The IMC formation and progress in the copper pillar Cu/Sn1.8Ag /OSP-Cu microbump structure upon current stressing

The present study investigated the evolution of intermetallic compounds on a microbump structure upon electric current stressing. The incorporation of Cu pillar gives rise to the microbump structure of TiCuCu/Sn1.8Ag/OSP (organic solderability preservative)-Cu. The current stressing was conducted at a current density of 1.0 × 10⁴ A/cm² under 125°C for various durations of 24hr, 100hr, 200hr, 300hr, and tested to fail(566hr). In order to have a good understanding to the progress and mechanism of IMCs growth, the bumps were examined in perpendicular directions. The growth of intermetallic compounds within the microbump was found to segregate on certain corner in the beginning of current stressing. The intermetallic compounds grew from the segregated area as induced by electric current flow. The intermetallic compounds grew to almost completely occupy the entire solder bump in most of the joints investigated. Cu₃Sn grew to occupy most of the solder volume of the microbump joint in the failed specimen, while the Cu₆Sn₅ and Ag₃Sn IMCs gathered at the center of solder joint. Cracks penetrated through the central region of the joint between the upper and lower Cu₃Sn IMC. The cross-section inspection revealed that the microbump structure failed through the voids and crack formation along the central region of the joints. The findings indicate the marvelous difference in intermetallic compounds formation behavior between flip solder bump and microbump as a result of the differences in current density and solder/UBM structure.