Relieving Hot-Spot Temperature and Current Crowding Effects During Electromigration in Solder Bumps by Using Cu Columns

Solder joints with Cu columns appear to be one of the best structures to resist electromigration. Three-dimensional thermoelectrical analysis was employed to simulate the current density and temperature distributions for eutectic SnPb solder bumps with 0.5, 5, 25, 50, and 100 μm Cu under bump metallization (UBM). It was found that the hot spots and current crowding effects in the solder were reduced significantly when the Cu thickness was over 50 μm, whereas the overall Joule heating effect remained almost unchanged. The mechanism by which the Cu column is effective in relieving the hot spot and current crowding effects is to keep the solder away from the heat source and crowding region. Simulated at a current of 0.6 A and 70°C, the estimated mean time to failure of the joints with a 50-μm-thick Cu column was 6.7 times longer than that of joints with a 0.5-μm-thick Cu UBM.