Military Conformal Coatings: Acrylic vs Urethane vs Parylene

Conformal Coatings for Military Electronics

Military electronics need to operate without fail under conditions of extreme duress. The role of conformal coatings — acrylic, urethane, and Parylene — as protective insulators of the printed circuit board (PCB) assemblies that guide automated military systems is well-documented and continually under review to develop enhanced performance parameters. This means ensuring the functional integrity of military systems in response to the impact of battlefield conditions, including such uncontrollable factors as:

• Incursions of chemicals, dust, rain, salt spray, soot, water or wind during operation
• Obstinate, severe vibrations affecting the device, internally or externally
• A wide range of temperature fluctuations

Acrylic Coatings
Liquid acrylic coatings are applied by brush, dip (immersion) and spray methods. They provide moisture protection, and although ineffective at higher temperatures, they retain a wide operating range of -65°C to +125°C. In addition, acrylic coatings possess:

• Excellent dielectric properties
• Humidity-resistance during component operation
• Low glass-transition temperatures
• Considerable post-application flexibility
• Repairability after treatment with mild solvents

In addition, acrylic coatings meet approval standards for UL 746C, the Standard for Polymeric Materials – Use in Electrical Equipment Evaluations, which covers performance criteria and material property considerations, for electrical, mechanical, thermal, testing and specialized military applications. Acrylics also meet IPC-610 film thickness requirements and those stipulated by IPC-CC-830 and MIL-I-46058C.

Quick-drying acrylic emits minimal heat during cure, protecting temperature-sensitive components’ integrity. It achieves maximum physical properties in minutes, making it a good coating choice for military applications requiring fast production turn-around. Acrylics work best as secondary protection material for military uses, minimizing component condensation during operation, while offering relative ease for repair/rework.

Urethane Conformal Coatings
Liquid urethane conformal coatings are applied similarly to acrylics. Very hard and resistant to mechanical wear, they have some useful military applications. However, products with outgassing oil-modified or alkyd chemistries can disrupt the coatings’ long-term performance. With an operative temperature range similar to acrylic, urethanes offer good humidity protection but are less optimal for condensation. Prone to cracking during prolonged thermal exposure, military coating applications are challenged by high-vibration/high-heat environments.

Urethane films are very resistant to chemical solvents, second only to Parylene conformal coatings in this regard. Applications requiring prolonged exposure to harsh chemical solvents benefit from urethane coating. It also displays dependable dielectric properties over extended durations. Unfortunately, urethane’s high solvent resistance makes it difficult to remove/rework.

One area where urethane excels is tin whisker mitigation, lessening the impact of the electrically conductive, crystalline structures within components. These can grow from electroplated tin surfaces used as final finish. Typically 1-2 millimeters (mm) in length, tin whiskers bridge closely-spaced circuit elements maintained at different electrical potentials. Their presence can cause short circuits and electronic system failures, as well as metal vapor arc and debris/contamination within an assembly. For military systems, these represent potential fail mechanisms.

Unfortunately, tin whisker growth cannot be entirely eliminated. However, an 11-year NASA study showed Arathane 5750 (a urethane resin) applied at 2mm thickness provides a viable tin whisker mitigation strategy, strong enough to prevent them from penetrating the coating and generating performance issues.

Parylene Coatings for Military Electronics

Dielectric and non-conductive, Parylene conformal coatings safeguard electronics from contaminants, corrosion, dust, fungus, moisture, salt spray, and temperature extremes. In comparison to liquid acrylic and urethane, Parylene’s specialized chemical vapor deposition (CVD) application method deposits gaseous parylene deep within targeted surfaces on a molecule-by-molecule basis. This insulating, dielectric film is exceptionally durable, yet flexibly ultra-thin, uniform and pinhole free. Completely conformal, it doesn’t decompose at upper-range temperatures or become brittle like liquid acrylic or urethane can under severe, frigid temperatures. Parylene coating remains adherent and intact, preserving the dielectric and insulation properties essential to military component performance.

Parylene coatings are RoHS compliant, meet IPC-CC-830 requirements and itemized for the MIL-I-46058 Defense Supply Center Qualified Parts List (QPL).

In addition, the MIL-STD-810F specification applies to Parylene’s use for commercial off-the-shelf (COTS) assemblies and product ruggedization, both used increasingly for military devices. It includes parameters for assuring a device’s ability to function under low pressure/high altitude situations, through fluctuating temperatures, in rain or humidity, salt, fog or fungus, as well as conditions of shock, vibration and acceleration.

Neither acrylic nor urethane display the versatility of Parylene conformal coatings but are useful for specified military electronics purposes. In comparison to liquid coatings, Parylene conformal films are recommended for military electronics where dedicated, reliable environmental protection is essential to maintaining functionality under punishing performance conditions. They represent the optimal, primary-service conformal choice for military electronics.