Microstructure and tensile mechanical behavior of In alloyed Sn-Bi solder

Currently, Sn–Pb alloy remains the primary solder used for interconnecting solar cell arrays. Amidst the growing pressure from environmental protection, the need for lead-free, low-temperature alloys in photovoltaics has emerged as a significant concern. This article explores the impact of xIn on the melting properties, microstructure, and mechanical properties of Sn-30Bi (x = 0, 0.5, 1, 2, 4, 6, 8, 10 wt.%, mass fraction) alloy, utilizing differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, and tensile testing. The findings reveal that as the In content increases, the solidus and liquidus temperatures of the Sn-30Bi alloy decline. The microstructure of the alloy, post the addition of 0.5, 1, 2, and 4 wt.% In, comprises β-Sn dendrites, Sn-Bi eutectic, and Bi particles. The incorporation of In homogenizes the distribution of Bi precipitates, leading to a complex eutectic mixture. Alloys containing 6, 8, and 10 wt.% In exhibit β-phase, BiIn-phase, and Bi-phase structures. At room temperature, In-doping enhances the tensile strength of the alloy. Notably, the ultimate tensile strength (UTS) of the 4 wt.% In doped alloy experiences the most significant increase, reaching 87 MPa, while the elongation rate peaks at 1 wt.% In doped alloy, attaining 29.8%. When tensile tests were carried out at high temperatures (60, 80, 100, and 120 °C), the maximum elongation of the alloy at 120 °C reached 150% after adding 1% In. This research finding holds significant importance for enhancing the mechanical properties of photovoltaic solder strip.