Impression creep of hypoeutectic Sn–Zn lead-free solder alloys

Creep behavior of the hypoeutectic Sn–Zn alloys containing 2.5–6.5 wt.%Zn was studied by impression testing and compared to that of the eutectic Sn–9Zn alloy. The tests were carried out under constant punching stress in the range 50–110 MPa and at temperatures in the range 298–370 K. Assuming a power law relationship between the impression rate and stress, average stress exponents of 6.2, 6.4, 7.3, and 7.4 and activation energies of 41.9, 43.9, 44.5 and 46.0 kJ mol−1 were obtained for Sn–2.5Zn, Sn–4.5Zn, Sn–6.5Zn and Sn–9Zn, respectively. These activation energies are close to 46 kJ mol−1 for dislocation climb, assisted by vacancy diffusion through dislocation core in the Sn. This, together with the stress exponents of about 7 suggests that the operative creep mechanism is dislocation climb controlled by dislocation pipe diffusion. Analysis of the data showed that for all loads and temperatures, the eutectic Sn–9Zn had the lowest creep rates and thus the highest creep resistance among all materials tested. The creep resistance of Sn–6.5Zn was only slightly lower than that of the eutectic alloy while the other two alloys showed much lower creep resistances. This is attributed to the higher volume fraction of Zn-rich second phase precipitates in the more concentrated alloys which acts as the main strengthening agent in the Sn–Zn alloy system.