Contradictory electrochemical migration behavior of copper and lead

In a condition of high humidity, high temperature, and high voltage applied, metals in electronics can cause insulation failure by making conductive filaments, which is called electrochemical migration (ECM). The existing of the electrochemical migration phenomenon is well known, but there are still many open questions and contradictions as well. One of these is the different ECM susceptibility and solubility parameters of the lead which can be found in the literature [1,3]. Therefore the ECM behavior of a leaded surface finish has been compared to other surface finishes applied by the electronics industry. The studied surface finishes were: bare copper (bCu), immersion tin (iSn), immersion silver (iAg) and Sn63Pb by Hot Air Solder Leveling (HASL). Water drop (WD) tests were carried out on comb patterns designed according to IPC-B-24 test board. The results were evaluated by the calculation of the Mean Time to Failure (MTTF) and by the investigation of the element composition of the dendrites. The results have shown contradicts relating bare copper, OSP and HASL compared to the migration ranking published in [1].