Parylene

Parylene refers to a family of polymer coatings based on poly-para-xylylene that are applied as conformal coatings to protect complex-shaped devices. The coatings are deposited by a chemical vapor deposition process carried out at ambient or near-ambient temperatures, producing uniform, pinhole-free films that conform to intricate geometries.

The coating process uses a di-para-xylylene dimer which is heated to sublimate, then pyrolyzed to generate reactive monomeric species. These species polymerize upon contact with the substrate, forming a continuous, adherent film. Common grades include parylene N, C, D and AF-4, each offering different properties. Parylene N is a basic form; C introduces chlorine to improve chemical resistance; D provides further barrier enhancement; AF-4 is fluorinated for additional chemical resistance and stability.

Key properties include excellent conformality on complex surfaces, a strong barrier to moisture and many chemicals, good dielectric performance, and biocompatibility. The coatings are chemically inert and can be applied over a wide range of substrates. Thickness is highly controllable, from tens of nanometers to tens of micrometers, with deposition completed at room temperature, minimizing thermal stress on sensitive parts.

Applications span electronics and MEMS packaging, medical implants and devices, pharmaceutical and sanitary equipment, optical components, and corrosion protection for sensitive substrates. Processing requires specialized vacuum equipment and surface preparation to optimize adhesion. Limitations include the need for controlled deposition conditions, substrate compatibility and adhesion considerations, and cost relative to conventional coatings. Overall, parylene coatings offer durable, uniform protection with minimal impact on device geometry.