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Electromigration of SnAg3.8Cu0.7 solder was investigated in flip chip solder bump. An under-bump metallization (UBM) of Cr/Cr-Cu/Cu tri-layer was deposited on the chip side and electroless Cu/Ni/Au pad was deposited on the BT board side. Electromigration damage was observed under the current density of 2×104 A/cm2 at 100°C. Voids were found at cathode side and crack was observed at solder/thin film UBM interface after current stressing, and the bump failed after 168-hour stressing. Copper atoms were found to move in the direction of electron flow. Intermetallic compounds of Cu-Sn and Ni-Cu-Sn were also observed to spread into the solder bump due to current stressing.