Indentation creep of lead-free Sn–3.5Ag solder alloy: Effects of cooling rate and Zn/Sb addition

Abstract

The effect of cooling rate and separate additions of 1.5wt% Zn and 1.5wt% Sb on the creep behavior of Sn–3.5wt% Ag lead-free solder alloy was investigated by indentation creep testing at 298 and 370K. All three alloys were prepared at two different cooling rates of 0.01Ks−1 and 8Ks−1, in order to examine the influence of cooling rate on their creep resistances. This affected the microstructure and thus, the creep behavior of the materials. Fast cooling enhanced the creep resistance by refining the coarse plate-like Ag3Sn particles. Both the ternary alloys showed creep resistances higher than that of the eutectic binary Sn–3.5Ag alloy. The higher creep resistance of the Zn-containing alloy is attributed to the refinement of the Ag3Sn precipitates, while solid solution hardening of Sb in the Sn matrix is responsible for the superior creep behavior of the Sb-containing alloy. The higher creep resistance of the ternary alloys is more pronounced at higher temperatures, where the binary base alloy weakens more readily. The stress exponents were found to be in the range 6.6–9.8 and 7.1–10.2 for the slowly cooled (SC) and the fast cooled (FC) conditions, respectively. These are in agreement with those determined by room temperature conventional creep and other creep testing methods of the same materials reported in the literature. The high stress exponents together with the activation energy of about 80kJ/mol may suggest that the dominant creep mechanism is dislocation creep.