Mark Mihalic

Thermal interface materials for power electronics applications

In a typical power electronics package, a grease layer forms the interface between the direct bond copper (DBC) layer or a baseplate and the heat sink. This grease layer has the highest thermal resistance of any layer in the package. Reducing the thermal resistance of this thermal interface material (TIM) can help achieve the FreedomCAR program goals of using a glycol water mixture at 105degC or even air cooling. It is desirable to keep the maximum temperature of the conventional silicon die below 125degC, trench insulated gate bipolar transistors (IGBTs) below 150degC, and silicon carbide-based devices below 200degC. Using improved thermal interface materials enables the realization of these goals and the dissipation of high heat fluxes. The ability to dissipate high heat fluxes in turn enables a reduction in die size, cost, weight, and volume. This paper describes our progress in characterizing the thermal performance of some conventional and novel thermal interface materials. We acquired, modified, and improved an apparatus based on the ASTM D5470 test method and measured the thermal resistance of various conventional greases. We also measured the performance of select phase-change materials and thermoplastics through the ASTM steady-state and the transient laser flash approaches, and compared the two methodologies. These experimental results for thermal resistance are cast in the context of automotive power electronics cooling. Results from numerical finite element modeling indicate that the thermal resistance of the TIM layer has a dramatic effect on the maximum temperature in the IGBT package.

Thermal Evaluation of Toyota Prius Battery Pack

As part of a U.S. Department of Energy supported study, the National Renewable Energy Laboratory has benchmarked a Toyota Prius hybrid electric vehicle from three aspects: system analysis, auxiliary loads, and battery pack thermal performance. This paper focuses on the testing of the battery back out of the vehicle. More recent in-vehicle dynamometer tests have confirmed these out-of-vehicle tests. Our purpose was to understand how the batteries were packaged and performed from a thermal perspective. The Prius NiMH battery pack was tested at various temperatures (0°C, 25°C, and 40°C) and under driving cycles (HWFET, FTP, and US06). The airflow through the pack was also analyzed. Overall, we found that the U.S. Prius battery pack thermal management system incorporates interesting features and performs well under tested conditions.

Battery usage and thermal performance of the Toyota Prius and Honda Insight during chassis dynamometer testing

This study describes the results from the National Renewable Energy Laboratorys (NREL) chassis dynamometer testing of a 2000 model year Honda Insight and 2001 model year Toyota Prius. The tests were conducted for the purpose of evaluating the battery thermal performance, assessing the impact of air conditioning on fuel economy and emissions, and providing information for NRELs Advanced Vehicle Simulator (ADVISOR). A comparative study of the battery usage and thermal performance of the battery packs used in these two vehicles during chassis dynamometer testing is presented. Specially designed charge and discharge chassis dynamometer test cycles revealed that the Insight limited battery usage to 60% of rated capacity, while the Prius limited battery usage to 40% of the rated capacity. The Prius uses substantially more pack energy over a given driving cycle but at the same time maintains the pack within a tight target state of charge (SOC) of 54% to 56%. The Insight does not appear to force the battery to a specific target SOC. The Prius battery contributes a higher percentage of the power needed for propulsion. The study also found that while both vehicles have adequate battery thermal management system for mild driving conditions, the Prius thermal management is more robust, and the Insight thermal management limits pack performance in certain conditions.

Single-Phase Self-Oscillating Jets for Enhanced Heat Transfer

In hybrid electric vehicles (HEVs), the inverter is a critical component in the power module, which conditions the flow of electric power between the AC motor and the DC battery pack. The inverter includes a number of insulated gate bipolar transistors (IGBTs), which are high frequency switches used in bi-directional DC-AC conversion. The heat generated in the IGBTs can result in degraded performance, reduced lifetime, and component failures. Heat fluxes as high as 250 W/cm2 may occur, which makes the thermal management problem quite important. In this paper, the potential of self-oscillating jets to cool IGBTs in HEV power modules is investigated experimentally. A full factorial design of experiments was used to explore the impact of nozzle design, oscillation frequency, jet flow rate, nozzle-to-target distance, and jet configuration (free-surface or submerged) on heat transfer from a simulated electronic chip surface. In the free-surface configuration, self-oscillating jets yielded up to 18% enhancement in heat transfer over a steady jet with the same parasitic power consumption. An enhancement of up to 30% for the same flow rate (and velocity since all nozzles have the same exit area) was measured. However, in the submerged configuration, amongst the nozzle designs tested, the self- oscillating jets did not yield any enhancements in heat transfer over comparable steady jets. Results also suggest that oscillating jets provide a more uniform surface temperature distribution than steady jets.

Thermal performance and reliability characterization of bonded interface materials (BIMs)

Thermal interface materials (TIMs) are an important enabler for low thermal resistance and reliable electronics packaging for a wide array of applications. There is a trend towards bonded interface materials (BIMs) because of their potential for low thermal resistance (<;1 mm2-K/W). However, due to coefficient of thermal expansion mismatches between various layers of a package, thermomechanical stresses are induced in BIMs and the package can be prone to failures and integrity risks. Deteriorated interfaces can result in high thermal resistance in the package and degradation and/or failure of the electronics. The Defense Advanced Research Projects Agencys (DARPA) Thermal Management Technologies (TMT) Program has addressed this challenge, supporting the development of mechanically compliant, low resistivity nano-thermal interface (NTI) materials. Prior development of these materials resulted in samples that met DARPAs initial thermal performance and synthesis metrics. In this present work, we describe the testing procedure and report the results of thermal performance and reliability characterization of an initial sample set of three different NTI-BIMs tested at the National Renewable Energy Laboratory.

