Corrosion failure analysis of printed circuit boards exposed to H2S-containing humid environments

The corrosion failure of a printed circuit board (PCB) with electroless nickel/immersion gold (ENIG) surface finish in a hydrogen sulfide-containing humid environment was analyzed in this work. To establish a comprehensive mechanism for the damage, the exposed surfaces were characterized by visual inspection, scanning electron microscopy/energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. It was realized that merely copper traces under the edge of soldermasks (on electrical junctions) suffer a galvanic-type corrosion reaction with hydrogen sulfide and moisture adsorbed, forming dominantly copper sulfides and a small amount of copper sulfate and oxide. The creep of the corrosion products on the surfaces of ENIG-plated layers, tin-based solders and adjacent soldermasked areas was also found to be responsible for creating short circuits on the outer layers of the miniaturized PCB.