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Thermal Management of Power Electronics Module Using Thermal Pyrolytic Graphite Based Substrate and Heatsink
Keywords: Thermal Pyrolytic Graphite, Power Electronics Module, Thermal Management
Thermal management of a power electronics module is very important. The device within a package will fail if the maximum junction temperature is not within the device’s permissible maximum temperature rating specified by the manufacturer. Modern electronic miniaturization demands multi-chip module (MCM) packaging; major benefits include different semiconductor technology integration, reduced number of component interconnects, and lower power supply. But the huge amount of heat generated by each chip produces thermal-coupling, leading to an increase in the junction temperature. The power device specifications in the datasheet list assume the devices being mounted on a suitable heatsink. As a result, the power module needs to be connected to a heatsink to effectively increase the surface area of the heat dissipation junctions. A high conductivity material based heatsink extracts heat effectively from the module as the thermal resistance value remains low. Usually aluminum is used for its high thermal conductivity and light weight. Wide bandgap (WBG) devices like silicon carbide (SiC) can generally sustain a maximum junction temperature of about 175 °C-200 °C. The junction temperature of the WBG devices becomes severe in a high-density high-power module. This highlights the need for a thermal management system to limit the maximum junction temperature within the device’s permissible limit. In this paper, thermal analysis is done for a high-density high-power module where the high in-plane thermal conductivity of thermal pyrolytic graphite (TPG) is exploited. TPG brings down the junction temperature to a considerably lower level, leading to a safer power module functioning. This paper focuses on the design and proper alignment of the heat-sink with respect to the module layout so that maximum junction temperature is reduced by proper heat extraction far below the operating temperature of the devices and also reduction of the thermal-coupling among the power devices placed next to each other on the same plane within the power module. TPG is a highly crystalline graphite with well-aligned graphene planes [1]. TPG, with its high in-plane thermal conductivity (~1700 W/m-K, which is seven times higher than that of aluminum with 235 W/m-K) can extract heat significantly [1]. In this study, the major applications to specifically exploit this property of TPG is highlighted. Other dominant features of TPG heatsink include its high reliability and light weight (density: 2.25 gm/cm3) [1]. Heatsink with TPG material, where the high conductivity graphene planes are aligned vertically to the substrate of a high- density power module, is designed. Heatsink structures are modelled and finite element analysis (FEA) simulations are done. Preliminary FEA simulation indicates a junction temperature drop of about 15 °C with TPG heatsink as compared to a traditional aluminum heatsink for a high-density power module. Further analysis will be done with improved heat sink design, modeling maximum number of high in-plane conductivity heatsink fins and this design can show five times higher power dissipation as compared to aluminum [1]. Heatsink made with composite structure of TPG loaded in co-efficient of thermal expansion matched enclosure like aluminum silicon carbide (AlSiC) or molybdenum copper (MoCu) can also provide thermal conductivity as high as 900 W/m-K [2-3]. The various heat sources or the switching devices integrated on the substrate demand proper design layout. These should be placed at a larger distance so that thermal-coupling does not occur but at the same time, a high-density power module is needed and also, electrical aspects need to be considered. Placing the power devices far from each other increases the parasitic inductance through the connections. The various power devices have separate heat flow paths to the heat-sink but as heat flows across the multiple layers like solder, DBC, and baseplate, to the heat sink, heat spreading occurs along the path. This heat from the closely spaced dies interfere with each other generating thermal-coupling. The datasheet values for a device do not consider thermal cross-coupling effect and they specify thermal resistance for a single active device. So, to determine cross-coupling effects accurately, the device junction temperatures are evaluated using FEA simulations. A reduced junction temperature is found when the devices are placed considering 45-degree heat-spreading angle rule [4-5], but again the high-density requirements of the power module are violated. And trade-off between high-density and low thermal-coupling is needed. Here, TPG can be applied to reduce the thermal-coupling effect of the large amount of heat generated by the closely placed chips in a multi-chip module. The geometry of the heat-sink fins is made such that maximum use of the TPG high in-plane conductivity can be done. The fins are designed with thickness aligned with the dimensions of the dies. The high thermal conductivity of the TPG fins constricts the heat flow direction and the heat flow spreading will be minimized to a huge extent. The cross-coupling is reduced which decreases the thermal resistance across the layers, leading to a lower maximum junction temperature. FEA simulations showing this decrease in junction temperature will be included in this study.
Riya Paul,
University of Arkansas
Fayetteville, Arkansas

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