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 process is unique in coating technology and is best described as a vapor deposition polymerization. It is carried out under vacuum and requires specialized equipment.
Parylene is applied at room temperature with specialized vacuum deposition equipment that permits control of coating rate and thickness. The deposition process takes place at the molecular level as the chemical, in dimer form, is converted under vacuum and heat to dimeric gas; pyrolized to cleave the dimer; and finally deposited as a clear polymer film.
The material is applied at .0002-in per hr. Coating thicknesses from .100 to 76 microns can be applied in a single operation. Typical coating thickness for circuit boards is .00 I-in.
Because it is a gaseous process every surface is coated identically unless masked to prior specification. It is perfectly uniform over and inside of all surfaces. No other process can make this claim.