Formation mechanism of porous Cu3Sn intermetallic compounds by high current stressing at high temperatures in low-bump-height solder joints

Abstract

Electromigration tests of SnAg solder bump samples with 15 μm bump height and Cu under-bump-metallization (UBM) were performed. The test conditions were 1.45 × 104 A/cm2 at 185 ºC and 1.20 × 104 A/cm2 at 0 ºC. A porous Cu3Sn 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 ºC will take more than 1000 h for porous Cu3Sn to form, and it will not form at 170 ºC 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 Cu3Sn when stressing with high current density and high temperature. Polarity effects on porous Cu3Sn formation is discussed.