Correlations between interfacial reactions and bonding strengths of Cu/Sn/Cu pillar bump

An investigation of the effects of thermal annealing on the growth of intermetallic compounds (IMCs) and the shear strength of Cu/Sn/Cu pillar bump structures is presented. The Cu₆Sn₅ phase formed at the Cu/Sn interface right after bonding and grew as with annealing time, while the Cu₃Sn phase formed and grew at the Cu/Cu₆Sn₅ interface with increased annealing time. IMC growth followed a linear relationship with the square root of the annealing time due to a diffusion-controlled mechanism. The shear strength was measured by the lap shear test and was observed to monotonically decrease with annealing until 150 h at 150 °C. It then stayed nearly constant with further annealing, which is correlated with the change in fracture modes from ductile to brittle at around the 150 h mark. This is ascribed not only to the increasing thickness of brittle IMCs but also to the decreasing thickness of Sn, as there exists a critical annealing time for a fracture mode transition in our thinly Sn-capped Cu pillar bump structures.