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 sigma/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 beta-Sn. By incorporation of a threshold stress, sigma(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, (epsilon)over dot can then be expressed as (epsilon)over dot = A(Gb/RT)(sigma - sigma(th)/G)(7) exp(-Q(C)/RT), 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, sigma is the applied stress, A is a material-dependent constant, and sigma(th) = sigma(OB) root1-k(R)(2), in which sigma(OB) is the Orowan bowing stress and k(R) is the relaxation factor.