Different failure modes of the electromigration behavior of SnAg3.5 solder bumps were investigated under different current densities. Damage occurred in both the anode/chip side and cathode/chip side under the stressing condition of 1 × 104 A/cm2. However, under the stressing of 5 × 103 A/cm2, failure happened only in the cathode/chip side. To understand the current-crowding behavior in the solder bump, a three-dimensional simulation of the current-density distribution was performed. The temperature increase and the thermal gradients were also measured for both the current densities. The failure in the cathode/chip side was found due to the current-crowding effect.