Consider Conformal Coatings for PCBs

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A conformal coating is a thin protective film that “conforms” to the surface of a printed circuit board (or PCB). This coating provides a layer of dielectric protection to circuitry, protecting the PCB against the environment (humidity/moisture, dust and conductive contaminants). It also provides extra resistance to chemicals/gases and aids in tin whisker mitigation, tiny, hair-like filaments of conductive tin that grow out of a component lead that may touch an adjacent devices lead, which could cause a short or equipment failure. Conformal coatings should be used when reliability and or safety is critical, when operating in an aggressive environment or when there is tight circuit spacing. There are multiple types of conformal coatings, but the most commonly used are acrylics, polyurethanes and silicones.

Acrylics are the most commonly used type of conformal coating. They’re used mainly in single component materials. Acrylics have excellent moisture resistance, dry very quickly and are easy to remove/repair. They are best suited for environments where moisture and debris are the main threats and in assemblies where future repair is needed. Acrylics should not be used where there is high solvent/chemical exposure.

Polyurethane conformal coatings have an increased chemical resistance over acrylics. They are easy to apply, but have longer cure times. There are varieties that can be cured with the use of heat or moisture. Polyurethanes are best suited for applications where extra chemical resistance is required, for example, industrial controls and marine electronics, where abrasion resistance is needed, and ease of repair is not as important.

Silicone conformal coatings are made of flexible elastomers and have excellent performance in high and low temperatures. They give decent moisture protection and have good solvent resistance. Silicone conformal coatings are the most difficult to repair or remove from PCBs. Like polyurethane conformal coatings, there are varieties that can be cured with the use of heat or moisture. Silicone is used in industrial drilling/mining equipment, under-hood automotive and in electronics where temperatures can reach greater than 150?C.

Other types of conformal coating materials are available including epoxies, parylene and UV curable. When selecting a conformal coating, there are some questions to consider when selecting the best type for the application. What are the main protection requirements? Is chemical resistance, re-work or industry qualifications (UL, MIL, IPC) required? What are the temperature extremes? Lastly, how should the conformal coating be applied (brush, dip, spray or automated application)?

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