Temperature levels effects on the thermomechanical behaviour of solder attach during thermal cycling of power electronic modules

The paper presents the effect of temperature amplitude and dwells level on the thermomechanical behaviour of power modules solder. Especially, we show the influence of these parameters on the crack initiation and propagation in the solder layer between direct bond copper and base plate of high power IGBT modules. For this purpose, thermal cycling tests have been performed on IGBT power modules with three temperature profiles. For cycle#l temperature vary between -40degC and 120degC, cycle#2 temperature vary from 40degC to 120degC and for cycle#3 temperature vary from -40degC to 40degC. These tests revealed that solder crack initiation and its propagation occur earlier for cycle#l where temperature variation is higher. But the unexpected results concern those of cycle#2 and cycle#3. In spite of the fact that its have the same temperature variations, solder crack initiations occur earlier and propagate faster in cycle#2 than in cycle#3 . These results show clearly that high and low temperature levels are important on solder lifetime. In order to understand the experimental results, FE simulations have been performed (with ANSYS). The first obtained results show that stress variation depends on the low temperature level, lower is Tmin higher is stress variation. On the other hand, strain variation depends on high temperature level, higher is Tmax higher is strain variation. Finally, Complementary simulations with various high and low temperatures have been performed and revealed tow mechanical behaviour of the solder. When upper temperature is below a homologous temperature of 0.74 Tm, shear strain variations remains in a relatively small range and shear stress variations have a linear dependence with the temperature variation. On the contrary, when Tmax is above 0.74 Tm, shear stresses variations reach a saturation value while inelastic shear strains increase significantly.