Parylene (XY)

Co-Authors and contributors

Dr Lee Hitchens, Nexus

Parylene (XY) is the trade name for a variety of chemical vapour deposited poly(p-xylylene) polymers used as moisture and dielectric barriers.

Parylene can be further divided into typically four chemical types. These are Parylene C, D, F and N.

They are vacuum deposited coatings. That is they are not like the liquid coatings.

They are applied in a vacuum chamber and deposited as a gas onto the surface of the circuit board.

Hence, this is a very different process application technique to the liquid conformal coatings.

Parylene Properties

The material properties of the XY coating are very different to the liquid coatings.

Parylene coatings are completely conformal to the surface of the Printed Circuit Board (PCB) or product. This means that the coating has a uniform thickness and is pinhole free. As a result, component configurations with sharp edges, points, flat surfaces, crevices or exposed internal surfaces are coated uniformly without voids.
Parylene coating provides an excellent barrier that exhibits a very low permeability to moisture and gases. This means that products coating in parylene generally are more “waterproof” than the same products coated in a liquid conformal coating.
Parylene is unaffected by solvents, has low bulk permeability and is hydrophobic. Coatings easily pass a 100hr salt-spray test.
Parylene has excellent electrical properties. These include low dielectric constant and loss with good high-frequency properties, good dielectric strength, and high bulk and surface resistance
Parylene is chemically and biologically inert and stable and make excellent barrier material.
Parylene has good thermal endurance: Parylene C performs in air without significant loss of physical properties for 10 years at 80°C and in the absence of oxygen to temperatures in excess of 200°C.
There are a few key points to consider with Parylene since it performs as a superior product to most other coatings.

The parylene process is a batch process. This means it does not lend itself easily to low cost high volume processing.
Parylene require masking of components that are not to be coated just like other liquid coatings. However, since this is a vapour being applied, the masking must be 100% effective or the coating will penetrate into the wrong areas.
There is no pigment or UV trace in a Parylene coating. This means inspection of the coating can be more challenging.
Rework of Parylene is normally achieved with abrasion techniques since it is chemically resistant.
Further information can be found on Parylene by clicking Alternative Coatings: Parylene