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Carbon Fiber-based Thermal Interface Materials for High Temperature Applications
Keywords: thermal interface, carbon fiber, high temperature
With the progress accomplished in compound semiconductor devices (SiC, GaN, etc.), there have been steep increases in the power densities in electronic devices. Some of the compound semiconductor power devices, in particular GaN are also capable of delivering high power densities in the smallest of footprints. Concurrently, power dissipations are also increasing in microprocessor chips, in particular graphical processor units (GPU). As a result, the electronics hardware designers and engineers are increasingly facing thermal management challenges upfront where all aspects of heat dissipation are required to be addressed. These include efficient design topologies, material choices and optimal design attributes to leverage the interaction of the two. Thermal management is has become critical and fundamental part of design revisions to ensure that electronic devices operate within their limits. Thermal interface materials (TIM) are one of the most important components of the ecosystem involving materials-design topologies-interactions in the development of today’s and tomorrow’s electronics systems. In most electronic assemblies, the dominant mode of heat transfer in the solid medium is by conduction whereas bigger drop in temperatures are realized by convection using heatsinks cooled by natural or forced convection in most terrestrial applications. In space applications, the heat dissipated is transferred to radiators using phase change method such as heat pipes and by liquid cooling. TIM is the essential interfacing medium connecting a heatsink or heat pipe in contact with the heat dissipating component. To minimize the thermal resistance across interfaces, many TIM require adequate pressure on the interface which are not sufficient to ensure optimal contact filling air gaps even at the higher end of the pressure scale. Many TIM are rigid, do not conform to the surface profile and are not reusable. For many applications requiring conforming to non-flat surfaces and skylines of components, there are very few choices of TIM available. Most importantly, more than 95% of the TIM products available in the market today are not suitable for operating at temperatures higher than 150degC. Almost all of the available TIM are only applicable post-surface mount assembly and none are suitable for automated processes using reflow ovens. Rework and reuse still remain as challenges to overcome. A majority of the TIM materials available in the market today are of low thermal conductivity that rules them out for applications with high heat flux as encountered in compound semiconductor devices, GPUs, etc. In this paper, we present the unique properties and advantages of Carbon Fiber Thermal Interface (FTI), an efficiently engineered TIM stack which is customizable to suit the applications in all product categories of electronics hardware. It offers many advantages such as high thermal conductivity and provides excellent contact resistance at low pressures. TIM thicknesses ranging from 0.3mm to as high as 4mm, FTI materials are compliant and are compressible to conform to the surfaces under pressures as low as 10psi. Furthermore, when the contact pressure is removed (for rework or changes), the compressible FTI fully recovers the original form exhibiting shape memory capability. Typical measured responses of interface pressure vs. thermal resistance vs. % compression of the interface are presented for low temperature (<85degC), medium temperature (85 to 125degC) and high temperature (125 to 225degC) FTI are presented. FTI is pliable, therefore has very low CTE which enhances the thermomechanical reliability of FHE products. Very low stress relaxation and nearly zero compression set of FTI results in no degradation of low contact pressure. The mechanical stress coupling between the interface and the component is thus greatly reduced. When used in skyline matching thermal interface applications, such pliable low compression TIM also minimize the stress interactions between components on the circuit board. Such attributes make FTI ideal suited for a variety of applications starting from flexible hybrid electronics (FHE), automotive, energy storage, consumer electronics, industrial, networking and datacenter, aerospace and space applications. We present examples of thermal simulation and design implementations in automotive power conversion using IGBT, lidless FPGA demonstrating the superior performance of FTI Carbon Fiber Technology. Temperature reduction by more than 10degC are realized in applications replacing Thermal Grease and other Silicone-based materials. Carbon fiber-based thermal interface materials therefore fill a very essential need in the market for TIM requiring high temperature operation, low pressure, no outgassing, low thermal expansion, low thermal resistance and high reliability.
Dr. MP Divakar, Consultant
KULR Technology
San Jose, CA

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