Mastering the Parylene Coating Process

While the Parylene deposition process is relatively straightforward, its implementation can pose challenges, especially in controlling coating thickness and ensuring a successful coating cycle. Because coating type and required surface thickness vary according to substrate material and coating project, deposition rates fluctuate. Processing times can vary from under an hour to over 24 hours, with a deposition rate of approximately 0.2 mils per hour. The slower substrate coverage rate contributes to Parylene’s exceptional conformal coating quality, surpassing other options, yet it does elevate costs. Proficiency in the Parylene coating process can mitigate these production expenses.

Deposition Described

Parylene’s complex and specialized vapor-phase deposition technique ensures the polymer can be successfully applied as a structurally continuous film, entirely conformal to the characteristics of the selected substrate. To master the process successfully, it’s crucial to ensure that every incoming order for the Parylene application contains all necessary information. This includes detailed drawings, specifications, and specific instructions that distinguish the order. These elements aid in creating tailored solutions for each item.

Parylene’s deposition method eliminates the wet application technique employed by coating materials like epoxy, silicone or urethane. It begins in a vacuum chamber with raw, powdered Parylene dimer placed in a loading boat and inserted into the vaporizer. The dimer is initially heated to between 100ºC – 150ºC, converting the solid-state Parylene into a gas at the molecular level. The process needs to maintain a consistent level of heat, gradually rising to the temperature to 680ºC. This causes the vaporous molecules to sublimate and split into a monomer.

Drawn by vacuum onto the selected substrate one molecule at a time in the coating chamber, the monomer gas reaches the final deposition phase, the cold trap. Here, temperatures are cooled drastically to levels sufficient to remove any residual Parylene materials pulled through the coating chamber from the substrate, between -90ºC and -120ºC.

Pre-deposition    

To master the Parylene process, detailed attention to the procedures below is essential before beginning deposition and coating.

• Thorough inspection of incoming items to-be-coated, verifying their quantity and condition.
• Preparation procedures are carried out as necessary. This includes actions such as cleaning and testing for cleanliness, or other specific processes, which are followed by the masking of connectors and electrical components. Contaminants on the substrate can reduce adhesion, so ensuring the surface is adequately clean is crucial for Parylene coating. Depending on the substrate’s surface, cleaning can be done manually or using batch, inline, or ultrasonic methods. Most materials, such as glass, metal and plastic, require treatment with A-174 silane for proper surface modification before applying Parylene. Typically, this involves manual spraying, soaking or using vapor-phase technology. After masking, A-174 silane forms a unique chemical bond with the substrate’s surface, enhancing Parylene adhesion.
• Masking is exceptionally labor-intensive. Exceptional care is required to ensure every connector is effectively sealed, so gaseous Parylene molecules do not penetrate their surfaces. All tape, or other covering materials, must thoroughly encompass the keep-out regions, without gaps, crevices or other openings, to ensure connector function is retained after coating.
• Further inspection assures masking is in compliance with customers’ specifications.
• The diversity of adhesion promotion methods requires a similarly diverse list of raw materials. Establishing best-adhesion practices is only part of mastering the Parylene coating process; once established, strict adherence standards need to be reliably enforced to ensure quality of the conformal coatings. Using industry best practices, such as substrate cleansing and A-174 silane application, appropriately combined with standard, repeatable processes, will ensure strong adhesion for Parylene coating. Adhesion promotion methods are typically used prior to the actual coating process; however, some can be integrated during the process itself.

Coating Requirements

The Parylene coating is applied through the deposition process described above. Once coating has been deposited, masking materials are removed; extreme caution must be exercised not to damage the thin layer of applied Parylene.

An important consideration of appropriate Parylene thickness is total required clearance. While an enclosure-PCB has few clearance issues, in many cases even an additional millimeter of Parylene coating can be sufficient to generate dysfunctional mechanical abrasion, damaging the Parylene surface and reducing its conformal qualities.

Regarding dielectric strength, items whose required levels of dielectricity are higher will need a thicker coat of Parylene. Balancing dielectric strength with clearance generally requires quality testing to determine their correct ratio. The end-item customer may not always provide these specifications; learning how to determine dielectric/clearance ratios without this data is integral to mastering the Parylene deposition process.

The Parylene process must generate a conformal covering explicitly meeting the customer’s precise specifications. If changes are necessary, making them to order and on time are essential elements of mastering the Parylene coating processes.  A final inspection ensures successful completion of all process phases, and that the final product complies with the customer’s drawings and specifications.