Interplay Between Interfacial Reaction and Solidification Kinetics in the Cu/Sn-58Bi System: Evolution of Microstructure, Crystallographic Texture, and Mechanical Properties

The thermo-mechanical reliability of Sn-58Bi eutectic solder joints is critically dependent on their solidification microstructure. This study systematically investigates the interplay between interfacial reactions and bulk solidification kinetics. Our results demonstrate that elevated reflow temperatures, up to 320 °C, accelerate the growth of interfacial Cu-Sn intermetallic compounds (IMCs) to a thickness of 3.87 µm and promote Cu dissolution, shifting the local liquid composition near the interface to a hypereutectic state with a Bi phase fraction as high as 61.1%. This compositional shift, amplified by slow cooling (0.125 °C/s), promotes the formation of coarse primary Bi phases. A key finding is the exclusive formation of a unique “lotus-leaf” eutectic morphology at the periphery of these Bi blocks, wherein the Sn phase exhibits a strong [010] crystallographic texture with an intensity of 89.5 M.R.D. (multiples of random distribution). This is attributed to heterogeneous nucleation on the primary Bi phase. By elucidating these fundamental mechanisms, this study provides insights for controlling solidification to tailor microstructures. Crucially, we establish a clear processing-microstructure-property relationship: the processing-induced microstructural coarsening, particularly the formation of interfacial primary Bi, serves as the weakest link, transitioning the failure mode from ductile which observed in rapidly cooled joints to brittle and severely degrading the joint’s shear strength by over 30%.