. Some of their excess energy is release as photons which interact with more electrons and thereby producing more photons

guidance [26]. The light produced by this laser can be guided using fiber optics

lower powers.

*2.2.2. Resonator*

power [25].

**2.3. Diode lasers**

arsenide (GaAlAs).

*2.2.3. Drawbacks of Nd:YAG laser for laser soldering*

delivered by the laser [21].

**1.** Relative replacement of the light bulbs is required.

#### *2.1.2. Resonator*

The design of the resonator has a very significant impact on the quality and the spatial power distribution of the emitted laser beam [25]. The carbon dioxide laser normally has a totally reflective rear mirror and a partly reflective output mirror. The cavity mirrors are often made of metal, with output coupling through a hole in the mirror rather than through a partly transmissive coating [24]. They are made with cavity optics reflective throughout the 9 to 11μm range in order to extract as much energy as possible from the co2 laser cavity.

#### *2.1.3. Drawbacks of carbon dioxide laser for soldering*

There are some technological and economical reasons which prevent the use of carbon dioxide lasers as a laser source for soldering applications [6, 8, 21 - 26].

#### **2.2. Nd: YAG laser**

This is a solid state laser. It uses Yttrium aluminum garnet doped with neodymium as a lasing substance. The neodymium is an impurity that takes the place of some yttrium atoms of roughly the same size. It emits at a wavelength of 1.06μm which is located in the near infrared region. Neodymium is an excellent lasing material as it produces the highest level of powers than any other doping element [25]. The small size of Nd:YAG rods and the optical properties of neodymium atoms limit the amount of energy that can be stored in a typical rod to about half a joule. Beam energies of 10-20 watts are normally used for soldering. Solder absorbs the thermal radiation of 1μm well which means that the light from the Nd:YAG has a high heating efficiency [23].

#### *2.2.1. The pump source*

The Nd:YAG being a solid state laser, has a solid crystal, and it uses light energy as the pump source. The thermal and optical properties of Nd:YAG allows it to be pumped continuously with an arc lamp or series of flash lamp pulses. Flash lamp pump is common for pulsed solid state lasers while continuous –wave solid state lasers can be pumped with an electric arc lamp. Energy pumping selectively energizes the Nd ions that subsequently lead to a cascading effect and stimulated emission of light. This light is bright enough to sustain population inversion in some solid state laser materials that are capable of continuous laser operation. The maximum average power from an Nd:YAG laser can exceed 1000W, although most operate at a much lower powers.

## *2.2.2. Resonator*

electrical discharge of the gas. An electric discharge passing through the laser gas excites the co2 laser. The energy of the accelerated electrons are transferred by collisions to the nitrogen molecules and then to co2 molecules. Thus the co2 molecules are raised to the excited state

The design of the resonator has a very significant impact on the quality and the spatial power distribution of the emitted laser beam [25]. The carbon dioxide laser normally has a totally reflective rear mirror and a partly reflective output mirror. The cavity mirrors are often made of metal, with output coupling through a hole in the mirror rather than through a partly transmissive coating [24]. They are made with cavity optics reflective throughout the 9 to 11μm

There are some technological and economical reasons which prevent the use of carbon dioxide

**1.** The energy at 10.6μm is strongly reflected by metals but undergoes about more than 90% absorption by the organic materials used for making flux, printed circuit boards and other substrates materials used for the soldering application and soldering alloys (e.g. tin-lead) have a reflective about 74% at the same wavelength. As a result, the risk of burning the

**2.** It has a high operating cost due to their electrical inefficiency (the amount of input

**4.** The dimension of this laser source is large and the light produced by this co2 laser cannot

This is a solid state laser. It uses Yttrium aluminum garnet doped with neodymium as a lasing substance. The neodymium is an impurity that takes the place of some yttrium atoms of roughly the same size. It emits at a wavelength of 1.06μm which is located in the near infrared region. Neodymium is an excellent lasing material as it produces the highest level of powers than any other doping element [25]. The small size of Nd:YAG rods and the optical properties of neodymium atoms limit the amount of energy that can be stored in a typical rod to about half a joule. Beam energies of 10-20 watts are normally used for soldering. Solder absorbs the thermal radiation of 1μm well which means that the light from the Nd:YAG has a high heating

