Investigation of Sn–Bi–In ternary solders with compositions varying from In–Sn eutectic to 59 °C ternary eutectic point

This study reports on the investigation of the melting behavior and mechanical properties of Sn–Bi–In ternary solders, which has two ternary eutectic points (59 °C and 76 °C), with six different compositions chosen from the liquidus line from the In–Sn eutectic point (118 °C) to the ternary eutectic point at 59 °C. We fabricated solder pastes with the compositions and characterized them using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), shear test, and solder balling test. The solder balling test disclosed that the solders with a wide pasty zone (T_liquidus−T_solidus) and a wide exothermic peak showed a poor morphology after solder balling test, while those with a composition close to the binary or ternary eutectic compositions exhibited a good morphology. The SEM analysis disclosed that the ternary solders were comprised of β-InSn, γ-InSn, and BiIn₂, and that the volume fraction of BiIn₂ increased as the composition moves toward the ternary eutectic point. The shear test unfolded that the microstructure change influenced the fracture mode, which changed from ductile to brittle possibly due to the increase of the phase fraction of BiIn₂. The shear strength was around 18 MPa for the In–Sn binary solder and was between 22 and 24 MPa for the ternary alloy samples.