Unlocking the potential of SiC power modules with graphite

Replacing the copper core in insulated metal substrates with highly oriented graphite boosts thermal conductivity of the SiC power module while trimming its weight

BY WEI FAN AND DAWN KRENCISZ FROM MOMENTIVE QUARTZ TECHNOLOGIES, GARRY WEXLER FROM HENKEL CORPORATION AND EMRE GURPINAR AND BURAK OZPINECI FROM OAK RIDGE NATIONAL LABORATORY

Choosing your next car is far from easy. As part of being environmentally responsible, you will certainly want to consider purchasing a vehicle that runs solely on battery power. While such a vehicle will undoubtedly help to curb your carbon footprint, will it have sufficient driving range? Or will your car run out of juice on one of your long trips?

To avoid agonising over this dilemma, what is needed is the development of vehicles with a longer range between recharges. Increasing energy storage in the battery will help, along with a hike in the performance of the power electronics.

Such efforts are already underway in the United States supported by a Department of Energy roadmap that details requirements for next-generation electric drive systems. Targets include: a doubling of power handling capability to allow the electric traction drive system to handle peak powers of up to 100 kW; and a hike in power density from 13.4 kW/litre in 2020 to 100 kW/litre in 2025. Research institutes and industrial partners have been pouring tremendous resources and efforts into the development of the next-generation power electronics for electrical vehicles (see Figure 1).

Increases in power density must be accompanied by improvements to the thermal management system. However, this must be accomplished without adding weight to the vehicle, as weight will negatively impact travel range. Meeting this requirement demands more advanced thermal management methods beyond traditional heat pipes, water cooling, and fans.

Figure 1. An EV test platform at ORNL that was used to evaluate power modules for wireless charging

Figure 2. Cross-sectional view, provided by a scanning electron microscope, of Momentive Quartz Technologies’ thermal pyrolytic graphite cores (left). An illustration of this material, showing well-aligned graphene basal planes (right).

When a direct-bonded copper substrate is used, it will typically provide good thermal conductivity and electrical isolation. Unfortunately, this usually comes at the expense of a thermal expansion mismatch, chemical instability, substrate rigidity, and a limit to layer thickness. These restrictions tend to hamper the performance of SiC power modules.

Figure 3. A substrate panel containing 8 thermal pyrolytic graphite (TPG) inserts (a); scanning electron microscope cross-section view of the TPG-core substrate (b), revealing a well aligned graphene stack and hermetic bonding; top-view of a thermal pyrolytic graphite-core substrate using ultrasound c-scan (c), showing a good bonding interface with little voiding.

The Group’s collaboration is able to draw on the expertise of Momentive Quartz Technologies, which, along with its legacy, has been an industry leader in thermal management in the aerospace, telecommunications and defence sectors for more than half a century. Within its portfolio, there is the: TC1050 heat spreader, deployed since the 1990s for board-level thermal management; the TMP-EX heat sink, introduced in 2011, that matches the coefficient of thermal expansion of the chip; and the TMP-FX thermal straps, which provide flexible heat conduction in tight spaces.

Figure 4. A seven-layer thermal pyrolytic graphite-core insulated metal substrate structure utilising proprietary materials and processing technologies from Momentive Quartz Technologies and Henkel.

For all these products, the key ingredient is Momentive Quartz Technologies’ thermal pyrolytic graphite, a material first produced in the 1960s when the current business was part of Union Carbide. This form of graphite, produced by CVD at over 2000 °C, is made up of millions of stacked layers that are highly oriented graphene planes. They equip the material with extremely high thermal conductivity (see Figure 2). Parallel to its basal plane, the thermal conductivity is 1650 W m^-1 K^-1 – four times the value of copper. Another merit of this material is that it is light weight. Its density is just 2.25 g/cm³ for thermal pyrolytic graphite, which is one-fourth the density of copper.

Encapsulating a core of this form of graphite into a copper substrate enables a high thermal conductivity to be combined with a low mass. The Group was able to accomplish this by drawing on Momentive Quartz Technologies’ proprietary bonding technology to ensure excellent thermal and mechanical joints between the graphite insert and the copper enclosure. The high quality of this bond is evident in the scanning electron microscopy and ultrasound c-scan images (see Figures 3 (b) and (c)). They reveal a hermetic, nearly void-free encapsulation.

