Microstructural evolution of Sn-Cu Based Bi Alloyed Lead-free Solder at Low Cooling Rate

Microstructural evolution of silverless Sn based lead-free solder alloys may be controlled by bismuth-alloying. In this work, an Sn-0.7Cu-0.05Ni-1.5Bi solder was analyzed and compared to a reference $\mathrm{Sn}-0.7 \mathrm{Cu}-0.05 \mathrm{Ni}$ solder alloy. The change in the microstructure due to $\mathbf{B i}$ alloying results in solid solution strengthening, a mechanism which is independent of intermetallic phase growth and the associated coarsening effect. It was shown, that the detected number and total volume of primary ($\mathbf{C u}, \mathbf{N i})_{6} \mathbf{S n}_{5}$ was significantly reduced by $\mathbf{1. 5 ~ w t} \% \mathbf{B i}$ doping. The analyses are based on DSC (Differential Scanning Calorimetry), EDX (Energy Dispersive X-ray analysis), SEM (Scanning Electron Microscope) and XCT (X-ray Computed Tomography) examinations and supported by simulations of phase transformations.