Significant waste heat generated in power electronics components such as inverters in EVs and hybrid vehicles due to the high power density, can cause material degradation. Further, constantly fluctuating power and torque demands in the electric drive train results in temperature gradients in the electronic components that exacerbate fatigue failures if thermal management is not addressed properly. Boron Nitride (BN) has one of the highest thermal conductivities coupled with excellent electrical insulation amongst ceramic fillers. Its low dielectric constant, dielectric loss, and high break down strength, along with its chemical inertness make it an attractive filler in thermal management applications pushing for higher power, higher voltages, and higher-speed transmissions, while maintaining lighter weight and smaller size. With push for higher voltage, coupled with increasing switching frequencies, dielectric loss can result in additional heat generation in the device. BNs low dielectric loss compared to other fillers enables good heat dissipation with very minimal energy loss.This work demonstrates providing improved thermal pathways for heat dissipation without fundamentally changing the package design by increasing thermal conductivity of the potting compound that encapsulates the dies. Pure resins such as silicones or epoxy-based systems are mostly used in these applications that provide protection from the elements, some mechanical support, but little thermal conductivity. The current work uses BN filled potting compounds that vastly improves thermal conductivity and can also improve stiffness of the resin while maintaining adequate compliance, providing additional mechanical support to the interconnects. Currently for single sided devices, heat generated is removed from the underside of the dies via a die-attach into a liquid or air-cooled substrate. This is the most efficient heat removal with the lowest thermal resistance (Rth). However, providing additional pathways through the potting compound enables heat extraction from top of the dies and interconnects/ wire bonds, spreading it to a larger area for improved heat dissipation.The current work describes a demonstrator device that mimics a liquid cooled single sided IGBT/ MOSFET device in terms of heat generation and cooling. Three different configurations were evaluated: (1) pure resin die attach and pure resin potting, (2) BN based thermally conductive die attach, but pure resin potting, which mimics most current single-side cooled devices, and (3) BN based thermally conductive die attach and thermally conductive potting.Multiple resistors (TO-247 thick film power resistor) were mounted on a water-cooled base plate with an epoxy based thermal interface material (TIM). A thermocouple was embedded to each resistor to monitor temperature during testing. Each resistor was compartmentalized with side walls to isolate them, providing equal area and coverage by the cold plate. They were then potted with different silicone based potting formulations to evaluate their effectiveness on device temperature. The temperature of the cold plate was kept constant at 25oC by a water-cooled chiller. The resistors were connected to a DC power supply and tested individually at different power levels. The results showed a 48% lower resistor temperature at max. power for the BN formulation compared with standard silicone potting between (2) and (3), dropping from 140oC to 84oC. This approach shows considerable promise of using BN based thermally conductive potting compound over standard silicone or epoxy systems. Boron nitrides high breakdown strength, chemical inertness, coupled with low loss make it an extremely attractive material for use in thermal management in power electronics in EV applications, maximizing the safety, performance, efficiency, and overall longevity of EVs.
Murugaiah et al. (Tue,) studied this question.