Parylene, chemically known as poly(p-xylylene), is a polymer which, when applied to printed circuit boards or virtually any substrate, acts as a barrier film with excellent mechanical, chemical and dielectric properties. It is chemically inert (unreactive) and non-toxic – a “green” chemistry.
Discovered in the 1940’s, parylene was first commercialized in the 1960’s and has seen widespread use in a variety of applications, most notably in the protection of electronic components and assemblies. Today parylene protective films are actively used in consumer electronics, automotive, aviation/aerospace, defense, medical and several new industry segments.
Parylene films are generally deposited using the well-known Gorham method that involves thermal cracking of dimers di-para-xylylene ( also known as [2,2] paracylophane) and other substituted dimers in vacuum. The first step of the deposition process involves the sublimation of the dimer in a sublimation furnace at temperatures in the 120 – 160 C range.The dimer vapors flow into a pyrolysis furnace which is heated to a high temperature. At high temperatures (> 600 C), the dimer is cleaved into two reactive monomer groups that are subsequently transported to the deposition chamber where they polymerize on all surfaces to form parylene thin films. The entire process takes place in vacuum and the deposition process is often referred to as Chemical Vapor Deposition Polymerization. See diagram A below.
The dimers have a general structure of [-CX2-Ar-CX2-]2, wherein, Ar is an aromatic moiety, X usually is H or F. Substitution of H on the aromatic ring yields the common variants of parylene dimers– parylene –C and parylene –D.The structures of the common parylene variants are shown below. They are all deposited as CVD films, but the physical, mechanical,
and electrical properties of the resulting parylene films are significantly different and the choice of chemistry is usually determined by the particular application.
A summary of the properties of the most commonly used parylene films is included in the properties section.
As a coating, parylene C is favored over all other types by a significant margin and is used in virtually all industry segments. Parylene C has a higher temperature resistance (80°C, in air, long term) as compared to parylene N, as well as a higher deposition rate and improved barrier properties. The gap-fill (crevice penetration) properties are not as good as parylene N. See Parylene Properties
Parylene N, although less frequently employed as barrier chemistry than parylene C, has certain properties that favor its use in several applications, particularly as a coating for select medical products and elastomers. Compared to parylene C, parylene N has superior conformality and crevice penetration properties. In addition, parylene N has a high dielectric strength and a dielectric constant value that is relatively independent of frequency, a property useful for RF applications. See Parylene Properties.
Parylene D, or poly-dichloro-p-xylylene, has two Cl atoms on the benzene ring, giving the polymer a higher thermal stability than parylene C. Parylene D films have excellent chemical resistance and barrier properties. See Parylene Properties.
The aliphatic fluorinated versions of parylene films have the highest thermal stability (>300 °C) of all the parylene variants. In addition to the thermal stability, parylene AF4 is most resistant to UV damage and is frequently used in outdoor applications. Low coefficient of friction and low dielectric constant value are attractive properties for several applications. One of the main disadvantages of the fluorinated parylene AF4 films is the high cost of the raw material.
Parylene VT-4, sometimes referred to Parylene-F, is the aromatically fluorinated version of parylene film. This variant of the parylene family also has a relatively high thermal stability and a low dielectric constant. The UV and oxidative stability of this variant is not as good as the parylene AF4 film. However, the relatively lower cost of the dimer material, as compared to the AF4 variant, makes this an attractive choice for many applications. See Parylene Properties.
As part of our ongoing R&D efforts, GAT is working on a new class of vapor deposited films that have improved film properties compared to several parylene variants. Initial tests have shown promising results. See Parylene Properties.
Although there are many features and advantages of parylene, those that are most notable are as follow: