Conformal Coating material is applied to electronic circuitry to act as protection against moisture, dust, chemicals, and temperature extremes that, if uncoated (non-protected), could result in damage or failure of the electronics to function. When electronics must withstand harsh environments and added protection is necessary, most circuit board assembly houses coat assemblies with a layer of transparent conformal coating rather than potting.
Precision analog circuitry may suffer degraded accuracy if insulating surfaces become contaminated with ionic substances such as fingerprint residues, which can become weakly conductive in the presence of moisture. The classic symptom of micro-contamination on an analog circuit board is sudden changes in performance at high humidity. For example, when a technician breathes on it. Furthermore, a suitably chosen material coating has proved to actually reduce the effects of mechanical stress and vibrations on the circuit and its ability to cope in extreme temperatures.
In a chip-on-board assembly process, a silicon die is mounted on the board with an adhesive or a soldering process, then electrically connected by wire bonding, typically with .001-inch-diameter gold or aluminum wire. The chip and the wire are very delicate, so they're encapsulated in a version of conformal coating called "glob top." This prevents accidental contact from damaging the wires or the chip. Another use of conformal coating is to increase the voltage rating of a dense circuit assembly; an insulating coating can withstand a much stronger electric field than air, particularly at high altitude.
With the exception of parylene, most organic coatings are readily penetrated by water molecules. A coating preserves the performance of precision electronics primarily by preventing ionizable contaminants such as salts from reaching circuit nodes, and combining there with water to form a microscopically thin electrolyte film. For this reason, coating is far more effective if all surface contamination is removed first, using a highly repeatable industrial process such as vapor degreasing or semi-aqueous washing in a special machine. Extreme cleanliness also greatly improves adhesion. Pinholes would defeat the purpose of the coating, because a continuous contaminant film would be able to make contact with the circuit nodes and form undesired conductive paths between them.
The coating material can be applied by various methods, from brushing, spraying and dipping, or, due to the increasing complexities of the electronic boards being designed and with the 'process window' becoming smaller and smaller, by selectively coating via robot. Different methods of curing / drying are available depending on the conformal coating material.
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