Conformal Coatings for Electronics

Withstanding operating environment challenges such as corrosion, fungus, oxidation, rain, salt water/mist, snow, temperature fluctuations or vibration is essential to the long-term performance of electronic devices. Without suitable protection, printed circuit boards (PCBs) and similar electronics will malfunction as they are not able to survive harsh environments exposure. Below are operating environments that electronics commonly endure:

• Aeronautic and automotive vehicles require PCBs to function as specified for years through an exceptional range of operating conditions
• Defense systems that power communications/surveillance equipment and high-tech weaponry in high-stress, life-threatening performance situations
• Personal devices – like handheld smartphones and tablets – required for personal and commercial communication
• Wearable devices and implants – fitness and medical trackers – which require circuitry that won’t fail during intense exercise sessions or when subjected to corrosive fluids within the body

Conformal coatings provide a reliably secure, non-conductive, dielectric covering. These coatings are applied to protect PCBs from damage that can result from contamination, dirt/dust, exposure to corrosive chemicals, fungus, moisture, salt spray or other potential deterrents to component functionality.

Protecting Electronics with Conformal Coatings

There are five basic types of conformal coating. Four of the coating types are applied as a liquid: acrylic, epoxy, silicone and urethane. Liquid coatings are applied to substrate surfaces by wet brush, dip or spray methods and are relatively simple and inexpensive to enact. Liquid coatings provide generally reliable exterior film protection for PCBs.

Parylene, the fifth type of conformal coating, is applied by a unique chemical vapor deposition (CVD) process that transforms solid dimer into a gas that penetrates deep within the surface of electronic assemblies. Protecting PCBs with Parylene is frequently more consistent than with liquid coatings, though the coating time may be longer.  

Conformal Coating Selection

Clearly defining the performance requirements of the PCB to be coated is key. Conformal film material selection should be based on factors such as operational moisture levels, temperature range and anticipated vibration. It is essential that the practical performance benefits of each coating material be considered to determine which type is most suitable for the assembly and its end use:

• Easily applied acrylic resin (AR) conformal coatings dry in minutes at room temperature. They are fungus resistant and have beneficial electrical and physical properties. Typically applied at a thickness between 0.002 – 0.005 inches, they withstand temperatures of no more than 125°C, limiting their use for many higher-heat applications. They remain a popular coating choice because many AR variations cure in 30 minutes, enhancing their value when a short turnaround time is the objective.
• Usually available as two-component compounds, epoxy resin (ER) systems deliver an extremely durable coating with good resistance to damage from chemicals, high-level abrasion and humidity. However, ER can shrink during polymerization, and temperature extremes diminish its stress resistance. The same rugged, long-lasting properties that distinguish epoxy’s performance also makes them difficult to rework and repair, so application processes need to be completed carefully.
• Silicone resin (SR) coatings are a popular choice for automotive applications because they maintain high-level performance in temperature environments exceeding 200°C. They also provide good corrosion and humidity resistance. Because they can be applied in thicker layers than other coatings, SR is exceptional at dampening vibrations during operation. Unfortunately, silicone provides lower resistant to abrasion and solvents than other conformal films.
• Delivering reliable chemical and humidity resistance, urethane resin (UR) coatings also provide excellent dielectric properties for long periods of time as well as better insulating signal traces from circuits placed in near proximity, a property that supports PCB miniaturization. Withstanding chemical solvents, UR also mitigates tin whisker growth. However, its solvent resistance makes the coating difficult to remove or rework. Long-term functionality in high-heat/vibration environments is limited.
• The unique CVD-application process of Parylene coatings (XY) delivers an exceptionally uniform, pinhole-free conformal film for PCBs. Parylene can be applied to virtually any surface and objects of any shape, including glass, metal, paper, resin, plastics, ceramics, ferrite and silicon. The coating’s chemical inertness enhances its utility for implanted biomedical devices. Parylene typically has a higher cost and can be tough to rework; the coating is also done in a batch process.

Most of these conformal coatings will provide some degree of protection for electronics, but the coating’s benefits will vary according to material, its application method and the PCB’s end use and operational environment. Understanding the basic properties of each coating type is critical when selecting which film material is best for an electronic device.