Understanding Parylene Deposition

Parylene’s deposition process is unique among conformal coatings. Unlike conventional processes where materials begin as a liquid, solid Parylene undergoes a distinctive transformation. The Parylene coating equipment converts solid Parylene powder into vapor, which subsequently adheres to the substrate. Although this innovative four-step approach presents certain challenges, it also offers significant benefits.

Sublimation

In the first step of the process, the powdery raw material, called dimer, is heated. As the temperature approaches 150°C, the dimer sublimates into a gas without passing through a liquid state. At this point, the dimeric gas is ready to pass into the next stage of the deposition process.

Pyrolysis

The dimer gas actually consists of two Parylene molecules, bonded together. In the pyrolysis stage, the gas is further heated in a furnace to temperatures as high as 700°C. The heat splits the molecules apart — creating a monomer. That monomeric vapor is what the Parylene conformal coating becomes when it reaches the item to be coated in the deposition stage.

Deposition and Polymerization

The container holding the items to be coated is depressurized to create a vacuum. That vacuum pulls the Parylene gas out of the pyrolysis chamber and into the deposition chamber. In that chamber, which is maintained at normal room temperature, the Parylene vapor turns into a polymer, coating everything that it touches. Throughout the deposition process, the Parylene gas undergoes cooling without significantly heating the items being coated beyond a few degrees.

Waste Collection

The vacuum pump that pulls the Parylene gas through the deposition chamber also plays an important role in disposing of waste vapor. As the gas exits the chamber, the pump pulls it though a cold trap. That trap, which is usually cooled to extreme subzero temperatures, causes the Parylene to solidify so that it can be disposed of.

Challenges in the Process

Adhesion poses a significant challenge with Parylene, particularly when applied to noble metals. This is often addressed by applying silane A-174 or one of SCS’ unique adhesion promotion technologies prior to or during the deposition process.

Due to Parylene’s distinctive deposition process, the quantities available for coating are restricted. Essentially, the number of items that can be coated is constrained by the capacity of the deposition system’s coating chamber. Although these systems may be sizable, there are practical constraints on the quantity of items that can be coated simultaneously.

Furthermore, while Parylene coatings are typically thin, they also deposit relatively slowly. The fastest-depositing variant of Parylene — Parylene C — typically deposits at a rate of 0.2 mils or 5 microns per hour. This means that a 75 micron coating would take approximately 15 hours. Parylenes N and D deposit more slowly.

The Power of Vapor

Despite the downsides of the Parylene deposition process, choosing a vapor-phase coating offers significant benefits. Notably, the dry nature of the vapor and the room temperature process allow even delicate items to be coated without harm.

Parylene vapor can go anywhere that air can. This means that it coats under parts, inside small crevices and can even coat inside semi-sealed areas. Its thinness means that it can coat without forming bridges in tight areas as well.

A Parylene coating offers exceptional conformality and avoids the running or dripping issues commonly associated with liquid coatings. Since it deposits on the entire surface at the same time, it also creates an extremely even coating of the same thickness, everywhere. This characteristic is one of the reasons that it is also typically the most pinhole-free of all of the conformal coating compounds.

Ultimately, the Parylene deposition process is an important part of what makes Parylene such a desirable coating choice. While its vapor-phase deposition can be time consuming and expensive relative to other coatings and their more traditional application methods, its results are also unparalleled for many applications.