Parylene Masking: Materials and Methods

Parylene deposition takes place at the molecular level. Applied at room temperature through chemical vapor deposition (CVD), the typical thickness of Parylene conformal film is in the microns-range.

Advantages of Parylene coatings include:

• Excellent material properties, adhering to a wide range of substrate surfaces, biocompatibility, chemical/corrosion resistance, reliable dielectric performance and thermal stability.
• Uniform, pinhole free coverage of all surfaces regardless of position or place on the assembly (component sides, lead surfaces).
• No harmful vapors during processing.

Despite its benefits, Parylene has some disadvantages. Among these are:

• Limited, batch-mode production volume.
• Expensive processing equipment/materials.
• Difficulty reworking ineffective coating.

A further disadvantage is the need for costly, time-consuming masking of selected assembly components. The Parylene masking process is necessary to protect designated areas of an assembly that could impede performance if coated.

Masking For Parylene                   

Masking assures selected assembly components are NOT covered by the applied Parylene film, which would inhibit their functionality. Integral to surface preparation, the masking process protects designated assembly components from the encapsulating effects of the Parylene itself.

For instance, Parylene’s excellent dielectric properties simultaneously disable a PCB’s contacts, rendering the unit inoperable, even as the substrate is protected from electrical interference.  Masking contacts resolves this issue, coating only those PCB parts that are not distressed by conformal protection. Components retain their capacity to accept an electrical charge and/or move as designed.

Parylene Masking Materials

Masking requires different materials such as peelable masking dots, tapes/contact pads, boots and either non-ammoniated or water-soluble liquid latex. It is important to recognize the properties of the various materials and how they are used in Parylene processing separately or in combination to ensure that masking failures do not occur.

Parylene masking dots and tapes generally use polyester or polyimide backings. In the vast majority of cases, properties like conformability and cost-effectiveness are as necessary to the masking process as strength and stability.

Essentially, masking dots are small stickers fastened over the contact before coating is initiated.
For masking tapes and contact pads, there are two widely used solutions:

• Polyester or Kapton tape
• Liquid peelable latex masking materials effectively prevent coating ingress into the component and peel easily from contacts after coating

Masking boots are typically custom-made, specific to the physical configuration of the component to-be-covered. Recyclable, boots’ capacity for repeat processing is a labor-saving initiative that also generates considerable cost-effectiveness during re-use, recouping the initial investment in short order.

Masking Methods for Parylene

Generally, the most labor-intensive part of the coating process, masking is done in accordance with the customer’s drawings and requests for coating keep-out areas. Because CVD causes Parylene molecules to penetrate any surface area accessible to air, considerable attention to adequately sealing every connector is mandatory to ensure all coating keep-out areas will resist and reject the Parylene film application.

Basic to the process is accurately identifying those sections of the assembly NOT to be covered during CVD. A general methodological outline includes these steps:

• Sizing boots, tapes and dots to the precise configurations of the masked regions.
• Preparing Parylene masking materials per instructions.
• Applying materials to selected areas, effectively covering contact points that need to remain uncovered during the assembly’s operation.
• Allowing peel-able masking compounds to fully dry before beginning Parylene CVD.
• Applying the Parylene, coating the entire substrate.
• Due to masking, the Parylene covers the masking materials, rather than the contacts or other protected areas of the substrate surface.
• After Parylene application, the masking is carefully peeled or otherwise removed.
• The peelable mask is subsequently removed, as soon as possible after the Parylene has dried, to prevent tearing the film, exposing the contacts or other masked regions.

These processes are exceptionally labor-intensive and can be costly. Nevertheless, they are essential to Parylene film administration. Touching up the periphery of the masked region with a small quantity of urethane after removing the masked materials lessens the threat of tears.

For most applications, use of conventional masking materials and techniques obstructs Parylene deposition on designated PCB keep-out regions. However, masking for MEMS/nano medical devices is more challenging. Laser ablation provides better options for these more difficult applications, enhancing adaptability to assembly’s alternate geometries, while providing greater precision during mask application. Masked regions are protected from coating, assuring they function as designed to generate appropriate assembly performance. Masking materials must thoroughly shelter the keep-out regions, without gaps, crevices or similar surface breaches, to provide reliable connector function after coating. Effective masking of each PCB connector demands concerted operator attention to appropriately seal it from contact with gaseous Parylene molecules during CVD. Masking preserves an assembly’s operational integrity and performance.