Microstructure, thermal analysis and stress relaxation behavior of Sn-1.5Ag-0.5Cu alloys with multiple strengthening mechanisms

Abstract High stress relaxation (SR) stability enables low-silver Sn−1.5Ag-0.5Cu (SAC155) to serve well as a Pb-free alloy for microelectronic industry. From the viewpoint of simultaneous regulation of microstructure and precipitates, micro-alloyed with In, Bi, and Te was proposed to improve the SR resistance and thermal behavior of SAC155 alloy in this work. The results suggest that SR can develop at low initial stress ( σ o ) levels, even below the yield strength (YS) at 25 °C. A peak effect of SR resistance η = 50.2% for Sn-1.5Ag-0.5Cu-3Bi-0.2Te (SAC-BiTe) and η = 56.3% for Sn-1.5Ag-0.5Cu-3Bi-2In (SAC-BiIn) solders was associated with formation of multiple coarsening-resistant strengthening phases: (i) Bi precipitated particles, (ii) Bi atoms in solid solution and (iii) SnTe, Ag 2 In, Ag 3 Sn, Cu 6 Sn 5 , and InSn 4 phases. This interesting high SR resistance is attributed to the significant atomic size difference provided by In, Bi, Te, and Sn atoms. Compared to SAC155 solder with η = 39.5%, this could result in a greater misfit strain. The inelastic deformation of SAC155, SAC-BiIn and SAC-BiTe alloys may be accurately described by the developed El-Daly empirical SR model, which is congruent with the Arrhenius power-law equation. Microalloying led to a considerable drop in SAC155 pasty range (∼ 4.4 °C), onset temperature (∼ 5.9 °C), and melting temperature (∼ 6.8 °C).