Just about every major type of conformal coating provides protection against moisture. If you get a printed circuit board coated with epoxy, acrylic, urethane, silicone or parylene wet, typically all that you have to do is wipe it off. Environments with high humidity pose a different set of challenges. Because moisture is omnipresent in humid environments, the conformal coating doesn't just have to resist water ingress. It also needs to completely seal the coated item. Given this additional requirement, the best choice will usually be either silicone or parylene.
Silicone Conformal Coatings and Humidity
Silicone carries a range of benefits, but its key one for humid settings is that it is an extremely good resister of moisture. While the silicone compounds used in conformal coatings are generally uniquely formulated, they share a basic structure with the silicone that you can find sealing bathrooms and kitchens against moisture. In fact, it is not uncommon for properly applied silicone caulking to last 20 years in the hot, wet and humid atmosphere of a shower. This indicates why it is a popular choice for conformal coatings as well.
Interestingly, silicone's key drawback is 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. More importantly, though, it also reduces the likelihood of any pinholes or other gaps in the seal forming. 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 has one drawback that makes it unsuitable for certain applications in high humidity. As you may know from touching other silicone-coated items, it is not a hard material. It can be easily scratched, dinged or even rubbed away. Furthermore, its softness means that it would be hard to form a stable coat over it with a different protective material. This means that it would not be suitable to protect items that will be subject to abrasion or other types of physical harm.
Parylene for Humid Environments
If silicone is not suitable for your project, parylene is the best alternative for high humidity environments. It can be applied in an exceptionally thin coat -- as little as 1/100th of the thickness of silicone -- giving you moisture and humidity proofing without meaningfully increasing the size of the coated item. Parylene is also hard enough to withstand knocks, dings and abrasions that would break down silicone or other soft coatings.
Parylene's key benefit for high humidity environments comes from the unique way in which it gets deposited on an item's surface. Most coatings are liquids that, whether brushed, sprayed or deposited by dipping, flow over the item. The coating process typically creates the opportunities for pinholes and other imperfections to form.
With parylene, the deposition is made while it is a vapor. This means that it touches everything that air touches. More specifically, it deposits anywhere that the moisture in humid air would be able touch. This truly conformal coating make the coated item truly moisture and humidity proof.
While parylene or silicone are usually the best options for high humidity environments, we understand that every project is unique and will require a thorough understanding of the project requirements.