Evaluation of Phase Transformation on Boron Decorated Sn-1.5Ag-0.7 Cu Joined Cu Pillar for Packaging Connection

Abstract The microstructure transformation, mechanical properties, and fracture behavior of boron (B) decorated tin (Sn)-1.5%Ag-0.7%copper (Cu) (SAC157) solder joints with Cu pillars were analyzed. Three concentrations of B (0.015%, 0.020%, and 0.025%) were evenly mixed into the SAC157 solders. The addition of B refined the primary β-Sn phase and transformed the eutectic phase Ag3Sn from a plate-like structure to a dense stripe configuration. During high-temperature storage, coarse intermetallic compounds (IMCs) that extruded into the primary β-Sn in SAC157 solder were observed, and the growth of IMCs was inhibited by B-pinning boundary motion. Furthermore, the B additive simultaneously increased hardness and improved ductility by reducing grain size. In shear strength tests, the SAC157 solder exhibited a brittle fracture surface with a shorter displacement of 0.78 mm, compared to 1.2 mm of elongation in B-decorated SAC157 solders, which displayed ductile properties characterized by a dimpled fracture surface. In conclusion, B particles acted as heterogeneous nucleation sites, effectively refining the grain structure, inhibiting the growth of the eutectic phase, and simultaneously enhancing ductility without compromising hardness.