Electromigration tests of SnAg solder bump samples with 15 µm bump height and Cu under-bump-metallization (UBM) were performed. The test conditions were 1.45ˆ10 4 A/cm 2 at 185˝C and 1.20ˆ10 4 A/cm 2 at 0˝C. A porous Cu 3 Sn intermetallic compound (IMC) structure was observed to form within the bumps after several hundred hours of current stressing. In direct comparison, annealing alone at 185˝C will take more than 1000 h for porous Cu 3 Sn to form, and it will not form at 170˝C even after 2000 h. Here we propose a mechanism to explain the formation of this porous structure assisted by electromigration. The results show that the SnAg bump with low bump height will become porous-type Cu 3 Sn when stressing with high current density and high temperature. Polarity effects on porous Cu 3 Sn formation is discussed.

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