Conformal coatings are applied to printed circuit boards (PCBs) to protect them from environmental stress such as salt, corrosion, humidity, and moisture, mitigate tin whiskers, and provide a barrier to electrically insulate components. They enable design of smaller electrical assemblies, while increasing mechanical support for components, and improving the fatigue life of solder joints.
A wide variety of conformal coating types are available, including silicones, acrylics, polyurethanes, epoxies, poly-para-xylylenes, and light-cure chemistries. It can be difficult to know which coating is right for the required application, but here’s a summary of their pros and cons to get you started:
* Acrylic conformal coatings are liquid conformal coatings that often require use of solvents. Acrylics are easy to rework but have poor chemical resistance. They’re typically used on printed circuit boards for moisture protection. However, in applications with possible exposure to solvents, acrylic conformal coatings do not provide the best protection.
* Polyurethane conformal coatings come as single-component, two-component, UV-curable, and water-borne systems. Although polyurethanes provide good chemical and moisture resistance, they are often hard to rework and have low tolerance in humid environments.
* Silicones are often used for very high- or low-temperature environments. They provide high humidity and corrosion resistance along with good thermal endurance but require thermal curing and have short pot lives. Silicone coatings are also prone to abrasion (low cohesive strength) and have high coefficients of thermal expansion.
* Epoxy-based coatings are usually two-part systems with limited pot life. Similar to polyurethanes, they provide good moisture and chemical resistance. A major downfall is that they are almost impossible to remove chemically for rework.
* Poly-para-xylylenes are applied at very high temperatures with a vacuum-coating process. Therefore, they cost significantly more compared to other technologies.
* Urethane conformal coatings are widely used for printed circuit boards since they provide protection against moisture as well as exposure to solvents. They are more durable and rigid than other types of coatings. Although urethanes have a high tolerance against solvents, removal of the coating to rework is possible. To read about possible ways of removing conformal coatings, check out this blog post.