Low-temperature interconnection of Sn–31Bi–19In alloy with high ductility and stable soldering interfaces

Low-temperature interconnection using low-melting alloys (LMAs) is essential for mitigating thermal warpage and enabling reliable packaging on flexible substrates; however, conventional Sn–Bi and Sn–In alloys suffer from either brittleness or insufficient strength. In this study, the microstructural evolution, thermal characteristics, mechanical properties, and interfacial reactions of Sn–31Bi–xIn alloys were systematically investigated, with particular emphasis on the golden-ratio composition, Sn–31Bi–19In (SBI3119). The results show that SBI3119 exhibits a unique combination of moderate strength and exceptional ductility, achieving a maximum room-temperature elongation exceeding 200%. This behavior originates from the elimination of brittle Bi phases, the formation of a softer BiIn phase, and the presence of In-enriched Sn(Bi,In) solid solutions. Thermodynamic and kinetic analyses reveal that the transformation from β-Sn to γ-InSn 4 is incomplete under the present thermal history. Furthermore, aging promotes solute redistribution and increases the area fraction of In-enriched regions. SBI3119 exhibits a low melting temperature (onset ∼76 °C) and good wettability, with a spreading ratio exceeding 100% at 140 °C. For Cu/SBI3119/Cu joints reflowed at 160 °C for 90 s, a thin Cu 6 (Sn,In) 5 interfacial layer (∼0.9 μm) forms. As a result, Cu/SBI3119/Cu joints exhibit higher shear strength than Sn–52In, with further improvement after aging. These results demonstrate that SBI3119 is a promising candidate for low-temperature interconnection in advanced electronic packaging.