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, ε̇, can then be expressed as ε̇ = A(Gb/RT)(σ - σ_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, σ is the applied stress, A is a material-dependent constant, and σ_th = σ_OB√1 - k_R², in which σ_OB is the Orowan bowing stress and k_R is the relaxation factor.