Parylene adhesion can be tricky to manage. Unlike other coatings that adhere to the surfaces they coat, parylene sticks to itself. This can cause trouble when it needs to be applied to smooth surfaces, like areas made of stainless steel or noble metals like gold or silver. However, since parylene has so many other advantages, it's worth looking into methods to improve adhesion. You can use the product; you just might need an extra step.
Introduction to Parylene Adhesion
There are two primary types of adhesion -- chemical and mechanical. Chemical adhesion occurs when a compound bonds to another compound. For this to occur, the two compounds have to be chemically active enough to form a bond between each other. Because parylene is inert, it adheres using the other method -- mechanical adhesion.
Parylene technically doesn't stick to surfaces. Instead, it sticks to itself and it fills in surfaces. Most coated items have small microscopic pits and cracks that the parylene fills. In essence, it works like snaps on clothing except that, once the parylene is snapped in, it doesn't come out. However, if parylene is deposited on a perfectly smooth surface, there aren't any pits and cracks for it to fill, and it may not form a reliable coating.
In applications where noble metals are used for their excellent conductivity, parylene adhesion is a non-issue. If you want to have the metal exposed in a connection, you won't want to have it coated with Parylene at all, especially given the compound's excellent dielectric properties. Masking off those areas eliminates having Parylene adhere (or not adhere) to them in the deposition process, completely sidestepping the problem.
The Silane Solution
While maintaining a perfectly clean surface may be enough to solve parylene adhesion issues on most materials, some surfaces, including noble metals, require additional treatment. Coating those surfaces with a compound like A-174 silane can solve the problem.
Technically referred to as Methacryloxypropyltrimethoxysilane, A-174 is a colorless compound related to silicone. When applied to surfaces like noble metals, it provides an uneven surface to which the Parylene can bond. The silane chemically adheres to the metal, providing a tight bond between it and the surface. When the Parylene is deposited, it adheres to the uneven surface of the A-174 silane.
Silane is relatively easy to apply. If an entire item needs to be coated, it can be soaked or applied in its vapor phase. The compound can also be selectively sprayed on to the surface.
With the addition of A-174 silane, parylene's primary challenge has been solved. This makes it a suitable choice for products containing noble metals that also need to leverage parylene's other strengths. To learn more about how we can help manage parylene adhesion issues, contact Diamond-MT today.