Interfacial Reaction Effect on Electrical Reliability of Cu Pillar/Sn Bumps

Thermal annealing and electromigration (EM) tests were performed with Cu pillar/Sn bumps to understand the growth mechanism of intermetallic compounds (IMCs). Annealing tests were carried out at both 100°C and 150°C. At 150°C, EM tests were performed using a current density of 3.5 × 10⁴ A/cm². The electrical failure mechanism of the Cu pillar/Sn bumps was also investigated. Cu₃Sn formed and grew at the Cu pillar/Cu₆Sn₅ interface with increasing annealing and current-stressing times. The growth mechanism of the total (Cu₆Sn₅ + Cu₃Sn) IMC changed when the Sn phase in the Cu pillar/Sn bump was exhausted. The time required for complete consumption of the Sn phase was shorter during the EM test than in the annealing test. Both IMC growth and phase transition from Cu₆Sn₅ to Cu₃Sn had little impact on the electrical resistance of the whole interconnect system during current stressing. Electrical open failure in the Al interconnect near the chip-side Cu pillar edge implies that the Cu pillar/Sn bump has excellent electrical reliability compared with the conventional solder bump.