Role of Ag additions on microstructure, phase transformation and thermal reactions of In-35Sn alloys

This study investigates the effects of minor silver (Ag) additions (1–3wt %) on the microstructure, reaction temperatures, and phase composition of an In-35Sn alloy, aiming to understand improved low-temperature solders for advanced electronic packaging. Samples were characterised using field emission scanning electron microscopy (FESEM), differential scanning calorimetry (DSC), synchrotron X-ray diffraction (synchrotron XRD) and synchrotron imaging X-ray fluorescence (µ-XRF). For chemical analysis, synchrotron soft X-ray photoelectron spectroscopy (XPS) was performed. Microstructure observations showed that the formation of AgIn2 fine particles and finer γ-InSn4 was obvious at higher concentrations of Ag. Synchrotron XRD at 25 and 115 °C showed the presence of β-In3Sn, γ-InSn4 and AgIn2 phases in ISA3510 alloy, indicating a preference for Ag to segregate and form intermetallic at In sites rather than Sn sites. Calculated average crystallite size of γ-InSn4 decreased approximately 25% with 1wt% Ag added to the system. The split spin orbitals of In3d and Sn3d of the alloy indicated the materialisation of In–Sn. Plasmon loss structure of In3d was found at approximately + 11.5 ± 0.1 eV from the main peak. Additionally, the thermal analysis revealed a decrease in melting and cooling onset temperatures, along with changes in pasty range and undercooling behaviour. The addition of Ag showed approximately 40% improvement of microhardness in In-35Sn-3Ag. The findings provide insights into the complex correlation between phase composition, microstructure, thermal properties and microhardness in In–Sn–Ag alloys. These results can guide the development of improved low-temperature solders with enhanced performance for next-generation electronic packaging applications.