The Nd:YAG being a solid state laser, has a solid crystal, and it uses light energy as the pump source. The thermal and optical properties of Nd:YAG allows it to be pumped continuously

range in order to extract as much energy as possible from the co2 laser cavity.

circuit board is high by primary or diffused scattered laser radiation.

**3.** The co2 laser relies on a constant gas supply and so it has to be maintained

*2.1.3. Drawbacks of carbon dioxide laser for soldering*

lasers as a laser source for soldering applications [6, 8, 21 - 26].

electrical energy converted into useful laser light),

be delivered by an optical fiber.

**2.2. Nd: YAG laser**

efficiency [23].

*2.2.1. The pump source*

*2.1.2. Resonator*

370 Materials Science - Advanced Topics

The most commonly used resonator design is composed of two spherical or flat mirrors facing each other and the beam propagation properties are determined by the curvature of the reflective mirrors as well as the distance these mirrors are apart [27]. The two ends of the medium which is a solid crystal is silver coated, one slightly and the other heavily and so acts as a resonant cavity. As the laser absorbs the pump energy, the laser rod heats up. If the frequency of the pump energy exceeds the thermal relaxation time of the crystal, the temper‐ ature of the crystal increases. This induces temperature gradients in the laser rod crystal that give rise to thermal lensing, whereby the crystal acts as a lens to diffract the laser-which reduces power [25].

#### *2.2.3. Drawbacks of Nd:YAG laser for laser soldering*

Nd:YAG laser emits at a wavelength of 1.06μm at near infrared and fortunately the near infrared spectrum is less reflective on metallic surfaces and less absorbent on organic materials thereby making Nd:YAG a more desirable candidate for laser soldering when compared to co2 laser [21]. The shorter emission also enables flexible and cheaper beam shaping and guidance [26]. The light produced by this laser can be guided using fiber optics

#### **2.3. Diode lasers**

This is just like a diode, i.e. a PN-junction which is operated in forward bias mode. They are sometimes called a semiconductor diode offering wavelength ranging from 790nm to 980nm. A population inversion is generated in the active region by injecting electrons. By recombining electrons and holes in the active region laser operation occurs above a characteristic threshold current density. The wavelength of the laser radiation can be tuned within certain limits by varying the Al-doping of the AlxGa1-xAs semiconductor, and so influencing the width of the bandgap [28]. They are mainly based on gallium arsenide (GaAs) and gallium aluminum arsenide (GaAlAs).

In this diode, electrons are injected and they combined with holes. Some of their excess energy is release as photons which interact with more electrons and thereby producing more photons in a kind of self sustaining process called resonance. This repeated conversion of incoming electrons into photons is actually similar to the process of stimulated emission that occurs in a conventional gas-based laser.

**3. Diode laser soldering**

sensitive components.

solder wire.

**Step 1: Pre-heating**

**Step 2: solderwire feeding**

a small volume of circulating water and a chiller.

locations and fine –pitch quad flat packs

**3.1. The process of a diode laser soldering**

30W-80W are sufficient for soldering applications [29].

system interfaces and this makes them easily adaptable to automation.

cation equipment manufacturers because of the following reasons [2, 29];

**3.** Easily tailored to specific joint design and consistent high quality joints.

formed at the solder joint and the contact is covered by the solder completely.

For soldering with solder wire, the process occurs in three steps [30]

the solder pad the surface of the PCB may get burned.

