Conformal coating on electronic assemblies protects sensitive components and copper tracks on circuit boards from the vagaries of the environment. Typical conformal coatings are epoxy-based, requiring a thick layer to be effective. Parylene conformal coatings, on the other hand, can be ultra-thin and pinhole-free as they are polymer based.
Parylene conformal coatings offer a number of high-value surface treatment properties. These include resistance to moisture and chemical ingress and an effective dielectric barrier. In addition, Parylene also offers excellent thermal conductivity, dry-film lubricity, and UV stability, very essential for electronic subsystems. These properties of Parylene conformal coatings make it an ideal choice for various applications in the fields of consumer electronics, medical electronics, transportation industry, and defense and aerospace industries.
Manufacturers of a unique polymer series use the generic name Parylene for their products. These variations or members of the Parylene family each offer their own, somewhat different properties of the coating. Parylene variants are commercially available, and they are Parylene N, C, D, HT, and ParyFree.
Parylene N or poly(para-xylene) is the basic member of the series. This is a totally linear and highly crystalline material. Being a primary dielectric, Parylene N exhibits a very low dissipation factor and high dielectric strength. It shows an exceptionally low dielectric constant, varying very little with frequency. It also exhibits substantially high crevice-penetrating ability, second only to Parylene HT.
The second commercially available member of the Parylene series is Parylene C, derived from the same raw material as Parylene N. The only difference from Parylene N is the substitution of a chlorine atom for an aromatic hydrogen atom. The useful combination of physical and electrical properties, in addition to very low permeability to corrosive gases and moisture, makes Parylene C useful as a conformal coating.
The third member of the Parylene series is Parylene D, also a derivative of the same raw material that produces Parylene N. The substitution of a chlorine atom for two aromatic hydrogen atoms differentiates Parylene D from Parylene N. Most properties of Parylene D are similar to those of Parylene C. However, Parylene D has the added ability for withstanding slightly higher use temperatures.
The newest addition to the Parylene family is Parylene HT, a commercially available variant. The difference from the other family members is in the replacement of the alpha hydrogen atom of the N dimer with fluorine. Parylene HT can withstand temperatures of 450 ℃, therefore suitable for high-temperature applications. It also has excellent long-term UV stability, a low coefficient of friction, and low dielectric constant. Among the four of the family above, Parylene HT shows the highest penetrating ability.
A unique member of the family series is ParyFree, and this is also the newest. The difference from Parylene N dimer is in the replacement of one or more hydrogen atoms with a non-halogenated substituent. Compared to other commercially available Parylenes, this halogen-free variant offers advanced barrier properties of Parylene C along with substantially improved mechanical and electrical properties. This allows ParyFree to offer robust protection against water, moisture, corrosive solvents, and gasses, as required by select industries.
