Effect of electromigration temperature on dominant migration and hillock phases of eutectic SnPb alloys

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 technique. The test temperature and the current density was either 100 or 50 degC and 6times10⁴ or 8times10⁴ A/cm², 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 100degC. 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