Effect of Zn addition on steady state creep characteristics of Sn–3·3Ag solder alloy

Abstract

The materials used in the present study are Sn–3·3 wt-%Ag and Sn–3·3Ag–1Zn (wt-%) alloys. Effect of stress σ, aging temperature T_a and aging time t_a, on the creep resistance of the alloys under investigation had been studied. Creep tests were performed at room temperature (300 K) under the effect of different constant loads ranged from 2·5 to 4 N on specimens pre-aged at the temperatures 353, 373, 393 and 413 K. At each aging temperature the specimens were heated for the different periods of time 1, 20 and 50 h. The steady state creep rate of the present alloys was found to be related to the applied stress with exponent n. This exponent was found to be decreased with increasing aging temperature T_a and/or time of aging t_a. Addition of 1 wt-%Zn to the binary Sn–3·3 wt-%Ag solder alloy affected its creep resistance. It was found to decrease with increasing both T_a and/or t_a. This was attributed to coalesce and enhancement of the particle sizes of the intermetallic compound Ag₃Sn during aging at high temperatures (0·6–0·7T_m) which subsequently led to the decrease in the pinning action of these particles and consequently leading to decreasing the creep resistance in the aged alloy. Moreover, increasing T_a and/or t_a increases the coarsening rate of grain sizes in both solder alloys from which it can be considered as another reason for the softening effect of increasing T_a and/or t_a. Microstructural changes were investigated by optical, electron microscopy and X-ray diffraction measurements.