Why does the Parylene Process take so Long?

Parylene is a transparent polymer that offers uniform and pinhole-free conformal coatings for printed circuit boards, medical devices and microelectronics. Common Parylene variants available through a modification of the molecular structure of para-xylylene include Parylenes N and C, ParyFree^® and Parylene HT^®. Each modification results in a slightly differing set of material properties that are applicable in different service conditions.

There are a number of points of consideration to be kept in mind regarding the Parylene process, which is an engineering intensive advanced process which results in a very high quality advanced conformal coating. Each step of the handling and processing is done with care and precision.

Reception of samples: The process starts with the reception of the sample substrates to be coated. The number of samples and surface area influence the deposition process time.

Inspection: After reception, samples are inspected for their surface quality, cleanliness and any damage. If incoming quality or cleanliness issues exist, labor and related costs will increase.

Cleaning and surface activation: If samples require a surface cleaning process such as a basic plasma cleaning processes (oxygen plasma) or UV-ozone treatment for surface activation, the labor added might take from a few minutes to an hour depending on the severity of issue. In some cases, a wet rinse might additionally be required. The quality of the bond surface is of very high importance in the final quality of the Parylene film. The adhesion strength is highly dependent on the surface energy of the substrate, and organic residues result in low surface energy which is not favorable for high integrity interface formation. In summary, dirty surfaces prevent Parylene molecules from attaching and penetrating to the substrate’s crevices and pores.

Masking: Masking is a critical step, especially in microelectronics, because this step determines the available surface areas for Parylene deposition. For example, the backside of the silicon or SOI wafers might be required to be left free of Parylene for MEMS processes. Connectors or ground planes of PCBs can be protected against Parylene deposition using a masking tape. Masking is applied according to pre-determined instructions, often called out in drawings.

Masking must be done by experts, ensuring a uniform application without any gaps, holes and paths where the Parylene can penetrate. Because Parylene molecules can enter and diffuse into very small holes and pores (of a few hundred nanometers), masked areas should be carefully examined prior to loading parts for coating. Masking can take a significant time depending on the number of samples.

Silane A-174 and surface adhesion promoters: To improve the adhesion of Parylene to the target surface, extra care is often taken to ensure a clean the surface. Subsequently, surfaces are treated using an adhesion promoter such as a silane (A-174). The adhesion promoter forms a monolayer of molecules in a way to provide a high strength bond interface between the surface and the Parylene. This step can be done in the vapor phase in the processing chamber.

Deposition: The vapor deposition polymerization (VDP) process of Parylene takes place in three steps (sublimation, pyrolysis and deposition). Overall, the process might take from a few hours to over 24 hours depending on the coating thickness and equipment-related processes such as pumpdown, temperature set-point attainment, etc. Another consideration for high-volume manufacturers is the throughput of Parylene due to equipment size limitations. In order to ensure thickness uniformity throughout the coating chamber, load density limits must be adhered to. This can also limit throughput.

A large variety of substrates can be coated using Parylene (elastomers, glass, metal, paper, plastic to name a few). Samples are coated in a vacuum deposition process. Process variables including temperature and pressure are pre-determined for the ideal coating conditions for each batch. The granular precursor, called dimer, is weighed and inserted into the sublimation section of the system. The coating process takes place once appropriate levels of vacuum are achieved. At this point, the Parylene dimer is sublimated and then pyrolized, forming the highly reactive monomer molecules that travel into the deposition chamber. The monomer molecules deposit onto all surfaces within the chamber and penetrate to the smallest voids, resulting in a uniform, void-free conformal coating. Thicknesses are tailored based on the application area and properties required by the application (dielectric strength, mechanical properties, chemical resistance, etc.).

De-Masking: Once the coating process is finalized, substrates are de-masked,. This step is also critical and must be done with care to avoid damage of the Parylene.

Final Inspection: Once the production processes are completed, a final inspection is performed. These inspections are done via different routes such as visual inspection for issues (uniformity, coating in keep out areas etc.). Methods may include optical microscopy and thickness control, using a series of thickness witness coupons.