A Review of Low-Temperature Solders in Microelectronics Packaging

The shift to lead-free solder alternatives led to the development of Sn–Ag–Cu (SAC) alloys as the most used solders due to their superior mechanical properties and reliability. However, their melting range (217 °C–222 °C) is much higher than that of lead (Pb)-based solders. With advances in microelectronics packaging at aggressive silicon nodes and complex heterogeneous architectures, the high melting temperatures of SAC-based solders significantly affect the package stress, leading to poor joint quality and early interlayer dielectric (ILD) delamination failures. Owing to this, lead-free low-temperature solders (LTSs) have gained momentum with focus on novel alloying compositions. Sn–Bi and In-based solders have emerged as the leading candidates for LTSs. This review article details the key requirements of LTS and evaluates the impact of alloy modifications to the microstructure, thermal/ mechanical properties, wettability, and reliability of Sn–Bi and In-based solder systems. This article also discusses the potential focus areas, especially hybrid LTS, as the bridge between SAC-based to full LTS joints.