Characterization of Contact and Bulk Thermal Resistance of Laminations for Electric Machines

The ability to remove heat from an electric machine depends on the passive stack thermal resistances within the machine and the convective cooling performance of the selected cooling technology. This report focuses on the passive thermal design, specifically properties of the stator and rotor lamination stacks. Orthotropic thermal conductivity, specific heat, and density are reported. Four materials commonly used in electric machines were tested, including M19 (29 and 26 gauge), HF10, and Arnon 7 materials.

Reliability of Bonded Interfaces for Automotive Power Electronics

In automotive power electronics packages, conventional thermal interface materials such as greases, gels, and phase change materials pose bottlenecks to heat removal and are also associated with reliability concerns. There is an industry trend towards high thermal performance bonded interfaces. However, due to coefficient of thermal expansion mismatches between materials/layers and resultant thermomechanical stresses, adhesive and cohesive fractures could occur, posing a problem from a reliability standpoint. These defects manifest themselves in increased thermal resistance in the package.The objective of this research is to investigate and improve the thermal performance and reliability of emerging bonded interface materials for power electronics packaging applications. We present results for thermal performance and reliability of bonded interfaces based on thermoplastic (polyamide) adhesive, with embedded near-vertical aligned carbon fibers, as well as sintered silver material. The results for these two materials are compared to conventional lead-based (Sn63Pb37) bonded interfaces. These materials were bonded between 50.8-mm × 50.8-mm cross-sectional footprint silicon nitride substrates and copper base plate samples. Samples of the substrate/base plate bonded assembly underwent thermal cycling from −40°C to 150°C according to Joint Electron Devices Engineering Council standard Number 22-A104D for up to 2,000 cycles. The dwell time of the cycle was 10 minutes and the ramp rate was 5°C/minute. Damage was monitored every 100 cycles by acoustic microscopy as an indicator of an increase in thermal resistance of the interface layer. The acoustic microscopic images of the bonded interfaces after 2,000 thermal cycles showed that thermoplastics with embedded carbon fibers performed quite well with no defects, whereas interface delamination occurred in the case of sintered silver material. Both these materials showed a superior bond quality as compared to the lead-based solder interface even after 1,000 thermal cycles.Copyright

Design of light-weight, single-phase liquid-cooled heat exchanger for automotive power electronics

Efficient thermal management is critical to increasing power density, improving reliability, and reducing the cost of automotive power electronics. In this paper, we present a heat exchanger design based on impinging jets (with 50%-50% mixture by volume of water-ethylene glycol as coolant) on the copper base plate with and without microfinned/enhanced surfaces, and a plastic fluid manifold. Finite-element analyses as well as computational fluid dynamics (CFD) modeling were utilized for the design. The performance of the jet-based heat exchanger is compared to the baseline channel-flow heat exchanger via CFD modeling. We also characterized the thermal performance of the channel-flow-based heat exchanger experimentally to validate the CFD predictions. CFD results indicate that the jet-based heat exchanger can provide up to 45% lower thermal resistance, 79% increase in power density, and 118% increase in specific power with respect to the baseline channel-flow heat exchanger. We also initiated experimental characterization of the reliability of jet impingement on a plain surface as well as on microfinned/enhanced surfaces. Results to date suggest that jet impingement does not degrade the thermal performance of the enhanced surfaces after six months of near-continuous impingement on the surface.

Investigation of thermal interface materials using phase-sensitive transient thermoreflectance technique

With increasing power density in electronics packages/modules, thermal resistances at multiple interfaces are a bottleneck to efficient heat removal from the package. In this work, the performance of thermal interface materials such as grease, thermoplastic adhesives and diffusion-bonded interfaces are characterized using the phase-sensitive transient thermoreflectance technique. A multi-layer heat conduction model was constructed and theoretical solutions were derived to obtain the relation between phase lag and the thermal/physical properties. This technique enables simultaneous extraction of the contact resistance and bulk thermal conductivity of the TIMs. With the measurements, the bulk thermal conductivity of Dow TC-5022 thermal grease (70 to 75 μm bondline thickness) was 3 to 5 W/(m·K) and the contact resistance was 5 to 10 mm2·K/W. For the Btech thermoplastic material (45 to 80 μm bondline thickness), the bulk thermal conductivity was 20 to 50 W/(m·K) and the contact resistance was 2 to 5 mm2·K/W. Measurements were also conducted to quantify the thermal performance of diffusion-bonded interface for power electronics applications. Results with the diffusion-bonded sample showed that the interfacial thermal resistance is more than one order of magnitude lower than those of traditional TIMs, suggesting potential pathways to efficient thermal management.

Advanced liquid cooling for a traction drive inverter using jet impingement and microfinned enhanced surfaces

This study evaluates a jet impingement based cooling strategy combined with microfinned enhanced surfaces as a means of improving thermal management for power electronic devices. For comparison, a baseline channel flow heat exchanger and jet impingement on plain surfaces are characterized. The jets, augmented with enhanced microfinned surfaces, provide localized cooling to areas heated by the insulated-gate bipolar transistors and diode devices. Lighter materials and simpler manufacturing while managing required pumping power increase the overall performance while reducing weight, volume, and cost. Computational fluid dynamics modeling validated by experiments was used to characterize the baseline as well as jet-impingement-based heat exchangers at typical automotive flow rates using a 50%-50% mixture by volume of water and ethylene glycol. The three cooling configurations were tested at full inverter power (40 to 100 kW output power) on a dynamometer. An increased thermal performance was observed for the jet-impingement configurations. Experiments were also performed to investigate the long-term reliability of the jets impinging on enhanced surfaces.