The diode laser is a semiconductor device that directly converts electrical energy into laser light. Mostly, higher power diode laser output in the near infrared, at either 808nm or 980nm. The size of diode laser is small and this makes them easier to be integrated into workstations. They produce their waste in a relatively small physical area and as a result, can be cooled with

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In diode laser soldering, a focused laser beam provides fast and controlled heating of the solder alloy to reflow the solder and form stable joints. Lasers with output power in the range of

A typical diode laser soldering system consists of a laser/control unit and a fiber cable is used to deliver the laser spot to any desired location. The laser/control unit provides diode laser temperature and current control and the output can be pulsed using internal and external

Higher power diode laser soldering has been adopted by some electronics and telecommuni‐

**1.** It provides temporal and spatial process control and this control extends to both the location and the metallurgy of connections, resulting in optimized joints for thermally

**2.** Automated solutions for complex applications – the complex three dimensional (3-D) circuit geometries with thermally sensitive or high value components. Difficult – to -reach

The laser beam is generated from the laser diode and is focused precisely on the solder joint with the use of an optical system. The required temperature at the solder joint is generated by absorption. This method is suitable for both reflow soldering with paste and soldering with

For selective reflow soldering, the soldering paste is dispensed first, then warmed slowly and the solder joint is preheated. Finally the soldering paste is melted totally, a meniscus is then

The laser is switched on and the solder wire enters the range of the laser beam. The direct radiating heats the wire up close to melting temperature. If the laser beam hits the PCB outside

The basic type which uses two semiconductor layers is very inefficient. Better designs have several layers which increase the power by combining more charge carriers and trapping more light inside the active layer which serves as a waveguide. The light is confined inside the waveguide where it is reflected and amplified until it exits through one end. The power of a single laser diode is low and many of it can be grouped to form a single high powered laser. When compared with co2 and Nd:YAG lasers, diode laser offers many technological advances in laser soldering [21] and this includes

#### *2.3.1. Pump source*

The source of pumping is by optical excitation. If a photon with more energy band gap strikes a semiconductor, it can raise an electron from the valence band to the conduction band, creating an electro-hole pair, a pair of current carriers. This effect can be used to detect light by generating an electric current proportional to the amount of illumination. The current passing through the semiconductor laser produces a population inversion in the junction which is incidentally the active layer. Inside the junction, the composition of the semiconductor changes. Population inversion in this case would be a large concentration of free electrons and holes in the same region.

#### *2.3.2. Resonator*

In conventional laser system, a laser beam is produced by pumping the light emitted from the atoms severally between two mirrors. In a laser diode, an equivalent process occurs when the photon bounces back and forth in the Junction (the active layer) between the N-type and Ptype semiconductor. This type of resonator is known as Fabry –Perot resonant cavity. The two parallel polished planes are used to produce multiple reflections in the cavity in order to achieve a very high intensity of laser beam. The amplified laser light eventually emerges from the polished end of the cavity.

## **3. Diode laser soldering**

in a kind of self sustaining process called resonance. This repeated conversion of incoming electrons into photons is actually similar to the process of stimulated emission that occurs in

The basic type which uses two semiconductor layers is very inefficient. Better designs have several layers which increase the power by combining more charge carriers and trapping more light inside the active layer which serves as a waveguide. The light is confined inside the waveguide where it is reflected and amplified until it exits through one end. The power of a single laser diode is low and many of it can be grouped to form a single high powered laser. When compared with co2 and Nd:YAG lasers, diode laser offers many technological advances

**1.** Due to its shorter wavelength, it has higher absorbance in metals and lower absorbance

**3.** The energy density distribution of the laser beam generated remains uniform through the

**6.** The maximum conversion efficiency of transforming the input electrical efficiency of about 59%, which translates into a total electrical efficiency of about 40% for a high power

