Abstract
Anodic dissolution characteristics and electrochemical migration (ECM) behavior of Sn-3.0Ag-0.5Cu solder in NaCl and Na2SO4 solutions were investigated using anodic polarization tests and water drop tests (WDT). The ECM lifetime of Sn-3.0Ag-0.5Cu solder in NaCl solution (42.4 s) was longer than that in Na2SO4 solution (34.8 s). The pitting potential of Sn-3.0Ag-0.5Cu solder in NaCl solution (135 mV, SCE) was higher than that in Na2SO4 solution (−367 mV, SCE). The passivity film (SnO2) formed on Sn-3.0Ag-0.5Cu solder during WDTs in NaCl solution was thicker than that formed in Na2SO4 solution. Therefore, the longer ECM lifetime of Sn-3.0Ag-0.5Cu solder in NaCl solution than in Na2SO4 solution can be attributed to the higher pitting potential in the NaCl solution, which is ascribed to the formation of a thicker passivity film (SnO2) in the former. It was confirmed that microelements such as Ag and Cu do not take part in ECM because they form chemically stable intermetallic compounds with Sn. We believe that Sn is the only element that contributes to ECM, and dissolution of Sn at the anode is possibly the rate-determining step of ECM of Sn-3.0Ag-0.5Cu solder.
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