Effect of Intermetallic Compounds Growth Characteristics on the Shear Strength of Cu pillar/Sn-3.5Ag Mcrobump for a 3-D Stacked IC Package

摘要

The effect of thermal annealing on the in-situ growth characteristics of intermetallics (IMCs) and the mechanical strength of Cu pillar/Sn-3.5Ag microbumps are systematically investigated. The Cu_6Sn_5 phase formed at the Cu/solder interface right after bonding and grew with increased annealing time, while the Cu_3Sn phase formed at the Cu/Cu_6Sn_5 interface and grew 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 measured by the die shear test monotonically increased with annealing time. It then changed the slope with further annealing, which correlated with the change in fracture modes from ductile to brittle at a critical transition time. This is ascribed not only to the increasing thickness of brittle IMCs but also to the decreasing thickness of the solder, as there exists a critical annealing time for a fracture mode transition in our thin solder-capped Cu pillar microbump structures.