The electromigration and thermomigration behaviors of Pb-fiee flip chip Sn-3Ag-0.5Cu solder bumps, with UBM (under bump metals) layers Al (0.3/m)/Ni(V) (0.3 μ m)/Cu (0.5 μ m) layers, upon current stressing at 120°C were investigated. The current density applied was 1.18×104A/cm 2 for all investigations. To investigate the change of morphology and composition in the bulk of solder bumps and the IMC layers at the interface between the UBM and solder, the solder bumps were examined at specified period of current stressing using SEM (Scanning Electron microscope), EDX (Energy Dispersive X-ray Spectroscopy) and WDS (Wave Dispersive X-ray Spectroscopy). There exists a thermal gradient between cathode and anode. The synergism effect of electromigration and thermomigration accelerated the growth and propagation of voids at the cathode. Voids formed on the Al trace/ solder interface regardless of the direction of electron flow. Voids also formed at the interface even though the current flowed along the Al trace. Thermomigration overwhelms electromigration and results in mass diffusion when counter flow of electron and thermal gradient exists. The evidence of mass transfer was further confirmed by the mass analysis throughout the bump and the hillock appeared at the lower portion of the bump. The formation of intermetallic compound at the substrate side (metallized with Cu/Ni-P/Au) was suppressed when the electron flow and thermal gradient were in the opposite direction. The solder mass was forced to migrate to the substrate when the current flowed toward the substrate or along the Al trace. In order to illustrate the importance of heat dissipation, a set of joints were stressed with current at an environmental temperature of -5°C. The comparison between the as produced bump and current stressed bump indicated that no visible defect exists either at the cathode or at the anode even after 600 hours. These observations further emphasized the significance of heat dissipation in reducing the migration defect.