Reliability-Oriented Design of Low-Temperature Sn–Bi Solder Interconnects for Advanced Packaging Systems

The rapid growth of artificial intelligence, electric vehicles, and high-performance computing has accelerated the demand for miniaturized and thermally reliable electronic packages. As front-end scaling approaches its physical limits, advanced packaging has become central to improving system performance. However, heterogeneous integration, 2.5D/3D packaging, and chiplet architectures intensify thermomechanical reliability concerns because materials with different coefficients of thermal expansion are integrated within confined structures. Conventional Sn–Pb solders offered low processing temperatures and excellent ductility, but their toxicity drove the transition to lead-free systems. Sn–Ag–Cu solders, especially SAC305, have been widely adopted for their mechanical strength and fatigue resistance; nevertheless, their high melting temperature aggravates warpage, interfacial delamination, and thermal stress. In this context, Sn–Bi solders have attracted attention as low-temperature candidates, since eutectic Sn–58Bi melts near 138–139 oC. Despite this advantage, they suffer from brittleness, Bi-rich phase segregation, microstructural coarsening, electromigration-related degradation, and excessive interfacial intermetallic growth. This manuscript reviews the evolution of conventional solder systems, the metallurgical characteristics of Sn–Bi alloys, and recent strategies including micro-alloying and nanocomposite reinforcement for improving the reliability of low-temperature solder joints in advanced packaging applications.