Effect of crystallographic anisotropy of β -tin grains on thermal fatigue properties of Sn–1Ag–0˙5Cu and Sn–3Ag–0˙5Cu lead free solder interconnects

Abstract

An effect of the crystallographic anisotropy of β-tin grains on thermal fatigue properties of Sn–1Ag– 0˙5Cu and Sn–3Ag–0˙5Cu lead free solder interconnects were discussed. From an orientation imaging microscopic observation, three types of microstructures (single crystal-like, fine grain type and large grain type) were observed in both solders. The single crystal-like microstructure disappeared and the large grain type occurred by further fatigue due to recrystallisation. Because single crystal-like microstructure had the {100} plane approximately parallel to strain concentrated areas, recrystallisation could be retarded if the slip systems of {100}<011> or {100}<010> operate and an amount of thermal strain decreases because these slip systems have the larger critical resolved shear stress due to an anisotropic nature of β-tin. One of the reasons Sn–3Ag–0˙5Cu had longer thermal fatigue life than Sn–1Ag–0˙5Cu can be the number of the single crystal-like or the fine grain type microstructures in Sn–3Ag–0˙5Cu were larger.