What is Parylene?

Parylene is the generic name for the poly-para-xylylenes. These materials form linear, highly-crystalline polymers but can be usefully produced only as coatings and films. The most commonly used is Parylene C, the mono-chloro substituted compound. Parylene N, the un-substituted compound, has better high-frequency dielectric properties, better penetrating power for coating the bore of very small diameter tubes, and is often preferred in medical applications. Parylene D, the di-chloro-substituted compound, has better high temperature endurance.

The Parylene Coating Process

The Parylene process is unique in coating technology and is best described as a vapour deposition polymerisation. It is carried out under vacuum and requires specialised equipment.

The Deposition Process
  1. The process begins with sublimation at about 150°C of the high purity crystalline dimer di-p-xylylene.
  2. The vapour is pyrolised at about 650°C to form the gaseous monomer which has an olefinic structure.
  3. The coating chamber is at room temperature. The vapour condenses on all surfaces equally and can pass through holes as small as 1μ. It then spontaneously polymerises to form a product with a high degree of crystallinity. The coating is absolutely conformal and can be laid down in thicknesses from a few angstroms to 50 microns or more depending on the requirements of the end use.

Substrates such as circuit boards are jig-mounted in a manner to ensure an even distribution of the monomer within the coating chamber and thus a consistent coating thickness. Small items such as ferrite cores are gently tumbled in the chamber, giving a very cost-effective process since many thousands may be coated at one time.

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