Military Conformal Coatings
Posted by Sean Horn
Friday, November 24, 2017 9:00
@ 9:00 AM
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,
· unexpected fluctuations in site temperature, and
· similar working circumstances that generate wear, stress, and abuse.
Liquid acrylic coatings are applied by brush, dip (immersion), and spray methods. They provide moisture protection, conformally-coating PCBs. Although ineffective at higher temperatures, they retain a wide operating range of -65° – + 125° C. In addition, acrylic coatings possess
· excellent dielectric properties,
· humidity-resistance during component operation, minimizing internal moisture-development,
· low glass-transition temperatures,
· considerable post-application flexibility,
· easy repair 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, Qualification and Performance of Electrical Insulating Compound for Printed Wiring Assemblies, and
- MIL-I-46058C, Insulating Compound (For Coating Printed Circuit Assemblies.
Quick-drying acrylic emits minimal heat during cure, protecting temperature-sensitive components’ integrity; it achieves maximal 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.
Liquid urethane conformal coatings are applied similarly to acrylic. 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, urethane offer good humidity protection but less 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. Military 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 a component; 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 2 mm. thickness provides a viable tin whisker mitigation strategy, strong enough to prevent them from penetrating the coating and generating performance issues.
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. Entirely 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:
· DoD Restriction of Hazardous Substances’ (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 spec 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:
among a variety of difficult operational circumstances.
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.