Effectiveness of Conformal Coat to Prevent Corrosion of Nickel-palladium-gold-finished Terminals

Nickel-palladium-gold-finished terminals are susceptible to creep corrosion. Excessive creep corrosion can result in device failure due to insulation resistance loss between adjacent terminals. The mixed flowing gas test has been demonstrated to produce creep corrosion on parts with nickel-palladium-gold finished terminals. Conformal coats are often used to protect printed wiring assemblies from failure due to moisture and corrosion. However, coating may not be sufficient to protect lead terminations from failure. In this study, acrylic, silicone, urethane, parylene, and atomic layer deposit (ALD) coatings were examined for their effectiveness at preventing corrosion of nickel-palladium-gold-finished terminals. The coverage of each coating was examined, and assemblies were subjected to eight hours of mixed flowing gas as well as temperature cycling. Non-uniform coating thickness was observed in the areas of the terminals. On some areas, little to no coating material was found for the acrylic, silicone, and urethane coatings. Parylene, which had the most uniform coating, was found to provide the best resistance to corrosion, while corrosion products were observed on the terminals of inspected parts protected by the other coatings.