One of the characteristics of the thermal pyrolytic graphite inserts is their anisotropic thermal conductivity. This allows the orientation and layout of the graphite within the copper enclosure to be tailored to the application. Measurements with a Netzsch Nanoflash 476 reveal a thermal conductivity of 760 W m^-1 K^-1 in a structure of thermal pyrolytic graphite sandwiched between copper. This conductivity is nearly double that of copper. At Momentive Quartz Technologies, researchers have subjected the metal-encapsulated graphite to a range of other tests, including thermal cycling and mechanical shock and vibration. This examination demonstrates the outstanding thermal performance and reliability of this hybrid structure.

Figure 5. Finished thermal pyrolytic graphite-core insulated metal substrate with patterned copper circuit layers and dielectrics on both sides.

During the fabrication of the insulated metal substrates, technologists at Henkel uses a proprietary coating technology to apply dielectric layers to the graphite-core substrate plane and the copper foil layers (see Figure 5). Adding these dielectric layers is critical to meeting the breakdown voltage requirements. Engineers at Henkel can also include features such as non-conductive, epoxy-filled grooves that provide electrical isolation between poles.

Figure 6. Electrical schematic of a half-bridge module (left) and insulated metal substrate circuit layout (right). More details are given in https://doi.org/10.1115/IPACK2019-6436

To demonstrate the power of this approach, the Group constructed a testing platform, a common half-bridge SiC MOSFET inverter module (see Figure 6). In this design, Momentive Quartz Technologies’ graphite inserts were positioned directly under the SiC dies. With this arrangement, waste heat was extracted through the thickness, towards the terminals, where it could transfer to Henkel’s thermally conductive dielectric layers (see Figure 7). In addition to reducing the thermal resistance of the copper substrate, the heat spreading provided by the thermal pyrolytic graphite increased the effective contact area of the dielectric layer, and thus its heat conductance. According to modelling by those on our team at ORNL, there was an immediate improvement in temperature uniformity for the insulated metal substrate with the thermal pyrolytic graphite core, associated with the higher in-plane heat conduction of this form of graphite (see Figure 8).

Figure 7. Conceptual module design using thermal pyrolytic graphite core to channel the waste heat away from the dies. More details are given in https://www.energy.gov/sites/prod/files/2019/06/f63/elt208_gurpinar_2019_o_4.9_3-41t.pdf

To validate their modelling, the scientists at ORNL conducted bench-top measurements to compare the performance of SiC power modules of insulated metal substrates with copper cores and thermal pyrolytic graphite cores (see table 1). Preliminary measurement of the assembled power modules confirmed the earlier modelling results. The testing is on-going, with those at ORNL assessing the final package performance, durability and reliability of insulated metal substrates with thermal pyrolytic graphite cores (see Figure 10).

Figure 8. Thermal analysis of a SiC module using insulated metal substrates with a copper-core (left) and thermal pyrolytic graphite core (right). The results demonstrate the improvement in heat spreading by the two thermal pyrolytic graphite cores. More details are given in https://www.energy.gov/sites/prod/files/2019/06/f63/elt208_gurpinar_2019_o_4.9_3-41t.pdf

Figure 9. Plotting the SiC MOSFET junction temperature during power cycling revealed a 1.8 °C reduction in temperature range using the thermal pyrolytic graphite-core insulated metal substrate.

Figure 10. Assembled SiC power module using a thermal pyrolytic graphite-core insulated metal substrate.

Table 1. Thermal performance comparison of insulated metal substrates with a copper core and a thermal pyrolytic graphite core. * Measured value using Nanoflash method.

W. Fan et al. “Improving Heat Conduction of Insulated Metal Substrate with Thermal Pyrolytic Graphite Core for SiC Power Module”, Materials Science Forum (2020)

US Department of Energy, Office of Energy Efficiency and Renewable Energy, “U.S. Drive Electrical and Electronics Technical Team Roadmap,” https://www.energy.gov/eere/vehicles/plans-and-roadmaps, October 2017

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E. Gurpinar et al. Proceedings of the ASME 2019 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems, V001T06A013. ASME. https://doi.org/10.1115/IPACK2019-6436

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