Available in five basic material types, conformal coatings can be readily adapted as protective, insulating films for electronics. However, there can be some confusion about which type is best-suited for a specific use. Clearly defining the performance parameters for the component[s] to-be-coated helps coordinate the conformal film material with a unit’s functional requirements. Accurate assessment of environmental conditions like anticipated levels of corrosion, contact with foreign particulates, expected concentrations of moisture/salt spray, temperature fluctuations and vibrational range determine which coating type is best-suited to your electronics’ applications. Without appropriate protection, printed circuit boards (PCBs) and similar electronics will not survive harsh environments, and malfunction.
Basic conformal coating materials include four liquid types -- acrylic, epoxy, silicone and urethane, and a fifth, vapor-phase polymerization parylene. Liquid coatings are applied to substrate surfaces by wet brush, immersion (dipping the component in a tank of liquid film material) or spray methods, which are relatively simple and inexpensive to enact; used correctly, each offers generally dependable external coating protection for electronic devices.
Slower to apply and more expensive to use, parylene’s unique chemical vapor deposition (CVD) process generates superior coating protection in most cases. Through CVD, solid parylene dimer is converted into a gas that penetrates deep within the surface of electronic assemblies, adding protection deeper than just a surface treatment.
To be successful, conformal coating assignments match the practical performance benefits of each coating material and its application method with the assembly’s function and operational environment.
Choosing the Right Conformal Coating
Each coating material functions optimally under specific conditions, which guide their use for electronics. Among liquid coatings:
Acrylic: Typically applied at thickness between 0.002 - 0.005 inches, fungus-resistant acrylic is easily applied and dries in as little as 30 minutes at room temperature, making it a good choice if the objective is rapid production. However, the fact that it cannot withstand temperatures above 125° C negates acrylic’s uses for electronics expected to perform in higher-heat environments.
Epoxy: In comparison to other liquid film materials, epoxy offers exceptional coating strength, dependably resisting abrasion, chemical incursion, humidity and vibration. Unfortunately, this same long-lasting surface durability also makes epoxy coatings very difficult to rework and repair. Epoxy can shrink during polymerization. These two factors make exceptionally careful film application a necessity. Temperature extremes diminish its stress resistance, further limiting epoxy’s use for electronics.
Silicone: In contrast to both acrylic and epoxy, silicone conformal coatings maintain high-level performance in upper-temperature environments of 200°+ C, making them a good choice for automotive electronics. Silicone responds well to thicker-layer application, reliably diminishing the impact of operational vibration on the functioning of electronic components. While corrosion and humidity resistance are good, silicone has lower resistant to abrasion and solvents than other conformal films, limiting electronics uses.
Urethane: Providing dependable dielectric functioning for extended periods, urethane supports MEMS/nano miniaturization through enhanced insulation of electronic signal traces from circuits situated in close-proximity on a PCB. Urethane also withstands chemical solvents, delivers humidity resistance, and mitigates development of tin whiskers on components. However, the benefits of solvent-resistance are somewhat minimized by difficulty reworking/removing the coatings, which also suffer from limited functionality under high-heat/vibration operating conditions.
In contrast, CVD-derived, non-liquid parylene generates a more comprehensive level of quality protection for electronics. Delivering consistent and continuous uniform, pinhole-free conformal films, its CVD application process facilitates gaseous parylene’s permeation within most substrate materials: ceramic, ferrite, glass, metal, paper, plastics and resin. The same quality allows exceptional coating adaptation to any shape on the unit’s surface without the pooling or dripping typical of liquid coating materials; corners, crevices, edges, even internal spaces are protected by parylene, which hardens to a durable, micro-thin film. Chemical inertness, a low dielectric constant and high dielectric strength enhance its utility for a wide range of electronic devices. Adaptable for MEMS/nano technology, parylene protects circuits from electrical interference while allowing the design of smaller, more compact boards. However, parylene can be expensive and difficult to rework; batch-sizes are small compared to other conformal coatings.
All conformal coatings offer some level of security for electronics, although specific protection depends on the film material used, its method of application, the function of the electronic device and its operational environment. In general, parylene provides conformal protection for a wider range of electronics’ purposes, but determining which coating material is best for your coating assignment depends largely on the conditions cited here.
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