Using primer with conformal coating

Co-authors and contributors

  • Dr Lee Hitchens, Nexus

Primer is an undercoat that is a preparatory coating put on circuits before conformal coating application.

Priming ensures better adhesion of the conformal coating to the surface. The primer is designed to adhere to surfaces and to form a binding layer that is better prepared to receive the conformal coating.

This is especially critical and useful where the conformal coatings do not bind easily to the substrate and problems such as delamination and de-wetting can be helped.

Using a primer with the different generic resin types

Acrylic and polyurethane conformal coatings

Most AR and UR coatings do not need priming before application of conformal coating. The wetting and adhesion of these coatings is normally sufficient to achieve a good result.

However, sometimes the substrate energy of the circuit may be very poor. This could be because of the solder resist used or the flux residues remaining due to a no clean process being use. This can lead to poor adhesion of the conformal coating to the circuit.

It is possible to use dilute version of the conformal coating (5% coating, 95% solvent) that can be used as a primer before coating application. This can lead to adhesion promotion.

Silicone conformal coatings

Many silicones have proprietary primers developed for them.

If adhesion is poor then these priming solutions can be used to enhance the process.

Parylene coatings

Parylene coatings can be notorious for adhesion. In normal production, adhesion promotion is generally required before the Parylene Process.

The priming process can be either a gas phase deposition or liquid process. A standard material used for liquid promotion is an organic silane, A-174.

Also, note that the important knowledge in Parylene processing is normally in achieving good surface adhesion promotion.