The advantages and disadvantages of FPC

A: Disadvantages:

1. Original cost is high


2. The modification and correction is difficult


3. Dimension limitation


4. Easier to be damaged if improper operation


 B: Advantages





FPC Application



The PCB market in Germany is recovering gradually



The packing method of FPC

The most common packing method of FPC is to pile up 10~20pcs flexible printed circuit boards (FPC) together, fix every part on the rigid board by using paper strips. We should avoid using tape, because if the chemical substances in the tape adhesives oozes, the terminal would be easy to be oxidized and change color. The base material is polyimide which is easy to absorb moisture. The flexible printed circuit boards together with silica gel, etc. the drying agents have to be put into the polythene bag and sealed. Then put it into the cartons together with cushion material. Due to special shapes of FPC, we should adopt different packing methods according to different shapes.

 Before punching the outline of flexible printed circuit boards, firstly stick the FPC on the polyester supporting spring printed with adhesives, using knife mold to cut the outline, and then send to our clients. The clients can get the flexible printed circuit boards and do the assembly, or they can do the assembly at first, take them from the polyester supporting spring after finishing the assembly. This method can be only applied on the products with small dimensions. Which can greatly improve the operation efficiency  whatever to flexible printed circuit boards manufacturers or to clients.

 The safest way is to use special tray, at first we prepare the trays according to the FPC types. Although the management is complex, the quality is guaranteed. They are convenient to use and helpful for client assembly, low cost and can be thrown away thereafter.

The structure, technique and design of FPC (2)

1. The structure of FPC:

 According to the layer of conductive copper, FPC is divided into single layer, double layer, multilayer, dual access, etc.

 The structure of single layer: the most simple. Base material + transparent adhesive + copper is one set of the raw material; coverlay + adhesive is another. Firstly, the copper needs to be etched to get the traces, while the coverlay needs to be drilled to expose corresponding soldering pad. After cleansing treatment two set of raw material are bonding together by rolling. Immersion gold or tin is done on the exposed soldering pads to protect from oxidization. At last, FPC is punched into smaller ones of corresponding shape.

 Some FPC doesn’t use coverlay but prints solder mask on the copper directly, which lowers the cost, but worsens the mechanical strength of FPC. Using coverlay is the best, unless at the occasion of low mechanical strength and the lower price the better,

 The structure of double layer FPC: when the circuits are too complicated, the single layer FPC can’t or needs copper to do ground shield, double layer even multilayer FPC is needed.

 The most typical difference between multilayer and single layer FPC is that the multilayer FPC is added with through-hole structure to connect the copper on every layer. The first workmanship of base material + transparent adhesive + copper is to make through holes. Holes are drilled o the base material and copper, after cleansing treatment they are plated with a certain thickness of Cu, then the through hole is done. We call it PTH (plated through hole). The technique after is almost the same with single layer FPC.

 The structure of dual access FPC: dual access FPC has soldering pads on its two sides, mainly used to connect with other circuit boards. Although it has similar structure with single layer FPC, its technique has great difference. Its raw material is base material + adhesive + copper, coverlay + adhesive. Holes are drilled on the coverlay according to soldering pads location, then stuck on the copper. After etching the soldering pads and traces another coverlay with drilled holes is stuck on that.

 2. The performance and selecting method of the materials

 (1) Base material:

 PI (Polyimide) is the most common used, which is a kind of macromolecule material of high temperature proof and high strength, invented by Dupont. The polyimide made by Dupont is called Kapton. Besides some polyimide made in Japan can also be found. Its price is much lower.

 Polyimide can bear 10 seconds of high temperature of 400 degree centigrade. Its tensile strength is 15,000~30,000 PSI.

 The base material with 25um thickness is the cheapest and the most popular one. If we need the circuit board to be more rigid, we should choose 50um. Otherwise, if we need the circuit boards to be more flexible, then choose 13um.

 (2) Adhesive:

 It is divided into epoxy resin and polyethylene, they are all thermosetting adhesive. The polyethylene’s strength is lower. If we hope the circuit boards to be more flexible, then choose polyethylene. The thicker the base material and the adhesive on it is , the more rigid the circuit boards are. If the circuit boards have to be bent in a large area, we should choose thinner base material and adhesive so as to reduce the stress on the surface of the copper, then lower appearance of micro crack.

 (3) Copper:

It is divided into rolled annealed copper foil and electrodeposited cooper foil. Rolled annealed copper foil is of high strength and can stand bending, but its price is higher. Electrodeposited copper foil is much cheaper, but of low strength, easier to be bent.

When we choose to use rolled annealed copper foil, we should pay attention to the direction of the copper. The direction of the copper has to be the same with the bending direction of the circuit boards.

 (4) Coverlay and its adhesive:

 25um coverlay would make the circuit board more rigid, but it is cheaper. Therefore to the circuit board needing to be bent greatly, we’d better choose 13um coverlay.

 The adhesive is also divided into epoxy resin and polyethylene. The FPC using epoxy is more rigid. After thermosetting, there are some adhesive squeezed out at the edge of the coverlay. If the soldering pads are larger than the hole on the coverlay, the adhesive squeeze out would reduce the size of the soldering pads and lead to irregular edge. Therefore, we’d better choose 13um adhesive.

The structure, technique and design of FPC (1)






The advantage of using FPC

The flexible printed circuits are designed to save room and improve product design flexibility, which can satisfy the design need of smaller and higher density installation, in favor of decreasing mounting steps and improving reliability, and become the only solution for satisfying the miniaturization and flexibility of electronic products.

 The flexible circuit boards are to etch the copper traces on the polymer base material, or to print polymer heavy layer circuit. In regard to thin, light, tightly and complicated structured components, their design scheme comprises from single sided conductive circuit to complex multilayer 3-D packing. The overall quality and volume of flexible packing decreases by 70% than using the traditional mega wire thread method. The flexible circuit can also increase its strength by using supporting material or lining board, so as to obtain extra mechanical stability.

 The flexible circuits can be moved, bent, twisted, without ruining the traces. They are with different shapes and special packing dimensions. The only limit is the problem of room and volume. Due to being able to bear millions of dynamic bending, the flexible circuits can be well applied in the inner connection system of continuous movement or regular movement, becoming one part of the final product function. Some products demanding for smaller shape modulus/packing dimensions because of the movement of electrical signal/power all benefit from the flexible circuits.

 The flexible circuit provides good electrical performance. Lower dielectric constant allows electrical signals to transmit quickly. Good thermal performance makes the assembly easy to cool. Higher glass transfer temperature or melting point makes the assembly work well at higher temperature.

 Because the quantity of the hardware needed for inner connection is reduced, such as the welding pots、trunk lines, base board traces and cables usually used in traditional electric packing, the flexible circuits can provide with higher installation reliability and yield. When the traditional inner connection hardware comprised of many complicated systems is mounted, higher rate of assembly out of alignment happens easier. As the emergence of quality engineering, a very thin flexible system is designed to be mounted in only one way, so as to eliminate the human error related to independent wiring installation。

 At early stage the flexible circuits are mainly applied in the fields such as the connection between the small or thin electronic products and rigid printed boards. In the end of 1970s, they were used in the electrical products such as computers, digital cameras, ink jet printers, car audio, CD-ROM and hard disk driver. Opening one 35mm camera, there are 9~14 different flexible circuits. The only way to decrease volume is smaller assembly, more precise lines, tighter spacing, and bendable objects. Heart maker, medical instruments, video camera, hearing aid, laptops, all most all the products we use comprise of flexible circuits today.

NB board and FPC: the only hope in 2009 PCB industry




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