Fatigue Life Predictions for Thermally Loaded Solder Joints Using a Volume-Weighted Averaging Technique

Fatigue lives of thermally loaded solder joints are predicted using the finite element method. An appropriate constitutive relation to model the time-dependent inelastic deformation of the near-eutectic solder is implemented into a commercial finite element code, and the stress-strain responses of different electronic assemblies under the applied temperature cycles are calculated. The finite element analysis results are coupled with a newly developed approach for fatigue life predictions by using a volume-weighted averaging technique instead of an approach based on the maximum stress and strain locations in the solder joint. Volume-weighted average stress and strain results of three electronic assemblies are related to the corresponding experimental fatigue data through least-squares curve-fitting analyses for determination of the empirical coefficients of two fatigue life prediction criteria. The coefficients thus determined predict the mean cycles-to-failure value of the solder joints. Among the two prediction criteria, the strain range criterion uses the inelastic shear strain range and the total strain energy criterion uses the total inelastic strain energy calculated over a stabilized loading cycle. The obtained coefficients of the two fatigue criteria are applied to the finite element analysis results of two additional cases obtained from the literature. Good predictions are achieved using the total strain energy criterion, however, the strain range criterion underestimated the fatigue life. It is concluded that the strain information alone is not sufficient to model the fatigue behavior but a combination of stress and strain information is required, as in the case of total inelastic strain energy. The superiority of the volume-weighted averaging technique over the maximum stress and strain location approach is discussed.