Towards an understanding of thermal cycling effects on lead free solder joint deformation mechanisms

Fatigue life predicting of lead-free solder joints in thermal cycling generally requires calculation of stresses and strains vs. time and temperature and knowing their constitutive relations. The challenge of this is the accurate prediction of the inelastic deformation during cycling. Common constitutive relations for solder were formulated based on experiments showing, among other, the rate of steady state creep to vary with the applied stress to a power of 4 or more. However, a recent experiment led to the suggestion that the rate of near-steady state creep of SAC305 solder joints during the high temperature dwell of each thermal cycle does in fact vary with the stress to a power near one.;This thesis presents generic numerical calculations, showing that experimentally observed dependencies of thermal fatigue life on different parameters are only compatible with each other if the steady state creep rate vary linearly with the stress. Followed by systematic measurements of creep rates of solder joints in an assembly after various levels of cycling. These show the steady state creep rate at room temperature to vary linearly with the stress up to about 22MPa and considerably faster above that. Stresses above 22MPa are not of relevance under typical long term cycling conditions.