Parylene Process

What's Parylene
Parylene is a conformal polymer film that has been used as a protective conformal coating for more than 50 years.It provides environmental and dielectric isolation in a variety of applications, including various medical substrates, electronic circuits, industrial and automotive sensors, and archival preservation of documents and archeological samples.
The most important performance specifications for a conformal coating are (1) uniformity and completeness of coverage, and (2) physical, electrical, chemical, mechanical, and barrier properties.Parylene offers significant performance capabilities in both of these areas.
As a dry process non-solvent based coating, parylene is not affected by volatile organic compound (VOC) restrictions, and it is not implicated in the ozone depletion concerns of the Montreal Protocol and other environmental legislation.
Poly-para-xylylene Versions
There are three common forms of the Parylene polymer:Parylene C, Parylene N, and Parylene D, each with unique properties that suit it to particular insulation applications.
For example, Parylene N provides particularly high dielectric strength and a dielectric constant that is independent of frequency.Because of its high molecular activity in the monomer state, Parylene N has the highest penetrating power of the Parylenes, with the ability to coat deep recesses and blind holes.
Parylene N has a very low dissipation factor, a low dielectric constant, and is ideal for high frequency applications where the coating is in the direct RF field.Unlike liquid coatings, the Parylene deposition process does not entrap air that can lead to high frequency corona problems.
Parylene C has a chlorine atom on the benzene ring, and this results in modified electrical and physical properties including very low permeability to moisture and corrosive gases.The Parylene C deposition rate is substantially faster than that of Parylene N, resulting in reduced crevice penetration ability.
Parylene D, with two chlorine atoms on the benzene ring, has the highest degree of thermal stability of the Parylenes and also possesses superior physical and electrical properties at high temperatures compared to Parylenes C and N.
The bulk electrical properties of the Parylenes make them good candidates for electrical and electronic applications. Dielectric constants and dielectric losses are low and unaffected by absorption of moisture, and the polymer's high purity, low moisture absorption and freedom from trace ionic impurities yield high bulk resistivities.

The Deposition Process

Vacuum deposited Parylene is applied to substrates in an evacuated chamber by means of gas phase polymerization.It grows as a conformal film on all exposed surfaces, edges, and crevices.Since Parylene is nonliquid, it does not pool, bridge, or exhibit meniscus properties during the application process.
The Parylenes are formed at a vacuum pressure of approximately 0.1 torr,Consequently, all sides of an object to be encapsulated are uniformly impinged by the gaseous monomer, resulting in a high degree of conformality in the applied coating.There is no liquid phase in the deposition process, and substrate temperatures are held near ambient.
The parylene precursor, a granular white powder, is first vaporized at approximately 150 °C and 1.0 torr vacuum.The resulting dimeric gas is further heated in a pyrolysis chamber -- to approximately 680 °C at 0.5 torr vacuum-- yielding the monomeric diradical para-xylylene.This monomer is thermally stable, but kinetically unstable.
Finally, this monomer gas enters the ambient temperature deposition chamber at just 0.1 torr vacuum where it simultaneously absorbs and polymerizes on all surfaces, producing a coating of high molecular weight.The mean free path of monomer gas molecules in the parylene deposition is on the order of 0.1 cm, and, unlike line of sight processes such as vacuum metallization, all sides of encapsulated objects are impinged by the gaseous monomer.

The coating grows from the substrate surface outward, and thus its thickness is infinitely controllable from as little as 100 Angstroms to over 75 micrometers (three mils).As a result, users can obtain the required protection with minimal coating mass.The cure cycle is automatic with parylene, and occurs before deposition.No testing or examination are required to confirm that parylene has cured fully.