When the design of a new PCB or electronic assembly is finished it’s time to consider conformal coating. The conformal coating (so-called because it conforms to the surface of the assembly) protects against hostile environmental conditions like humidity or abrasion. Such coatings are very cost-effective, and even if a design is not targeted for aerospace, medical or automotive deployment, coatings can still provide a benefit because they reduce the risk of failure.
Consider these four points when deciding which of the five types of conformal coating – acrylic, epoxy, urethane, silicone and Parylene – is the best choice for the design:
- Will the product need reworking?
- Is using the least expensive coating a driving factor?
- Are the environmental conditions known?
- Will there be any issues with coating and component compatibility?
Particularly during prototyping it’s common for a board or assembly to require reworking. That usually means removing the conformal coating locally, if not over the full surface. Acrylics are notably easy to remove while epoxy is almost impossible.
Cost is always important, but should be weighed against required functionality. Parylene is not the cheapest conformal coating but provides excellent dielectric properties as well as humidity, chemical and abrasion resistance, making it the coating of choice for aerospace, medical, automotive and instrumentation applications.
Environmental factors include exposure to UV as well as humidity (including salt spray) vibration, abrasion and temperature variation. Silicone offers good UV resistance while the humidity resistance of epoxy is somewhat mediocre. Acrylics suffer from poor abrasion resistance. Not identifying environmental conditions during coating selection is a common factor in premature failure.
Compatibility is complicated as the component materials depend so much on the product and its application. In many cases opting for a Parylene coating is a good way of avoiding problems as it’s sufficiently inert for medical device application.
Selecting a conformal coating can seem minor in relation to all the design decisions that have gone before. However, it affects both durability and performance. Consideration of conformal coating requirements is necessary to prevent premature failure of the product due to an inappropriate coating selection. If in doubt discuss the options with an NPI Services, Inc. specialist.