The electromigration in SnAg solder bumps with and without Cu column UBM have been investigated at 2.16×10 4 A/cm 2 at 150°C. Different failure modes were observed for the two types of samples. When the SnAg solder bump with only Ni UBM was stressed by 2.16×10 4 A/cm 2, open failure occurred in the bump that the direction of electron flow was from the chip side to the board side. However, the crack formed along the interface between the IMC Cu 6Sn 5 and the solder on the substrate side in the Sn-3.0Ag-0.5Cu solder bump with Cu column UBM. A three-dimensional simulation of the current density distribution was performed to provide a better understanding of the current crowding behavior in the solder bump. The current crowding effect was found to account for the failure in the chip/substrate side.