Formation Mechanism of Porous Cu3Sn Intermetallic Compounds by High Current Stressing at High Temperatures in Low-Bump-Height Solder Joints

Electromigration tests of SnAg solder bump samples with 15 m bump height and Cu under-bump-metallization (UBM) were performed. The test conditions were 1.45 x 10(4) A/cm(2) at 185 degrees C and 1.20 x 10(4) A/cm(2) at 0 degrees C. 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 degrees C will take more than 1000 h for porous Cu3Sn to form, and it will not form at 170 degrees 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 Cu3Sn when stressing with high current density and high temperature. Polarity effects on porous Cu3Sn formation is discussed.