The source of pumping is by optical excitation. If a photon with more energy band gap strikes a semiconductor, it can raise an electron from the valence band to the conduction band, creating an electro-hole pair, a pair of current carriers. This effect can be used to detect light by generating an electric current proportional to the amount of illumination. The current passing through the semiconductor laser produces a population inversion in the junction which is incidentally the active layer. Inside the junction, the composition of the semiconductor changes. Population inversion in this case would be a large concentration of free electrons and

In conventional laser system, a laser beam is produced by pumping the light emitted from the atoms severally between two mirrors. In a laser diode, an equivalent process occurs when the photon bounces back and forth in the Junction (the active layer) between the N-type and Ptype semiconductor. This type of resonator is known as Fabry –Perot resonant cavity. The two parallel polished planes are used to produce multiple reflections in the cavity in order to achieve a very high intensity of laser beam. The amplified laser light eventually emerges from

in organic materials usually used for making PCBs and other substrates.

**2.** It gives manufacturers a flexible and powerful tool to solve heating problems.

diode laser system and this actually caused the operating cost to be low.

a conventional gas-based laser.

372 Materials Science - Advanced Topics

in laser soldering [21] and this includes

spot regardless of its size.

**5.** High powered laser diode is very compact

**4.** It is maintenance free.

*2.3.1. Pump source*

holes in the same region.

the polished end of the cavity.

*2.3.2. Resonator*

The diode laser is a semiconductor device that directly converts electrical energy into laser light. Mostly, higher power diode laser output in the near infrared, at either 808nm or 980nm. The size of diode laser is small and this makes them easier to be integrated into workstations. They produce their waste in a relatively small physical area and as a result, can be cooled with a small volume of circulating water and a chiller.

In diode laser soldering, a focused laser beam provides fast and controlled heating of the solder alloy to reflow the solder and form stable joints. Lasers with output power in the range of 30W-80W are sufficient for soldering applications [29].

A typical diode laser soldering system consists of a laser/control unit and a fiber cable is used to deliver the laser spot to any desired location. The laser/control unit provides diode laser temperature and current control and the output can be pulsed using internal and external system interfaces and this makes them easily adaptable to automation.

Higher power diode laser soldering has been adopted by some electronics and telecommuni‐ cation equipment manufacturers because of the following reasons [2, 29];

#### **3.1. The process of a diode laser soldering**

The laser beam is generated from the laser diode and is focused precisely on the solder joint with the use of an optical system. The required temperature at the solder joint is generated by absorption. This method is suitable for both reflow soldering with paste and soldering with solder wire.

For selective reflow soldering, the soldering paste is dispensed first, then warmed slowly and the solder joint is preheated. Finally the soldering paste is melted totally, a meniscus is then formed at the solder joint and the contact is covered by the solder completely.

For soldering with solder wire, the process occurs in three steps [30]

#### **Step 1: Pre-heating**

The laser is switched on and the solder wire enters the range of the laser beam. The direct radiating heats the wire up close to melting temperature. If the laser beam hits the PCB outside the solder pad the surface of the PCB may get burned.

#### **Step 2: solderwire feeding**

This process is important for an accurate soldering process. The solder wire is fed during the solder feeding time at a specific speed. The solderwire with temperature close to melting temperature hits the solder spot and melts at the preheated solder pad and the preheated pin. If the solder pad and pin are not warm enough, the wire gets burred or bends. The solder wire feeder is driven by a DC-Motor and an encoder is used to control the feeding rate. The feeding length and feeding rate are programmable.

**Large, 1 to 3 mm (0.04 to 0.12 in.) Pads**

**3.3. Laser soldering parameters**

energy delivered to the joint by a laser.

minimum soldering times, is therefore preferred.

**4. Laser soldering as a selective soldering process**

determines the rate of the soldering process.

laser beam in real time.

include [31]:

**•** Increased circuit density

**•** Lower costs in volume application

**•** The reduction in the size of the board

**•** The reduction in the size of the components

following

In some cases, it is very essential to scan several joints simultaneously and this can be done by expanding the spot size of the diode laser. This technique is a viable solution for soldering multiple joints on a densely populated printed circuit board (PCB). To increase the scan time, higher average power diode laser systems (up to 80 watts continuous wave) can be employed.

