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Parylene Masking – General Concepts Explained

Posted on January 28th, 2022 by

Do you need to produce a reliable electronic product? If you want to ensure your electronics will remain resilient throughout exposure to harsh environments, applying conformal coatings to PCBAs and other components may be your best bet. However, conformal coating protection can be too much of a good thing without processes to ensure certain areas remain coating-free.

When specific components are covered with these polymeric films, it can inhibit critical functions. Thankfully, the masking process resolves this issue so that you can benefit from conformal coatings without losing functionality.

 

What is Conformal Coating Masking?

Conformal coating masking is the process of applying a temporary film, boot, tape or plug to designated assembly components, preventing them from being coated. While the masking process is essential for any conventional coatings such as acrylics, epoxies, urethanes, and silicones, Parylene has unique qualities that create a need to ensure that masking is done right.

 

Quick Introduction to Parylene Masking Techniques

For general guidance on conformal coating masking, the IPC-HDBK-830A handbook provides a good reference. However, because the Parylene coating process causes Parylene molecules to penetrate any surface area accessible to air, operators must ensure that all coating keep-out areas resist Parylene coating application, which requires domain expertise.

HZO uses proprietary masking equipment and techniques our engineers build and customize for every project. However, a short description of general methods follows below.

Parylene masking techniques typically involve precisely sizing materials such as tapes, form in place gaskets, and boots to designated masked regions. After applying materials to the appropriate areas, a masking operator will allow peelable masking materials to dry before starting the CVD process, coating the entire component. After coating, the masking material is carefully removed.

 

Why Proper Masking for Parylene Coating is So Critical

Parylene’s exceptionally high dielectric strength requires you to be diligent about masking the right areas. A PCBA coated with Parylene would benefit from electrical insulation protection. However, if the connectors on that same PCBA get covered, you won’t be able to plug it in or make connections.

Download HZO’s Parylene datasheet

Another reason you must get your Parylene masking process correct is due to Parylene’s incredible conformality. Where Parylene is allowed to penetrate, pins and moving parts will be completely encapsulated. Although this cohesive and thorough presence of coating is typically considered a benefit, additional cost and labor to remove the Parylene from keep-out areas will be required with incorrect masking implementation. In other words, it’s best to prevent instead of rework.

 

Why It Can Also Be More Complicated

When you mask for Parylene application, the stakes are higher. Once it’s on, removing the coating will be a struggle without a knowledgeable partner with tools and techniques to make rework easier. The fact that masking for Parylene is also more complex, requiring more skill, raises further complications.

Without skill and experience, masking for Parylene is more challenging than general conformal coating masking due to the chemical vapor deposition process (CVD) used to apply the coating.

Download our infographic for an illustrated guide to CVD

Unlike conventional conformal coatings applied as liquids, Parylene is applied as a vapor, which means you are masking against a gas. Therefore, a much better barrier is required with the Parylene masking process, as vapor can more easily bypass masking materials.

Other masking considerations include:

  • The need to validate if all components are hermetically sealed or not due to the vacuum nature of the CVD process
  • Chamber maintenance
  • Masking more than just keep-out locations may be required
  • The need for specific fixturing
  • Materials need to be thoroughly cured before coating and outgassing should be avoided

The list below includes components that in many cases should not be coated:

  • MEMs
  • Actuators
  • Mating connectors and pins
  • Test points needed prior to assembly
  • Ground contacts

Parylene Coating With HZO

Parylene Masking with HZO

 

If you are masking for Parylene, you will need to implement the process correctly. Further, Parylene masking also requires considerable expertise and experience to identify factors affecting masking success. These might include Parylene masking materials, the Parylene type, and the different areas that must be coated. Proper timing is also essential, as there is a relationship between the masking and coating processes. Any timing issues can impede coating quality. Managing electrostatic discharge is also a concern when masking.

Fortunately, our engineers have years of this experience, creating custom masking plans for customers with electronic products of all sizes, shapes, and functions. Additionally, we have developed industry-leading masking equipment which can automate and semi-automate the process for larger productions.

Reach out today if you would like a consultation to discuss masking for Parylene and the many ways HZO can help you reduce time and cost and to ensure your product will work reliably through proper masking with our proprietary techniques.

Mallory McGuinness

As a veteran writer with over a decade of writing experience, Mallory McGuinness has spent the last two years at HZO learning about coating technology from the best minds in the industry. Professionally, Mallory is especially interested in the process of problem-solving and watching how the engineering team develops solutions that ensure business requirements are met. In her free time, you can find Mallory walking her dog Ebbie, fueling up on coffee, watching the Simpsons, and referencing the Simpsons.

All of Mallory’s blogs are reviewed for accuracy before publication.

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