Fluxes have several serious drawbacks:
* Corrosivity, which is mostly due to the aggressive compounds of the activators; hygroscopic properties of the flux residues may aggravate the effects
* Interference with test equipment, which is due to the insulating residues deposited on the test contacts on electronic circuit boards
* Interference with machine vision systems when the layer of flux or its remains is too thick or improperly located
* Contamination of sensitive parts, e.g. facets of laser diodes, contacts of connectors and mechanical switches, and MEMS assemblies
* Deterioration of electrical properties of printed circuit boards, as soldering temperatures are above the glass transition temperature of the board material and flux components (e.g. glycols, or chloride and bromide ions) can diffuse into its matrix; e.g. water-soluble fluxes containing polyethylene glycol were demonstrated to have such impact
* Deterioration of high-frequency circuit performance by flux residues
* Deterioration of surface insulation resistance, which tends to be as much as three orders of magnitude lower than the bulk resistance of the material
* Electromigration and growth of whiskers between nearby traces, aided by ionic residues, surface moisture and a bias voltage
* The fumes liberated during soldering have adverse health effects, and volatile organic compounds can be outgassed during processing
* The solvents required for post-soldering cleaning of the boards are expensive and may have adverse environmental impact
In special cases the drawbacks are sufficiently serious to warrant using fluxless techniques.