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.
The real-time microstructural evolution during electromigration of eutectic SnPb solder lines with an edge drift structure was examined using an in-situ scanning electron microscope (SEM) technique. The test temperature and the current density were either 100°C or 50°C and 6 · 10 4 A/cm 2 or 8 · 10 4 A/cm 2 , respectively. In-situ microstructural observation of the depleted phases and quantitative analysis of the number of hillock phases made it clear that the dominant migrating element and dominant hillock phase were Sn and Pb at room temperature, respectively, while both dominant migrating element and dominant hillock phase were Pb at 100°C. Such temperature dependence of the dominant hillock phase in the eutectic SnPb solder can be understood by considering the atomic size factors of the metallic solid solutions. We suggest that at high temperature, it is easier for Pb atoms to be injected into the Pb phase (Pb-phase hillocks); while at low temperature, Pb-phase hillocks were squeezed by Sn, which penetrated the Pb phase.
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