Description of Parylene: Parylene is the trade name for a variety of polyxylylene polymers marketed by several providers. Parylene N is a polymer manufactured from di-p-xylylene, a dimer synthesized from p-xylylene. Di-p-xylylene, more properly known as paracyclophane, is made from p-xylylene in several steps involving bromination, amination and elimination.
Parylene is a common generic name for a unique series of polymers based on paraxylene. The three most common types of parylene are referred to as: Parylene N, Parylene C, and Parylene D. In actual usage, Parylene C is the greatly predominant type of parylene used for almost all types of applications and as such is usually the type of material associated as “Parylene.” The basis for the parylene family is the polyp-xylene monomer which comprises Parylene N. Parylene C and D are created by the substitution of a single chlorine molecule (C) or two (double) chlorine molecules (D). See Figure 1.
There are a number of derivatives and isomers of Parylene, but only a few are used commercially, e.g., Parylene N, Parylene C and Parylene D. This article discusses the molecule which isn’t substituted, which produces Parylene N. Heating paracyclophane in a partial vacuum gives rise to a diradical species which polymerizes when deposited on a surface. Until the “monomer” comes into contact with a surface it is in a gaseous phase and can access the entire exposed surface. It has a variety of uses. In electronics, chemical vapor deposition at low pressure onto circuit boards produces a thin, even conformal polymer coating. Parylene coating has very high electrical resistivity and resists moisture penetration. It is used as a dielectric in certain high-performance capacitors for precision measurement. It has uses in preserving archival paper.
The parylenes are formed by the pyrolysis of a di-p-xylene (dimer) in a vacuum environment which is then deposited on a cooler (i.e. room temperature) substrate under continuous vacuum. Vapor phase deposition of the parylene polymer allows it to be formed as a structurally continuous film which is truly conformal to the design and structure of the substrate upon which it is being deposited. Parylene can be effectively deposited with excellent accuracy in the thickness range of 0.1 mils to over 2 mils.
The most widely used dimer, providing a useful combination of properties, plus a very low permeability to moisture, chemicals, and other corrosive gases.
Provides high dielectric strength and a dielectric constant that does not vary with changes in frequency. Best selection where greater coating protection is required.
Maintains its physical strength and electrical properties at higher temperatures.