Reliability assessment of immersion silver finished circuit board assemblies using clay tests

This paper discusses issues regarding the reliability of immersion silver (ImAg) surface finish on printed circuit boards (PCBs) in high-sulfur environments with a focus on creep corrosion. The test approach used clay with sulfur to drive corrosion. It was found that silver sulfide had formed on the ImAg surfaces, but dendrite-shaped creep corrosion products were formed on the edges of the ImAg finished copper traces by galvanic corrosion. The creep distance and the content of CuS and Ag₂S in the corrosion products were dependent on the relative location of the pad edges and the solder mask, which caused different extents of coverage of ImAg finish on the copper traces. A Weibull distribution with two parameters was used to analyze the length of the creep corrosion products, the values of which were used along with corrosion probability to assess the corrosion resistance of ImAg finished PCBs.