Effect of cooling rate on the room-temperature impression: creep of lead-free Sn-9Zn and Sn-8Zn-3Bi solders

The influence of cooling rate on the creep behavior of Sn-9%Zn and Sn-8%Zn-3%Bi solder alloys was studied by impression testing. The tests were carried out under constant impression stress in the range 60-190 MPa at room temperature. The alloys were prepared at two different cooling rates of 0.01 K s^-1 and 8 K s^-1. This affected the microstructure and thus, the creep behavior of the materials. In the binary Sn-9Zn alloy, the fast cooled (FC) microstructure, characterized by a uniform distribution of fine Zn precipitates, was much more creep resistant than the slowly cooled (SC) condition containing coarse needle-like Zn phase. The introduction of 3% Bi into the binary alloy enhanced the creep resistance and decreased the difference between SC and FC conditions. This is due mainly to the strong solid solutioning effect of Bi in Sn, which decreases the steady state creep rate, especially in the FC condition. The stress exponents, depending on the cooling rate, were found to be 6.2 and 8.5 for Sn-9Zn, and 9.3 and 9.8 for Sn-8Zn-3Bi, which are in agreement with those determined by room temperature conventional creep testing of the same materials reported in the literature.