Base Plate Shape Optimisation for High-Power IGBT Modules

Abstract:
The thermo-mechanical reliability of high-power IGBT modules is strongly connected with the thermal resistance between the transistors and the heat sink which is directly influenced by a deep contact between the module and the cooling equipment. Due to thermal mismatch between different materials applied for an IGBT module, the base plate changes its contour depending on temperature. A spherical shape is often applied to force the module to be flat during operation. But this simplification does not take into account whether the board arrangement at the base plate, the structured copper at the top of the boards, nor the real die distribution, and ignores a non-uniform thickness of the solder gaps. This paper reports a numerical approach, based on finite element (FE) analyses, which takes into account the effects neglected up to now. An improved base plate contour is found by an initial inverse computational step followed by a second one including a contact formulation between the base plate and the cooling device. Pure elastic as well as visco-plastic solder properties can be taken into account. The numerical results have been compared with warpage measurements at real parts. A good agreement was found.