Research on the interface structure composite behavior and performance influence of Ni and Cr elements in Sn–Cu–Bi–in solder microstructure

Herein, the influence of Ni and Cr as additive elements on the properties of lead-free and silver-free Sn–Cu–Bi–In solder was investigated for the first time. Particularly, a systematic investigation was conducted to determine the strengthening mechanisms of Ni and Cr in the internal phases of Sn–Cu–Bi–In solder and at the interface of BGA solder joints. The bulk solder's tensile strength experienced an approximate 19 % enhancement, and its elongation capabilities improved by 20 %, aligning with the improved mechanical properties observed in Sn–Ag based alloys when alloyed with specific elements. The shear strength at the bonding interface also increased by 24 %. Microstructural analysis and characterisation revealed that Ni and Cr refined the crystal structure and simultaneously dissolved into Sn and Cu to form different strengthening layer interfaces, thereby enhancing the tensile strength and toughness of the solder. Furthermore, a substitutional solid-solution reaction was observed, which contributed to solid-solution strengthening. Various strengthening phases equivalent to twin strengthening behaviours were discovered in the solder microstructure. These phases, which have not been observed in conventional solder alloys, are expected to considerably enhance their overall service performance.