The electromigration behavior of SnAg solder bumps with and without Cu column under-bumpmetallizations (UBMs) has been investigated under a current density of 2.16 Â 10 4 A/cm 2 at 150 C. Different failure modes were observed for the two types of samples. In those without Cu column UBMs, when SnAg solder bumps that had implemented 2 lm Ni UBMs were current stressed at 2.16 Â 10 4 A/cm 2 , open failure occurred in the bump that had an electron flow direction from the chip side to the substrate side. However, in those with Cu column UBMs, cracks formed along the interface of Cu 6 Sn 5 intermetallic compounds and the solder on the substrate side in the Sn-3.0Ag-0.5Cu solder bump that had an electron flow direction from the substrate side to the chip side. A three-dimensional simulation of the current density distribution was performed in order to obtain a better understanding of the current crowding behavior in solder bumps. The current crowding effect was found to account for the void formation on both the chip and the substrate side for the two kinds of solder bumps. One more important finding, as confirmed by infrared microscopy, is that the alleviation of current crowding by Cu column UBMs also helped decrease the Joule heating effect in solder bumps during current stressing. Therefore, the measured failure time for the solder joints with Cu column UBMs appears to be much longer than that of the ones with the 2 lm Ni UBMs. V

Temperature-dependent electromigration failure was investigated in solder joints with Cu metallization at 126 C, 136 C, 158 C, 172 C, and 185 C. At 126 C and 136 C, voids formed at the interface of Cu 6 Sn 5 intermetallic compounds and the solder layer. However, at temperature 158 C and above, extensive Cu dissolution and thickening of Cu 6 Sn 5 occurred, and few voids were observed. We proposed a model considering the flux divergency at the interface. At temperatures below 131 C, the electromigration flux leaving the interface is larger than the incoming flux. Yet, the incoming Cu electromigration flux surpasses the outgoing flux at temperatures above 131 C. This model successfully explains the experimental results. V

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