The morphological evolution of Sn-9wt.%Zn solder under electromigration at a current density of about 105 A/cm2 was examined. Sn extrusion was observed, suggesting that Sn is the dominant moving species under electromigration. In contrast, Zn appeared to be immobile. It was also found that the microstructure of the solder had a significant effect on the electromigration behavior. For the solder with fine Zn precipitates, the surface morphology of the solder was almost unchanged except for the formation of Sn extrusion sites at␣the anode side after electromigration. However, for the solder with coarse Zn precipitates, more Sn extrusion sites were observed, and they were located not only at the anode side but also within the solder. Coarse Zn precipitates appeared to block Sn migration, thus Sn migration was intercepted in front of the Zn precipitates. The Sn atoms accumulated there, which led to its extrusion. The blocking effect was found to depend strongly on the size and orientation of the Zn precipitates.