Masking Before Parylene Conformal Coating
Release Time：2022-01-17
Conformal coating with parylene is a very different process compared to coating with traditional solvent-borne or UV curable materials. Parylene coating has some advantages including
• a very thin application profile,
• uniform and complete surface coverage, and
• generally excellent protection and minimal outgassing potential.
On the other hand, parylene coating also presents unique challenges such as
• a significant investment in complex equipment,
• batch processes that limit productivity and increases costs, and
• the user’s inability to selectively control where the coating is applied.
It is this final challenge that we will concern ourselves with within the following discussion.
Why does the parylene process require protective masking?
The use of parylene is a specialized and often expensive method to protect PCBs. A simple schematic of the basic process is shown above. The “deposition chamber” in the drawing is where PCBs are placed to be coated. The parylene is vaporized in a near-vacuum, and the vessels including the deposition chamber must be closed, resulting in a batch process.
Unlike traditional conformal coatings using selective application equipment, the vaporized parylene coating process will deposit the polymer evenly across all exposed surfaces. As a result, it is necessary to completely mask and protect any sensitive surfaces or devices that will be affected negatively by the presence of parylene on their surfaces.
What are the surfaces or components that may require protective masking?
On nearly all printed circuit boards, there are components and sections where conformal coating would not be acceptable. It is probably not necessary to discuss in detail all of the various reasons, but the most common areas that shouldn’t be coated include
• connectors, test sites, and pins where electrical continuity must be maintained;
• sensitive or unsealed components, such as open inductors or relays;
• large through-holes or “vias” where coating would leak through;
• LED surfaces if the light output is affected by coating; and
• in some cases, certain integrated circuit packages, such as BGAs and QFNs, if wicking under the component is unacceptable.
Link:https://blog.humiseal.com/masking-before-parylene-conformal-coating
