Improving the thermal reliability of large area solder joints in IGBT power modules

Due to power losses as well as alternating external operating conditions in IGBT power modules thermo-mechanical strain arises from the different coefficients of thermal expansion. Especially for Cu baseplates in the substrate-to-baseplate solder layer this cyclic straining leads to the formation and propagation of cracks. As a result, the thermal resistance between chip and heat sink increases with increasing number of cycles. In this paper the influence of numerous design and material parameters, such as solder material, thickness of the solder layer and substrate layout, on the long term reliability of large area solder connections is discussed.