A conformal coating is a specialty polymeric film forming material that protects circuit boards, components and other electronic components from adverse environmental conditions arising from corrosive atmospheres, humidity, heat, fungs and contamination such as dust and dirt, they also mitigate the risk of short circuits stemming from the expansion of tin whiskers.
It is important to emphasize that while conformal coatings offer moisture resistance, they do not provide waterproofing. Therefore, they are not suitable for applications exposed to prolonged wet conditions. In such cases, it is advisable to employ a waterproof enclosure, with the boards coated conformally inside.
Conformal coating serves well in safeguarding against issues like condensation, especially when transitioning from a cold environment to a warm, humid one. Thoroughly cleaning the PCBA is mandatory to ensure proper coating adhesion. To delineate the areas designated for coating application and those meant to remain uncoated, the use of masking techniques is typically required. This may involve the application of masking tapes or adhesive circles to safeguard flat surfaces and openings from receiving the coating.
Latex masking material is employed to effectively mask surfaces with irregular shapes or contours. Typically, a coarse substrate with elevated surface energy will demonstrate superior wetting and coating adhesion compared to a smooth substrate with lower surface energy
5 common types of conformal coatings are available. How do you choose the right one for your application?
Acrylic Adhesive
Acrylic Adhesives can be easily dissolved in many organic solvents and thus make it very easy to have repair work done to the board, and in general provides only selective chemical resistance. Acrylic Adhesive dries rapidly, has good fungus resistance, does not shrink during cure and has good humidity resistance. They suffer from low abrasion resistance, easily leading to scraping, chipping and flaking.
Epoxy, 2 Part
Epoxies are usually 2-part compounds that start curing upon mixing. Epoxy exhibits good abrasive and chemical resistance, as well as reasonable humidity resistance.This coating is difficult to remove and rework. A buffer is recommended around delicate components, since film shrinkage occurs during polymerization. The shrinkage can be minimized by curing at a low temperature.
Polyurethane
Polyurethane provides good humidity and chemical resistance. Due to its high chemical resistance, removal of the coating requires the use of stripping agents which may leave ionic residues. These may need to be thoroughly cleaned to prevent corrosion on the underlying board. Polyurethane can be soldered through for rework, but usually results in a brownish residue which affects the appearance of the product.
Silicone
Silicone is usually a single compound that begins curing upon exposure to moisture in the air, along with temperature. Silicone can provide a good wetting and adhesion of the cured silicone to all surfaces of the electronic device or module. It can be widely used in high temperature (>120C), moisture sensitive, chemical resistant, corrosive, antifungal environments.
Urethane
Urethane has great strength in protection, hardness and high level of solvent resistance. It provides superb resistance to abrasion and a low level of permeability of moisture. In addition, it has good low temperature adaptability, however it does not perform well in high temperature environments and for the most part cannot be repaired or reworked.
Reference - Chemtronics: ruggedizing electronics best practices for bullet-proof devices