Influence of Indium on Cu6(Sn,In)5 Formation and Microstructure Refinement in Sn-10wt.%Cu Solder Alloy

This study investigates the effects of indium (In) on the phase stability, microstructural evolution, and the thermal and mechanical behavior of Sn-10Cu-xIn (x = 0.5–5 wt.%) bulk solders. Synchrotron X-ray diffraction was employed to study phase formation and transformation at 100°C and 200°C, while synchrotron micro-X-ray fluorescence mapping revealed the distribution of Sn, Cu, and In. Tensile testing was conducted to evaluate mechanical properties. Increasing In content suppresses the peritectic formation of the primary ε-Cu3Sn phase. The Sn-10Cu-5.0In alloy stabilizes the hexagonal η-Cu6(Sn,In)5 phase and prevents polymorphic transformation during thermal exposure. Differential scanning calorimetry analysis shows a continuous decrease in eutectic melting and undercooling with increasing In content, resulting in finer Cu6(Sn,In)5 formation. The growth restriction factor (Q) increased from 224.86 to 259.59, and 273.15 for 3.0 and 5.0 wt.% In, with increases of approximately 14.2% and 20.1%, respectively, highlighting In as an effective grain refiner. Tensile testing revealed that 3.0 wt.% In significantly enhances tensile strength from 51.54 MPa to 63.63 MPa but excessive In lead to γ-InSn4 formation and reduced strength. Overall, controlled In addition improves microstructural refinement, phase stability, thermal properties, and mechanical strength of Sn-10Cu solder for advanced electronic applications.