The effects of annealing, electromigration, and thermomigration on volume shrinkage and voiding mechanisms of Cu/Ni/Sn-2.5Ag microbumps are systematically investigated by using in-situ scanning electron microscopy under current stressing of 1.5×105 A/cm² at 150 °C. The resistance increases rapidly in the initial stage due to formation of intermetallic compounds (IMC)s followed by a gradual increase in resistance. Growth of Ni₃Sn₄ IMCs is controlled by a diffusion-dominant mechanism, and voids and volume shrinkage are closely related to IMC phase transformation of (Au, Ni)Sn₄ to Ni₃Sn₄ in microbumps.