While parylene is an extremely effective conformal coating, its benefits only come into play when it is properly applied. When parylene is either applied incorrectly or is deposited on a surface that is not prepared for adhesion, the coating can become compromised. Luckily, common parylene defects can be identified, planned for and mitigated through proper procedures.
Problems Parylene Lacks
While Parylene is prone to defects related to adhesion in some instances, the unique nature of the coating and its deposition methods means that it avoids many of the common challenges that other coatings poses. The vapor deposition process that transfers Parylene from a gaseous state to an eventual solid coating typically allows it to form a truly conformal coating. This means that problems like uneven coatings, bubbles, capillary action-formed pools or rivulets or failures to coat in small areas -- like under components -- do not happen.
The nature of parylene deposition means that the item does not have to be heated or cooled -- expanding or shrinking it -- to be coated. This also reduces stress on the Parylene coating. The fact that it deposits as a dry vapor also helps to ensure that the coated items goes through as few changes as possible, removing another risk factor for the coating to become defective.
Parylene Adhesion Challenges
Many of the challenges with Parylene comes from its chemical structure and its mechanical method of adhesion. Parylene sticks well to itself, but technically, does not form chemical bonds with any items to which it is applied. Instead, its mechanical method of adhesion means that it flows into the small cracks and crevices on most substrates, and ends up locking itself into the coated item. However, because it does not form chemical bonds, it is less able to stick to surfaces that do not have these microscopic gaps. Typically, this causes a problem when using Parylene to coat noble metals -- like gold used on circuit boards.
Parylene's chemical inertness is one of its most desirable properties. However, while it doesn't react to many chemicals, it also doesn't stick to them. Parylene is particularly bad at adhering to solvents and fluxes. This means that it may not be able to be used on items that are not carefully cleaned. In fact, Parylene will even adhere to the oils that are left behind in a fingerprint. Unfortunately, this means that if the oils break down, the Parylene coating could delaminate.
These two adhesion challenges are easy to fix. the first step is to clean any items that are to be coated with Parylene with alcohol or with appropriate detergents or solvents. Even "no clean" fluxes have to be removed prior to coating. Items with noble metals need a different treatment. They need to first be treated with an adhesion supporting compound like A-174 silane. This compound chemically adheres to the noble metals and gives the Parylene something to mechanically bond to.
The other common parylene defect is technically not a defect -- coating more than the end user intends. Since it deposits as a vapor, it can coat anything that air can touch. Unfortunately, this means that it could end up coating areas of the item that are not supposed to be coated. This problem is also solvable by having those items masked prior to coating. That way, the Parylene sticks to the mask, allowing it to be removed by peeling away the masking material.
Unlike other coatings that pose a wide range of challenges to achieving a perfect coating, the challenges inherent in using Parylene are relatively limited. Properly preparing the surface for adhesion -- or for non-adhesion in the case of items to be masked -- can go a long way to ensuring a perfect and truly conformal coating.