It is imperative to obtain proper adhesion of the coating to the substrate in order to truly reap the benefits of parylene conformal coating. Poor parylene adhesion, after all, can negate some of parylene's most-prized properties, including corrosion resistance, chemical resistance, moisture resistance, and dielectric strength. So, it's in an engineer's best interest to understand the importance of parylene adhesion and how to obtain it.
Parylene adheres mechanically, rather than chemically, to the surfaces to which it is applied. This means that it flows into small pores on the substrate, bonds with itself inside those pores, and essentially gets stuck on itself through the process of cohesion. The problem with this method of adhesion, however, is that it's only effective when there is something for parylene to flow into. Parylene frequently does not adhere effectively to noble metals, for example, especially the gold that is prevalent in circuit boards where high levels of conductivity are important.
Parylene will deposit uniformly on virtually any surface, topography, or geometry. This feature is why parylene is a truly conformal coating and why it's such an effective barrier. But this ability to coat virtually any substrate does have limitations: contaminants on a substrate surface can affect parylene adhesion. As such, all surfaces—even solder fluxes that are designed not to require cleaning—must be perfectly devoid of any contaminants. Oils and other organic compounds, whether natural or artificial, also can negatively impact parylene adhesion.
Before coating any substrate with parylene, adhesion problems can be managed by conducting cleanliness testing. This process identifies the contaminants that are present on the material, if any, so that they can be removed prior to the parylene deposition process. If the material requires cleaning, it can be cleaned with manual, batch, or ultrasonic processes, as appropriate.
Once a surface is clean, it can be modified to promote better parylene adhesion. Typically, treatment with A-174 silane helps make stainless steel and other metals more receptive to parylene coating. To apply the adhesion promoter, the substrate is usually exposed to silane vapor, although it can also soak in the compound. The silane then chemically bonds to the surface, thereby providing something to which parylene can mechanically adhere.
Researchers are currently exploring additional options for improving parylene adhesion in instances for which A-174 silane is either unsuitable or ineffective. Parylene, for instance, has demonstrated improved adhesion to silicon in response to surface roughening with xenon difluoride gas. For medical devices, on the other hand, the use of plasma surface-treatment methods appears to reduce problems with delamination.
When parylene fails to properly adhere to a substrate, the plethora of parylene's positive properties are threatened. However, the challenges inherent in the process of achieving a perfect, pinhole-free, and long-lasting coating are manageable with preparation, care, and knowledge.