Effect of indium on microstructure, phase stability, anisotropic lattice behavior and mechanical performance of Sn-58Bi solder alloy

The present work investigates the influence of Indium (In) additions at 4, 8, and 12 wt.% on the microstructure, phase stability, anisotropic lattice behavior and mechanical performance of Sn-58Bi solder alloy. Alloys were fabricated via casting and characterized using differential scanning calorimetry (DSC), tensile testing, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX), and synchrotron-based techniques including in situ X-ray diffraction (XRD) and micro-X-ray fluorescence (XRF). Indium addition progressively reduced the melting temperature from 142.52 °C (Sn-58Bi) to 116.33 °C (Sn-58Bi-12In), while broadening the pasty range. Microstructure analysis revealed the formation of BiIn intermetallic compounds and coarsened Bi-rich regions with increased In content. Synchrotron micro-XRF mapping confirmed the elemental segregation of Bi and In. In situ synchrotron XRD demonstrated that the BiIn phase is stable at room temperature but dissolves above 80 °C. Lattice parameter refinement of the rhombohedral Bi phase indicated anisotropic stabilization, with suppression of contraction along the a-axis upon In addition. Solderability assessment showed that Sn-58Bi-12In achieved the shortest wetting time of 1.356 s, and a thinner Cu6Sn5 intermetallic layer was observed at the solder/Cu interface. Mechanical testing revealed a tradeoff between strength and ductility where tensile strength decreased from 69 to 53 MPa, while elongation improved to 15.5% at 12 wt.% In. Fracture surface analysis indicated a transition from brittle to ductile failure with increasing In content. These results demonstrate that Indium addition effectively tailors the microstructural, thermal, and mechanical behavior of Sn-58Bi solder, making Sn-58Bi-xIn a promising candidate for low-temperature electronic packaging applications.