In this paper a wide range of temp. dependent kernel function ρ(Z) and the strain-rate function h(, T) in the Endochronic viscoplasticity were established first by using experimental steady hysteresis loops of Wei et. al., for Sn/3.9Ag/0.6Cu Solder at fixed 298K or 373K and strain rate at 10−3, 10−4 and 10−51/s; and then extended to predict the thermomechanical behavior of solder under constant low strain rate (10−51/s), from 0 to 1% max. strain fatigue loading with in-phase thermal cycling between 298K and 373K.
A novel approach in the theory to account for the microstructural changes enhanced by the stressassisted grain boundary diffusion mechanism during thermal cycling was proposed by using a partial relaxation function h1(T) of back stress (i.e. ρ0 in ρ(Z)).The theoretical results were in very well agreement with experimental data. These results and the out-phase results from the present theory were new and might be used as references to compare with results derived from other theories.