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375

http://dx.doi.org/10.5772/51527

Hoult [2] has also identified a list of laser soldering parameters to be considered while developing a soldering process in order to achieve a high quality joint. They include the

**1.** Average power: The average power of the laser controls the rate of heat delivered to the joint being soldered. High average power is preferred since it reduces the soldering time, but the excess power causes vaporization and also reduces the quality of the joint.

**2.** Pulse time/length: Just like average power, pulse time/length controls the amount of

**3.** Pulse duty cycle: This modifies the rate at which heat is delivered to the joint thereby increasing the control of the soldering process. A high duty cycle, because it permits a

**4.** Laser power density (intensity): The laser power density (intensity) controls the response of the material to the laser beam and in association with the average power, generally

**5.** Laser focus position: The accurate placement of the laser focus spot is very critical in order to secure a good quality joint. This is best achieved by using precision x-y positioning tables coupled with a CCD camera and an imaging accessory that allows viewing of the

Years back, most of the components in all electronic products are through-hole (TH) compo‐ nents but today about 90% of the through –hole components in these electronic products are being replaced by their surface mount counterparts [1]. This may be to the benefits of the surface mount technology (SMT) with regard to through-hole technology (THT) and they

#### **Step 3: Hold time**

The solder wire having melted, can spread evenly to build the typical shape of a solder spot. Liquid solder has a significant lower absorption rate. It acts like a mirror. In this phase of the soldering process the laser beam can be partially deflected and can damage possible sur‐ rounding components or the plastic housing.

The beams of many single diodes must be bundled to the focus point in order to have the energy of approximately 30 W to a focus point of 0.8 mm diameter the and this is only possible with complex optics (see figure 3). The beam of the laser diodes is at first collimated in one axis by a cylinder lens and is then coupled into the optical fiber.

**Figure 3.** Laser diode with the optics delivery beam. Adapted from [30]

#### **3.2. Soldering with high power diode lasers**

Hoult [2] stated that in the microelectronics industry, for soldering tasks to be done, an average laser power of 2 to 80 watts can used. This laser average power depends on solder joint dimensions and the required speed. The soldering tasks are categorized by size into small, medium and large soldering areas [2].

#### **Small, 40 to 100 μm (0.0016 to 0.004 in.) Pads**

Typical applications for soldering these small pads are in high-density packaging. A few watts of average power are normally sufficient to solder these joints. Specific spot size and working distance requirements can be addressed by a host of commercially available optical imaging accessories (OIAs).

#### **Medium, 100 to 500 μm (0.004 to 0.02 in.) Pads**

In these cases, 25 watts delivered from an 800-μm-diameter optical fiber with an OIA capable of reducing the source size by a factor of 1.8:1 offers some benefits. Dwell time is approximately 1 second per solder joint.

## **Large, 1 to 3 mm (0.04 to 0.12 in.) Pads**

This process is important for an accurate soldering process. The solder wire is fed during the solder feeding time at a specific speed. The solderwire with temperature close to melting temperature hits the solder spot and melts at the preheated solder pad and the preheated pin. If the solder pad and pin are not warm enough, the wire gets burred or bends. The solder wire feeder is driven by a DC-Motor and an encoder is used to control the feeding rate. The feeding

The solder wire having melted, can spread evenly to build the typical shape of a solder spot. Liquid solder has a significant lower absorption rate. It acts like a mirror. In this phase of the soldering process the laser beam can be partially deflected and can damage possible sur‐

The beams of many single diodes must be bundled to the focus point in order to have the energy of approximately 30 W to a focus point of 0.8 mm diameter the and this is only possible with complex optics (see figure 3). The beam of the laser diodes is at first collimated in one

