ENERGY-BASED METHODOLOGY FOR DAMAGE AND LIFE PREDICTION OF SOLDER JOINTS UNDER THERMAL CYCLING

Thermal fatigue of solder joints is critical to electronic package performance and life considerations. It is also difficult to predict because of complex time temperature dependenc of solder behavior. Strain based solder fatigue descriptions, like Coffin-Manson, are not adequate as the fatigue life of the solder may also be function of stresses. When the relationship between stress and strain is not unique, which is the case here because of strain rate dependence, a Coffin-Manson type of strain based correlation will be valid only over a limited range. A nonlinear finite element method (FEM) based simulation methodology, has been developed, for predicting the life of solder joints when subjected to thermal cycling. This methodology uses a hysteresis energy based damage function approach for damage and fatigue life prediction of the solder joint. The nonlinear solder behavior response inclusive of elastic, time independent plastic and time dependent viscoplastic response is accounted for. The methodology has been used for correlating the fatigue life of flip chip package designs with silicon die, alumina substrate and 95Pb5%Sn peripheral bumps subjected to thermal cycling. Two different bump designs and six temperature cycles with different ramp and dwell times were used from the literature to correlate the life prediction methodology with experimentally determined mean cycles to failure. A multiple cycle response was simulated to determine the stable cycle response. The observed correlation between the hysteresis energy based damage function and experimentally determined fatigue life is extremely encouraging.