Failure Mechanism for Fine Pitch Microbump in Cu/Sn/Cu System During Current Stressing

Current-induced failures in fine pitch Sn microbump with Cu pillar have been investigated under a current density of 3.2 × 10⁴ A/cm² and temperature of 150 °C. This process takes place in 2000 h of electromigration test. From the focused ion beam image and energy dispersive X-ray analysis, we observed the intermetallic compound formation, Kirkendall effect, and crack contributed to this failure. There are two stages of failure process for Cu pillar with microbump during current stressing. In the first stage, the whole Sn solder was transformed into intermetallic compound and Kirkendall voids were formed at the interface between the Cu pillar and Cu₃Sn intermetallic compound. In the second stage, the Kirkendall voids coalesced into larger porosities then formed continual crack by current stressing, leading more bump resistance increase.