Advancing the solidification of Sn–Bi alloys for electronic packaging: challenges, progress, and future directions

MetalMat

2025-08-18

Low‐temperature soldering (LTS)

Mechanical properties

Microstructure

Sn–Bi alloys

Solidification

Wettability

In the early 2000s, the soldering industry faced a major shift due to regulations restricting Pb usage. As a result, four main alloy systems emerged as alternatives: Sn–Bi, Sn–In, Sn–Cu, and Sn–Ag. However, for reasons not initially evident, higher melting point alloys were the first to be widely developed, most notably the Sn–Ag–Cu (SAC) alloys, which became the industry standard due to their balanced performance in reliability, mechanical strength, and process compatibility. Over the past decade, increasing emphasis has been placed on low‐temperature soldering (LTS), requiring studies not only on defects such as warpage, interfacial pores, and joint strength but also on the fundamental melting and solidification behavior of these alloys. Sn–Bi alloys have emerged as a commercial alternative, particularly for consumer products such as clients and server computers, while maintaining compatibility with surface mount technology (SMT) technology for high‐volume manufacturing. The thermal fatigue reliability of Sn–Bi is also well‐recognized. This short review will provide an overview of various studies conducted on the solidification behavior of Sn–Bi based alloys. The solidification paths, eutectic formation, morphologies, and properties will be explored. Future research directions comprise microalloying insights to improve ductility, interaction between Sn–Bi solder balls and SAC (Sn–Ag–Cu) pastes to mitigate PCB warpage, and exploring advanced characterization techniques such as X‐ray microtomography (XMT) and nanohardness testing. These developments are essential for optimizing LTS alloys and ensuring their reliability in next‐generation electronic packaging