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Integrated Gate Bipolar Transistors (IGBTs) generally have a high output power and generate significant amounts of heat, which needs to be removed from the chip to ensure continued operation. Since IGBT chips are commonly mounted on a layered assembly structure which is in turn mounted onto a heat sink assembly, the thermal dissipation properties of the layered structure are crucial in keeping temperatures within operational boundaries. Traditionally, the selection of materials for the layered structure has been largely influenced by the thermal conductivity for heat dissipation, the similarity of the coefficient of thermal expansion for physical integrity of the structure and to a lesser extent, the weight of the material. These principles of material selection are indeed adequate for steady state operation of IGBTs. However, IGBTs are often installed in applications where they are subjected to pulsed operation, which is predominantly transient. During transient operation, it was found that thermal conductivity was not necessarily the best criterion to use for material selection within the layered structure. In certain instances, materials that absorbed heat rather than conducting it yielded lower temperatures and higher cooling rates, which in turn resulted in lower start temperatures in the next pulse. This study therefore proposes an additional material selection criterion, one based on the density-specific heat capacity product that should be used in conjunction with thermal conductivity to guide the material selection process, opening the door to material combinations for specific applications that could enhance chip lifespan and reduce deliamination. Materials with high density-specific heat capacity products could also potentially be used to compensate for the thermal "bottlenecking" effect. This study was conducted with numerous simulations based on the well validated Transmission Line Matrix Modelling method. |
