Introduction
During the 2009 U.S. Department of Energy, Annual Merit Review & Peer Evaluation Meeting, the National Renewable Energy Laboratory (NREL) presented under the U.S. DOE Hydrogen Program and Vehicle Technologies Program highlighted the continuous development of Thermal Interface Materials (TIMs) in Power Electronics application.
The paper by Narumanchi et al., which was presented, addresses the need for cutting-edge thermal control technologies to enable high-power density applications and reduce the system cost wherein TIMs still represented a significant heat removal bottleneck. One of the objectives of the study is to identify TIM which meets the target thermal performance of 5 mm2K/W for 100 microns bondline thickness (BLT) , as well as meets the reliability and cost constraints for power electronics application.
In this article, they discussed the thermal resistance data of the various commercial TIMs evaluated through thermal performance by the NREL study. Among the 40 materials tested, the NREL study has identified the materials which have the potential of meeting the target performance including Honeywell PCM 45G, an identical material to Honeywell PCM 45F.
Applications
The main application focus of the thermal interface materials (TIMs) is on automotive power electronics cooling, specifically for IGBT Package used in an inverter. All the state of the art thermal interface materials have been evaluated.
The development of thermal greases/gels, phase-change materials (PCMs), solders, and carbon nanotubes (CNTs) as interface materials have all made significant strides. As of today, greases, gels, and PCMs are the most widely used with thermal performance reaching up to 10 mm2K/W. It can also be noted the ongoing research on CNT materials as TIMs in which Double-sided CNT has significant potential due to its thermal resistance performance, which is as low as 4 mm2K/W.
Results and Discussion
Since grease is the primary interface material in the automotive power electronics industry, NREL conducted tests on different commercially available thermal greases. Figure 2b shows the thermal resistance as a function of grease thickness at 75°C, in which the resistance is directly proportional to the grease thickness as expected. Based on the findings presented, it is evident that the commercially available greases examined in this study can yield satisfactory performance; however, none of them achieve a value as low as 3 mm2K/W.
In correlation to the resistance-thickness relationship, Figure 2a shows the dominance of bulk resistance over contact resistance. It can be shown here that the five commercial materials having low resistances both on bulk & contact resistance are: Chomerics XT8030, Honeywell PCM45G/PCM45F, Dow Corning TC 5022, Shinetsu X23-7783D-S, and 3M AHS1055M.
Figure 3, indicates the relationship between temperature, pressure, and resistance. Figure 3a shows that as the temperature increases, thermal resistance increases as well. The temperature range during tests is subjected up to more than 120°C which is the operating temperature for IGBT packages showing potential in the application. This exhibits the relatively lower performance of Shinetsu X23-7783D-S on the temperature test which had the highest resistance of 30-50 mm2K/W while the rest only had 5-20 mm2K/W.
Figure 3b shows that as the pressure increases the resistance lowers. This indicates that the compressibility of the material up to a certain BLT is also reducing the resistance. This highlights the superb performance of Honeywell PCM45G/PCM45F having the lowest resistance of <10 mm2K/W among all the other commercial TIMs.
Among the TIMs considered in this study, Chomerics XT8030, Dow Corning TC 5022, Shinetsu X23-7783D-S, and 3M AHS1055M had promising performance in terms of thickness. Honeywell PCM45G/PCM45F stood out and showed a huge difference in performance during pressure tests. Industry trends also showed future work that needs to be done with bonded, sintered, and possibly semi sintered interfaces.
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