Whether the application is a medical device, a printed circuit board (PCB), or a light-emitting diode (LED), a parylene conformal coating is typically applied to protect the product. Sometimes, however, the product actually has to be protected from the parylene conformal coating—or at least parts of it do.
That's where parylene masking comes in. The parylene masking process serves to protect designated areas of an assembly that could negatively impact performance if coated. And because masking parylene coating preserves the functionality, integrity, and performance of an assembly, it is among the most crucial steps in the parylene coating process.
Reasons for Masking Parylene Coating
Some of parylene's key properties can be both desirable and detrimental to an assembly, depending on where the conformal coating is applied. For example, parylene is an excellent dielectric. As a result, a parylene-coated PCB will be protected from electrical interference. If the PCB's contacts are coated, however, the board won't work. Parylene masking solves this conundrum by allowing the board to be selectively coated in order to reap the benefits of parylene without rendering the PCB inoperable.
Masking parylene coating is also performed when an area of a device or component has a moving part. If these parts are coated, the parylene coating could be damaged by the motion of the part. Masking these parts serves to protect the integrity of the parylene coating.
Although masking parylene coating is among the most time-consuming and labor-intensive steps in the parylene conformal coating process, it also among the most important. Improper or poor parylene masking, after all, can compromise the coating or cause failure of the finished part.
When working with a contract coatings company, the first step in the parylene masking process is to generate drawings that clearly identify the areas of the product that should not be coated during parylene deposition. Once the product has been delivered, the mask has been designed, and the substrate has been cleaned in preparation for parylene deposition, masking materials are applied to the areas that must remain uncoated.
Once the product is masked, it is placed into the vacuum chamber for parylene deposition. During this stage of the process, parylene vapor envelops the entire product, covering any surface of the substrate that is exposed to air. On the masked areas, however, the parylene is deposited on the masking materials instead of on the actual substrate. The masking material is then removed when the product is dry, taking the parylene coating with it.
Alternatives to Masking Parylene Coating
While parylene masking and mask removal are time-consuming and exacting processes, the alternative approach is to coat the entire product and then selectively remove the parylene coating from the specified "coating-free" areas. And while this approach can yield success, it poses its own challenges.
Parylene removal can be accomplished through thermal, mechanical, and abrasion methods. Thermal and mechanical methods, however, carry a high risk of damaging the substrate. Abrasion—which uses a technique that is similar to sandblasting, albeit on a near-microscopic scale—can produce accurate and cost-effective results. On the other hand, it is still more labor intensive than applying a mask and selectively coating the substrate.
Depositing and masking parylene conformal coating are complicated processes with many variables and challenges. But when performed correctly, parylene coating offers a degree of protection and performance that few other types of conformal coatings can match.