FATIGUE BEHAVIOUR OF LEAD-FREE SOLDER INTERCONNECTIONS IN LIFE PREDICTION OF TRILAYER STRUCTURES SUBJECTED TO THERMAL CYCLING

Trilayer structures such as flip chip plastic ball grid array (FC-PBGA) packages are bodies made of a large variety of dissimilar materials. Due to the coefficients of thermal expansion (CTE) mismatches between and temperature gradients within the layers, thermally induced interaction becomes a typical type of the loads for the joint layer made of lead-free solder joint interconnections. Thermal stresses and strains at the interfaces of solder joints and neighboring adhesive layers are the cause for solder joint fatigue failures, which account for the most common package failures. The current study puts forward a fatigue life prediction method for a trilayer structure using the critical plane-energy fatigue damage parameter in combination with the modified Coffin-Manson life model. The proposed method of calculated fatigue damage parameter for the samples of study, along with their experimental life (N-f50%) under two different thermal conditions is presented. The values of life in (0-100 degrees C) condition and (25-25 degrees C) with the same temperature ramp rate and dwell conditions are found to differ by a factor of 1.3 where the structures tested under (0-100 degrees C) condition show lower lives. The present study further correlated the fatigue damage parameters with the Coffm-Manson type equation to calculate/predict the fatigue life of structures under (25-125 degrees C) condition. The results of the N-f50 fatigue life prediction versus the experimental cycles show that the predicted lives of samples with SAC305 solder joints fall apart with a factor ranging from (1.24)similar to(-1.45). The advantage of the proposed method in comparison with the existing method in life prediction of the trilayer structure with solder alloy is that there are no empirical parameters involved in energy-critical plane damage parameter in life prediction of the trilayer structure. Parameters within the proposed approach purely involves mechanical and fatigue properties of the midlayer alloy.