Creep Deformation of Lead-Free Sn–3.5Ag–Bi Solders

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Creep rupture properties of lead-free Sn–3.5Ag-based alloys with varying amounts of Bi were investigated using dog-bone-shaped rolled and heat-treated bulk specimens. Nominal compositions of Bi additions were 0, 2.5, 4.8, 7.5, and 10 wt%, respectively. The minimum strain rates ($\dot{\varepsilon}_{\text{min}}$) were lowest for the 2.5Bi specimens. The stress exponents ($n$) of $\dot{\varepsilon}_{\text{min}}$ were usually around $4\pm 0.6$, with the exception of the 10Bi alloys, which showed $n\sim 2$. Additions of Bi reduced the creep ductility of Sn–3.5Ag-based ternary alloys, and fractographic analyses revealed typical creep rupture by the nucleation and growth of cavities on the grain boundaries except for the Bi-free alloy which had cavities in the grains. Subsequent AES analyses revealed that Bi segregation to grain boundaries facilitated the cavity nucleation, thereby increasing the propensity for the brittle creep rupture.
2003-03-15