Authored By:
Michael Osterman
Center for Advanced Life Cycle Engineering, University of Maryland
College Park, MD
Summary
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.
Conclusions
Nickel-palladium-gold-finished copper terminations are susceptible to corrosion when subjected to sulfur and chlorine gases. With the exception of parylene, the common conformal coating materials and the atomic layer deposition coating do not prevent corrosion of the terminal metallization in corrosive environments. Corrosion products were observed at regions of thinned coating thickness, in particular on the edges of the terminals.
Temperature cycling may play a role in the degradation of coatings, particularly ALD, and increasing their susceptibility to corrosion. Product manufacturers with corrosive environment considerations should consider using parylene. Uniform thickness or greater thicknesses of coatings along the edges of the terminals may result in better performance for the AR, UR, and SR coatings. Manufacturers should focus on terminal coverage and not rely solely on coating thickness on a flat surface when qualifying a coating process. For this study, ALD is not recommended for mitigation of corrosion-induced failure.
Initially Published in the IPC Proceedings
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