Physical and mechanical properties of synthesized low Ag/lead-free Sn-Ag-Cu-xBi (x = 0, 1, 2.5, 5 wt%) solders

Highlights: • Adding Bi with different percentages to SAC257 lead-free solder. • Reduced melting point and undercooling with the addition of Bi. • Microstructure refining and formation of fine precipitates by adding Bi. • Wettability of SAC257-xBi solder alloys was significantly improved by adding Bi. • Improving the mechanical properties of the SAC257 lead-free solder by adding bismuth. - Abstract: In order to reduce the price of the SAC357 solder, the percentage of silver was reduced and simultaneously the Bi-element was added to improve the properties of the solder. For this purpose, the bismuth element was added to the base solder Sn-2.5 wt% Ag-0.7 wt% Cu (SAC257) with weight percentages of 1, 2.5 and 5. To evaluate the effects of the addition of bismuth on the physical properties of the new solders, microstructure, XRD, thermal properties, wettability, and density have been investigated. The specimens experienced micro hardness and uniaxial tensile tests. Finally, to evaluate the failure behavior, the fracture surface was studied. The finer microstructure, size reduction of the β-Sn dendrites, expansion of the eutectic region and formation of small precipitates (about 500 nm in size) in the β-Sn matrix were among the most important changes in the microstructure of SAC257-xBi solders. Improvement in wettability, no significant changes in density, and reduction in melting point, solidus and liquidus temperatures were other physical changes in the new solders. The mechanical properties of the new SAC257-xBi solders (x = 0, 1, 2.5, 5) such as hardness, yield strength and ultimate tensile strength increased with the addition of bismuth.