Temperature-Level Effect on Solder Lifetime During Thermal Cycling of Power Modules

In this paper, we show that, during thermal cycling, the solder lifetime of power modules is not only dependent on temperature variation, but we also highlight the influence of some other key parameters such as upper and lower dwell temperature levels. In particular, we show the influence of these parameters on the solder crack initiation and propagation in the solder layer between the direct copper bonding and base plate of high-power insulated gate bipolar transistor modules. For this purpose, both experimental and numerical investigations have been carried out. Concerning thermal cycling tests, three temperature profiles have been done: -40degC/120degC, 40degC/120degC, and -40degC/40degC. Results have shown that stress values in the solder are monitored by the low temperature level and that the strain is monitored by the high-level one. We observed that the relative magnitude of strain variations is larger than that of stress variation. In order to understand experimental results, finite-element simulations with various high and low temperatures have been performed. Results have pointed out that the solder exhibits two different mechanical behaviors, depending on whether the upper dwell temperature (Tmax) exceeds or not a homologous temperature of approximately 0.74 T_m. When Tmax is below this value, shear strain variations remain in relatively small range values, and shear stress variations have a linear dependence with the temperature variation. In these conditions, only energy-based models should be used for solder lifetime estimation. On the contrary, when Tmax is above 0.74 T_m, shear stress variations reach a saturation value while inelastic shear strains increase significantly. Therefore, in these conditions, either strain- or energy-based models could be used for solder lifetime estimation. Finally, the thermal cycling behaviors of a lead-free solder (SnAg3Cu0.5) and a lead-based one (SnPb37) are numerically compared.