Microstructural stability of Sn–1Ag–0.5Cu–xAl (x = 1, 1.5, and 2 wt.%) solder alloys and the effects of high-temperature aging on their mechanical properties

The effects of Al addition on the microstructural stability and mechanical properties of Sn–1Ag–0.5Cu (wt.%) solder alloy under high-temperature aging were investigated. The addition of Al refines the primary β-Sn dendrites and enlarges the interdendritic regions. The addition of Al also suppresses the formation of Ag3Sn and Cu6Sn5 intermetallic compounds and leads to the formation of Ag3Al and Al2Cu intermetallic compounds. Moreover, the addition of Al leads to the segregation of an Al-rich phase. Tensile test results indicated that the addition of Al significantly increased the yield strength and ultimate tensile strength, whereas the elastic modulus increased only slightly and the total elongation decreased. Similar to the Sn–1Ag–0.5Cu solder alloy, the microstructures of Al-modified Sn–1Ag–0.5Cu solder alloys become notably coarsened after aging, which in turn significantly degrades their mechanical properties. Fracture surface analysis indicated that the addition of Al to the Sn–1Ag–0.5Cu solder alloy leads to a brittle fracture instead of a ductile fracture.