Common Parylene Defects

While Parylene is an extremely effective conformal coating, applications only benefit when the coating 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 adhesion defects, the unique nature of the coating and its deposition methods mean that it avoids many of the common challenges that other coatings pose. The vapor deposition process that transfers Parylene from a gaseous state to an eventual solid coating allows it to form a truly conformal coating. This means that problems such as uneven coatings, bubbles, capillary action-formed pools or rivulets or failures to coat in small areas, like under components, do not occur.

The nature of Parylene deposition means that the item does not have to be altered in size, accomplished through heating or cooling, to be coated. This also reduces stress on the Parylene coating. Being deposited as a dry vapor also helps to ensure that the coated items experience as few physical alterations 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 the coated item. Instead, Parylene’s mechanical method of adhesion means that it flows into the small cracks and crevices on most substrates, ultimately locking itself into the coated item. However, because it does not form chemical bonds, it is more challenging to stick to surfaces that do not have these microscopic gaps. Typically, this causes a problem when using Parylene to coat noble metals, such as gold.

Chemical inertness is one of Parylene’s most desirable properties. However, Parylene is particularly bad at adhering to solvents and fluxes. This means that Parylene is not recommended to be used on items that are not carefully cleaned. Parylene will 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 easily fixed. The first step is to clean any items that are to be Parylene coated with alcohol or with other appropriate detergents or solvents. Even “no clean” fluxes have to be removed prior to coating. Items containing noble metals need a unique treatment. They need to first be treated with an adhesion supporting compound like Silane A-174. This compound chemically adheres to the noble metals and gives the Parylene something to mechanically bond to.

Parylene Thickness

The other common Parylene defect is coating more than the end user intends. Since Parylene deposits as a vapor, it can occupy any space that air can reach. 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 masking these components before applying the Parylene coating. That way, the Parylene sticks to the mask instead of the component, allowing Parylene 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 when there is a demand for masking, can go a long way to ensuring a perfect and truly conformal coating.