Effect of isothermal aging on room temperature impression creep of lead free Sn-9Zn and Sn-8Zn-3Bi solders

The influence of isothermal aging on the creep behaviour of Sn-9Zn and Sn-8Zn-3Bi solder alloys was studied by impression testing. The tests were carried out under constant impression stress in the range from 90 to 230 MPa at room temperature. Aging affected the microstructure and, thus, the creep behaviour of the materials. The binary Sn-9Zn alloy, with an as cast microstructure characterised by an almost uniform distribution of fine Zn precipitates, was much more creep resistant than the aged condition containing a more sparse precipitate in a softer matrix. In the ternary Sn-8Zn-3Bi alloy, however, isothermal aging enhanced Bi precipitation with almost no change in the distribution and density of Zn particles, the result being an improved creep resistance of the aged material. Irrespective of the processing condition, Sn-8Zn-3Bi showed much lower steady state creep rates than the Sn-9Zn due to both precipitation and a solid solutioning effect of Bi in the Sn matrix. The stress exponents of the as cast and aged conditions were found to be respectively 8.5 and 7.7 for Sn-9Zn and 9.8 and 7.8 for Sn-8Zn-3Bi. These values are in agreement with those determined by room temperature conventional creep testing of the same materials reported in the literature.