Hoult [2] stated that in the microelectronics industry, for soldering tasks to be done, an average laser power of 2 to 80 watts can used. This laser average power depends on solder joint dimensions and the required speed. The soldering tasks are categorized by size into small,

Typical applications for soldering these small pads are in high-density packaging. A few watts of average power are normally sufficient to solder these joints. Specific spot size and working distance requirements can be addressed by a host of commercially available optical imaging

In these cases, 25 watts delivered from an 800-μm-diameter optical fiber with an OIA capable of reducing the source size by a factor of 1.8:1 offers some benefits. Dwell time is approximately

length and feeding rate are programmable.

rounding components or the plastic housing.

axis by a cylinder lens and is then coupled into the optical fiber.

**Figure 3.** Laser diode with the optics delivery beam. Adapted from [30]

**3.2. Soldering with high power diode lasers**

**Small, 40 to 100 μm (0.0016 to 0.004 in.) Pads**

**Medium, 100 to 500 μm (0.004 to 0.02 in.) Pads**

medium and large soldering areas [2].

accessories (OIAs).

1 second per solder joint.

**Step 3: Hold time**

374 Materials Science - Advanced Topics

In some cases, it is very essential to scan several joints simultaneously and this can be done by expanding the spot size of the diode laser. This technique is a viable solution for soldering multiple joints on a densely populated printed circuit board (PCB). To increase the scan time, higher average power diode laser systems (up to 80 watts continuous wave) can be employed.

#### **3.3. Laser soldering parameters**

Hoult [2] has also identified a list of laser soldering parameters to be considered while developing a soldering process in order to achieve a high quality joint. They include the following

## **4. Laser soldering as a selective soldering process**

Years back, most of the components in all electronic products are through-hole (TH) compo‐ nents but today about 90% of the through –hole components in these electronic products are being replaced by their surface mount counterparts [1]. This may be to the benefits of the surface mount technology (SMT) with regard to through-hole technology (THT) and they include [31]:

Though many through-hole components are being replaced by their surface mount counter‐ parts, printed circuit boards (PCBs) are still being designed with both technologies. This is because despite the benefits of SMT in relation to THT, the through-hole components will remain in electronic industry for many years because of the [1]:

nents that cannot withstand the high temperature involved in the wave soldering process are soldered by hand. This method is not only slow and expensive but unfortunately the quality

Laser Soldering

377

http://dx.doi.org/10.5772/51527

For cost effective selective soldering of through-hole components when either wave, hand or convection reflow is either not technically possible, desirable or too expensive, laser soldering comes into play. Laser soldering does not need any special fixture and is capable of meeting the requirements of mixed SMT and TH fabrication in terms of the quality, repeatability aswell as the flexibility [1]. The miniaturization and integration of electronic components and the use of temperature-sensitive components used in telecommunication equipments also contributed

Laser soldering is a desirable process used to solder areas that are difficult to reach or parts which cannot be soldered by either wave or reflow soldering. These can be parts that are heat/ temperature sensitive and is repeatable [22] or need a special temperature program due to their size. The power control of the laser source and the steadiness of the output power are necessary to guarantee a steady repeatable process [3]. The use of optics developed for the soldering process produced conically shaped beams and this allows it to be moved and focused on a solder joint. The use of laser technology for selective soldering permits precise heating, thereby avoiding the subjection of high thermal stress on the heat-sensitive components at high

**1.** It allows the user to optimize the solder process down to pin level verses the compromise

**2.** It improves the quality of the solder and reduces the thermal coefficient of expansion issues with minimalistic surface mount components that do not tolerate the thermal shock

**3.** Selective soldering is ideal for thermally challenged through-hole components since the flux deposition solder dwell time and peel-off parameters are fully programmable. **4.** It is used to solder printed circuit board assembly (PCBAs) with high component density since it can maintain clearances between the through-hole pads and adjacent surface mount pads that are not achievable with masking pallets and permits the soldering of through –hole components on both sides of the PCBAs without restricting the components

**5.** It offers the ability to reduce solder defects, hence improving the first pass yields.

**5. Comparison of laser soldering with most of the common soldering**

Laser soldering is mainly characterized by short duration heating and high intensity radiation which can be focused on a very small spot. There are other types of soldering techniques - Iron

of the joint being soldered depends on operator's capability.

