Anodic dissolution characteristics and electrochemical migration (ECM) behavior of Sn-3.0Ag-0.5Cu solder in NaCl and Na 2 SO 4 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 Na 2 SO 4 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 Na 2 SO 4 solution (-367 mV, SCE). The passivity film (SnO 2 ) formed on Sn-3.0Ag-0.5Cu solder during WDTs in NaCl solution was thicker than that formed in Na 2 SO 4 solution. Therefore, the longer ECM lifetime of Sn-3.0Ag-0.5Cu solder in NaCl solution than in Na 2 SO 4 solution can be attributed to the higher pitting potential in the NaCl solution, which is ascribed to the formation of a thicker passivity film (SnO 2 ) 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 ratedetermining step of ECM of Sn-3.0Ag-0.5Cu solder.
The existence of an incubation stage before edge drift occurs was found by examining the electromigration characteristics of eutectic SnPb solder in an edge drift structure using in situ scanning electron microscopy and the interruptive test method. During this incubation stage, the depletion of Pb was observed at the cathode end. From the change in resistance, the activation energies for the incubation and edge movement stages were calculated to be 0.88 and 1.02eV, respectively. Based on a comparison of the activation energies for each stage with the previously reported values, it is believed that, during the incubation stage, Pb migrated before Sn and that the edge movement resulted from the migration of Sn. These results suggest that Pb depletion is a prerequisite for electromigration-induced void nucleation in eutectic SnPb solder.
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