Conformal coating

Conformal coating material is a thin polymeric film which ‘conforms’ to the contours of a printed circuit board to protect the board's components. Typically applied at 25-250 μm(micrometers) it 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.

Because of the vast range of electronics in today's world, a printed circuit board is expected to perform in a huge range of environments. Circuit boards then are exposed to a vast array of different conditions such as moisture, salt, chemicals and temperature changes to name but a few, conformal coatings are designed to protect the electronic components from these varying conditions and prevent the board from corroding. More recently, conformal coatings are being used as a mitigation to reduce the potential of tin and zinc whisker short circuit risk on electronic assemblies using lead-free finishes and solders.

Conformal coatings are also “breathable”, allowing trapped moisture in electronic boards to escape while maintaining protection from environmental contaminates. However these coatings are not sealants, and prolonged exposure to vapors will cause transmission and degradation to occur. There are typically four classes of conformal coatings: Acrylic, Urethane, Silicone, and Varnish. While each has its own specific physical and chemical properties each are able to perform the following functions:

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.

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