A Comparison Between Pressure-less and Pressure-assisted Cu Sintering for Cu Pillar Flip Chip Bonding

As the most promising candidate for heterogeneous and 3D integration, Copper (Cu)-based interconnect are gaining prominence, due to superior characteristics (e.g., low resistivity, high electromigration resistance, high-temperature performance and low cost). Cu pillars allow for finer pitch, by providing higher control of bump diameter and height when compared to solder bumps. Flip chip bonding of Cu pillars can be conducted directly through thermocompression bonding or indirectly by employing solder caps. While the former imposes high temperature and pressures on the interface, the latter face reliability issues due to the formation of brittle intermetallic and thermal mismatches. Correspondingly, in this study, an alternative approach for solder-free Cu pillar bonding using Cu sinter pastes was investigated. To achieve an all-Cu interconnect between Cu pillars and Cu pads, a recently developed pressure-less Cu sinter paste was used in this study. The electrical and mechanical properties of the flip-chip bonded Cu pillars using pressure-less Cu paste were further compared with a pressure-assisted Cu paste. The long-term reliability of the bonded samples was assessed using thermal cycling tests.