The benefits of a selective soldering includes but not limited to [33]

to the rapid growth of this field.

techniques in flux application.

of wave soldering.

temperatures.

height.

**methods**

The hot air and convection reflow soldering methods are primarily used for SMT components. The hot air is used for low volume application while the convection reflow is used for high volume application. Hand and wave soldering are primarily used for TH components, though a few SMT components can be soldered by these methods.

When confronted with mixed assembly board, there is always a challenge to determine the optimal method that can be used to solder these boards, and then selective soldering becomes the answer.

In selective soldering, only the through-hole components are selectively soldered after the surface mount components have been reflow-soldered. Therefore selective soldering is defined as a process of soldering only through-hole components onto a printed circuit board (PCB) that has surface mount components on the underside.

In a mixed- assembly board, through-hole components can be selectively soldered by three methods [1].

Wave soldering, with or without specially designed fixtures: This can be used for throughhole whenever the board's secondary side contains devices that can go through the wave solder process. The through-hole components must be selectively soldered without impact to the adjacent components already soldered in the reflow oven. Depending on the application, selective wave soldering using text fixtures can be very expensive.

Convection reflow using paste-in-process: This allows reflowing of both the through-hole and surface mount device (SMD) at the same time but the difference and distribution of thermal masses on circuits may lead to different results in terms of the quality for different components. When the component is temperature-sensitive, has too many rows of pins to allow sufficient solder paste deposition, or the board is not designed for the paste-in-hole process to begin with, the paste-in-hole process using convection reflow cannot be used [1].

Hand soldering: This has always been the backbone of electronics assembly for decades. They can be used to complete connections that are not possible using other methods. The compo‐ nents that cannot withstand the high temperature involved in the wave soldering process are soldered by hand. This method is not only slow and expensive but unfortunately the quality of the joint being soldered depends on operator's capability.

For cost effective selective soldering of through-hole components when either wave, hand or convection reflow is either not technically possible, desirable or too expensive, laser soldering comes into play. Laser soldering does not need any special fixture and is capable of meeting the requirements of mixed SMT and TH fabrication in terms of the quality, repeatability aswell as the flexibility [1]. The miniaturization and integration of electronic components and the use of temperature-sensitive components used in telecommunication equipments also contributed to the rapid growth of this field.

Laser soldering is a desirable process used to solder areas that are difficult to reach or parts which cannot be soldered by either wave or reflow soldering. These can be parts that are heat/ temperature sensitive and is repeatable [22] or need a special temperature program due to their size. The power control of the laser source and the steadiness of the output power are necessary to guarantee a steady repeatable process [3]. The use of optics developed for the soldering process produced conically shaped beams and this allows it to be moved and focused on a solder joint. The use of laser technology for selective soldering permits precise heating, thereby avoiding the subjection of high thermal stress on the heat-sensitive components at high temperatures.

The benefits of a selective soldering includes but not limited to [33]

**•** Shorter pins

**•** Shorter interconnections

**•** Improved electrical performance.

remain in electronic industry for many years because of the [1]:

a few SMT components can be soldered by these methods.

that has surface mount components on the underside.

selective wave soldering using text fixtures can be very expensive.

with, the paste-in-hole process using convection reflow cannot be used [1].

Though many through-hole components are being replaced by their surface mount counter‐ parts, printed circuit boards (PCBs) are still being designed with both technologies. This is because despite the benefits of SMT in relation to THT, the through-hole components will