Comprehensive Evaluation of Microstructure–Property Relationships in Al-Added Sn-Zn Eutectic Solder Alloys from Thermal, Electrical, and Mechanical Perspectives

(Sn-8.8Zn)-XAl (X = 0, 0.5, 1.0, 2.5, 5.0 wt.%) solder alloys were produced using a vacuum muffle furnace. The microstructural images, chemical compositions, and phase structures of the alloys were characterized by field emission scanning electron microscopy (FESEM), field emission scanning electron microscopy–energy dispersive x-ray spectroscopy (FESEM–EDX), and x-ray diffraction (XRD) analyses, respectively. According to FESEM images, a fully eutectic microstructure was observed in the Sn-8.8Zn alloy. With increasing Al content, dendritic structures formed and became denser. The mechanical properties of the alloys (ultimate tensile strength σUT, tensile yield strength σTY, compressive yield strength σCY, and Vickers hardness HV) were measured as a function of composition. The highest strength and hardness values were obtained for the (Sn-8.8Zn)-2.5Al alloy, whereas the (Sn-8.8Zn)-5.0Al alloy exhibited the highest ductility. The melting enthalpies (ΔH) and the specific heat differences between the solid and liquid phases (ΔCP) of the alloys were measured by differential scanning calorimetry (DSC) analysis. It was determined that an increase in the Al content of the eutectic alloy resulted in higher measured thermophysical properties. Finally, electrical resistivity (ρ) values at T = 300 K, measured using the standard four–point probe method (FPPM), revealed an increase in resistivity up to 1.0 wt.% Al content, followed by a decrease beyond this value. In contrast, thermal conductivity values calculated using the Wiedemann–Franz law exhibited an opposite trend, decreasing up to 1.0 wt.% Al, and increasing thereafter. Based on microstructure–property relationships, the results were compared with previous studies, highlighting that (Sn-8.8Zn)-XAl alloys represent promising alternatives to lead-free solders.