Best Conformal Coating for Humid Environments

Liquid conformal coatings such as epoxy resins (ER), acrylic resin (AR), polyurethane (UR) and silicones (SR) as well as Parylene conformal coatings (XY) are well known for protecting printed circuit boards (PCBs) against moisture. In the case of high-humidity environments; however, a different set of challenges can arise. Since moisture is omnipresent in a humid environment, conformal coatings need to be able to resist water ingress while completely coating the application. Given this additional requirement, both silicones and Parylene possess distinct coating advantages.

Silicone Conformal Coatings and Humidity

Silicone carries a range of benefits, but its key advantage for humid settings is as a resistor of moisture. While the silicone compounds used in conformal coatings are generally uniquely formulated, they share a basic structure with the silicone found in sealing kitchens and bathrooms against moisture. It is not uncommon for properly applied silicone to last 20 years in these types of environments. 

Interestingly, silicone’s drawback is also its key advantage for dealing with high humidity. Silicone typically needs to be applied in a relatively thick coat. While silicone applications can vary in thickness, it is not uncommon to see it applied in thicknesses approaching 0.01 inches. A thick coat protects the surface and can even cushion it, but, more importantly, it also reduces the likelihood of any pinholes or gaps forming in the seal. With high humidity, the only way to fully protect the item is to achieve a perfect seal, and silicone is so thin that, even if it is imperfect in spots, the rest of the coating will cover the imperfections.

Silicone is not a hard material; it can easily be scratched or rubbed away. Furthermore, silicone’s softness means it would be hard to form a stable coat over it with a different protective material. In certain situations, this means that silicone would not be suitable to protect items that will be subjected to abrasion or other types of physical harm.

Parylene for Humid Environments

Aside from silicone, Parylene is the best conformal coating alternative for high humidity environments. Applied in an exceptionally thin coat – as little as 1/100th of the thickness of silicone – Parylene provides moisture and humidity proofing without meaningfully increasing the size of the coated item. Parylene is also hard enough to withstand abrasions that would break down silicone or other soft coatings.

Parylene’s key benefit for high humidity environments comes from the unique way it is deposited on an application’s surface. Most liquid coatings are brushed, sprayed or deposited by dipping, causing them to flow over an application. This coating process creates opportunities for pinholes and other imperfections to form. Parylene is applied through the vapor deposition process and is deposited anywhere the moisture in the air is able to touch, making the application both moisture and humidity-proof.

While silicone and Parylene are usually the best options for high-humidity environments, every project is unique challenged and require a thorough understanding of project requirements.