Mechanics-based acceleration for estimating thermal fatigue life of electronic packaging structure

A simple mechanics-based approach is extremely anticipated to efficiently perform the equivalent estimations for thermal fatigue life of electronic packaging structures. In this paper, a cyclic loading in the form of four-point bending is applied and the mechanical fatigue life are measured for five groups of wafer level chip scale packaging (WLCSP) structures with different displacement amplitudes at a constant temperature of 125 °C. By conducting the conventional thermal cycling experiments, the thermal fatigue lives of the packaging structures are obtained for four stress levels due to different constraint schemes. With consideration of temperature effect within the temperature range of thermal cycling, the constitutive parameters of the lead-free Sn-3.0Ag-0.5Cu (SAC305) solder material are reasonably assessed by the framework of the unified creep-plasticity model to reflect the plastic and creep deformation at different temperatures, which is the essential perquisite of finite element simulations for fatigue life analysis. The increment of equivalent accumulative plastic strain is numerically calculated for each loading cycle for both four-point bending and thermal cycling by finite element simulations. By following the same slope in the double logarithmic coordinates, both the mechanical and thermal fatigue life are found to linearly decrease with regard to the increment of equivalent accumulative plastic strain, despite of certain discrepancy in intercept. This make possible the equivalence of accelerations between mechanics-based four-point bending and thermal cycling method.