<p>As the global electronics manufacturing industry transitions towards lead-free and low-temperature soldering processes, mixed solder joints (Sn–Ag–Cu/Sn–Pb and Sn–Ag–Cu/Sn-Bi), crucial technologies bridging traditional processes and novel materials, have emerged as a focal point in the aerospace and defence sectors. This review paper aims to systematically examine research progress on microstructural evolution, interfacial reaction mechanisms, and reliability assessments under multi-stress conditions in these two mixed solder systems. For the Sn–Ag–Cu/Sn–Pb backward-compatible system, this study covers an in-depth analysis of the effects of reflow profiles, lead content, and distribution uniformity on recrystallisation behaviour and thermal cycling life. Consequently, the mechanism by which the interfacial lead-rich layer inhibits the excessive growth of intermetallic compounds (IMCs) is thoroughly discussed. Regarding the Sn–Ag–Cu/Sn-Bi low-temperature mixed system, the underlying failure mechanism of Bi phase segregation is investigated in detail. Furthermore, the paper highlights the significant impact of microalloying (Ni, Co, Pd) and pad surface treatments on enhancing the shear strength and drop test reliability of the solder joints. The study indicates that the durability and service life of mixed solder joints are governed not by a single factor but by the synergistic interaction among material composition, packaging architecture, and processing parameters. The findings offer systematic theoretical guidance for the design of processes for high-reliability electronic assemblies, suggesting that future research should focus on in-depth exploration of multi-field coupled failure mechanisms to advance solder joint performance.</p>