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 150h at 150C. It then stayed nearly constant with further annealing, which is correlated with the change in fracture modes from ductile to brittle at around the 150h 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.
cu/sn/cu pillar bump intermetallic compound shear strength fracture mode