Creep behavior of eutectic Sn–Ag lead-free solder alloy

Precipitation-strengthened tin-based eutectic Sn–3.5 Ag alloy was investigated for its creep behavior at three temperatures ranging from 303 to 393 K, under the tensile stress range of σ/E = 10⁻⁴ to 10⁻³. The steady-state creep rates cover seven orders of magnitude (10⁻³ to 10⁻⁹ s⁻¹). The initial microstructure was found to have Ag³Sn intermetallic compound finely dispersed in the matrix of β–Sn. By incorporation of a threshold stress, σ_th, into the analysis, the creep data of eutectic Sn–Ag at all temperatures can be fitted by a single straight line with a slope of seven after normalizing the steady-state creep rate and the effective stress, indicating that the creep rates are controlled by the dislocation-pipe diffusion in the Sn matrix. The steady-state creep rate, , can then be expressed as , where Q_C is the creep activation energy, G is the temperature-dependent shear modulus, b is Burger's vector, R is the universal gas constant, T is the absolute temperature, σ is the applied stress, A is a material-dependent constant, and , in which σ_OB is the Orowan bowing stress and k_R is the relaxation factor.