Use Of Thermal Interface Materials Research Articles

Optimization of freeze granulation and sintering behavior of MgO granules for thermal interface materials

Heat management of high-performance secondary batteries, used in various applications that require high charging and discharging rates, such as electric vehicles and electronic devices, has recently been attracting increased attention. One of the most popular heat management technologies involves the use of thermal interface materials (TIMs) for heat dissipation. TIMs are composites of thermally conductive granulated fillers uniformly dispersed in a polymer matrix. In this study, the freeze granulation process is optimized to prepare MgO granules with high thermal conductivity as an alternative to commercial alumina fillers for TIMs. The heat dissipation characteristics of TIMs are directly related to their thermal properties, size distribution, shape, density, and filler content. Therefore, a suspension is optimized with high solid content and low viscosity for proper spraying. The size distribution and sintered granule density are analyzed for various spraying distances and pressures to optimize the process for producing high-quality TIMs. Finally, a TIM with MgO granules formed by freeze granulation dispersed in silicone-based resin is fabricated, with a high thermal conductivity exceeding 5.425 W/m∙K (with an interfacial thermal resistance of 0.343 K∙ cm2/W) and low density of 2.78 g/cm3.

Radio frequency-assisted curing of on-chip printed CNT/silicone heatsinks produced by material extrusion 3D printing

With the rapid development of high-power integrated electronic devices, many polymer-based thermal management devices have been developed to address the problem of overheating and to improve the reliability and lifetime of electronic devices. Here we demonstrate the material extrusion 3D printing of carbon nanotube (CNT)/silicone heatsinks directly onto electronic devices. CNTs were used as a conductive nanofiller and a rheological modifier to improve thermal and electrical conductivities and the printability of the silicone inks, respectively. Additionally, CNTs are also a radio frequency (RF) susceptor, so the integration of CNTs into the silicone matrix allowed for rapid out-of-oven curing by applying RF heating. Specifically, at a CNT loading of 14 wt%, the CNT/silicone heatsinks could be printed and RF cured with shape retention; the heatsinks’ performance was comparable to that of commercial heatsinks. More importantly, the CNT/silicone heatsinks could be printed and cured directly onto a chip attached to a circuit board. This printing approach can eliminate the use of thermal interface materials and the step of heatsink assembly while reducing the number of thermal interfaces between the chip and the heatsink. The results suggest that material extrusion 3D printing combined with RF-assisted curing can be an effective approach to fabricate high-performance polymer-based heatsinks with reduced production time and energy requirements.

Theoretical and experimental evaluation of thermal interface materials and other influencing parameters for thermoelectric generator system

Thermal interface resistance of Thermoelectric Generator (TEG) plays a vital role in power production. Improving surface finish of contact surfaces, applying pressure between the contact surfaces and use of Thermal Interface Material (TIM) are few methods of reducing thermal resistance and thereby improving the efficiency of TEG. There is a need to evaluate the influence of these methods and use them optimally for TEG system. Experiments were carried out to study the influence of parameters such as thermal conductivity of TIM, contact pressure, surface roughness and heat source temperature on the voltage and power outputs from TEG. Experimental results are validated with simulations using mathematical heat transfer model and COMSOLTM Multiphysics numerical model. Appreciable agreement is seen between the experimental observations and model outputs. Experimental and model results indicate 0.6 W/mK as optimum thermal conductivity for TIM material. Hence, use of costly TIMs like MWCNT (Multi Wall Carbon Nano Tube) and copper nanoparticles may not be required for the selected application. The contact pressure and surface roughness have appreciable influence when air is used as TIM. These factors have insignificant influence for TIMs with higher thermal conductivity. Increase in heat source temperature increases voltage and power output of TEG.

Metallic TIM Testing and Selection for IC, Power, and RF Semiconductors

Metallic TIM Testing and Selection for IC, Power, and RF Semiconductors

Effect of Load and Interface Materials on Thermal Contact Resistance between Similar and Dissimilar Materials

The thermal contact resistance (TCR) between similar and dissimilar materials (copper, aluminium, brass and type 304 stainless steel) was assessed. Copper foil, aluminium foil, lead and Sn-9Zn lead free solder were selected as thermal interface materials (TIMs). The interfacial contact pressure was varied by application of load on materials in contact. The interfacial heat flux was estimated by solving the inverse heat conduction problem (IHCP). At higher load, the temporal variation of thermal contact resistance showed early occurrence of peak and lower values of TCR. The use of thermal interface materials reduced the contact resistance between the hot source and cold sink materials. Among the interface materials used, Sn-9Zn lead free solder showed the lowest contact resistance. This was attributed to the solid to liquid phase transformation at higher temperatures and the conformance of the interface material to surfaces in contact. Factorial experiments were carried out to determine the significance of experimental variables on TCR. The analysis showed that the effect of applied load on TCR was significant compared to other parameters.

Transient Liquid Phase Sintering (TLPS) Conductive Adhesives for High Temperature Automotive Applications

Power electronics products such as inverters and converters involve the use of Thermal Interface Materials (TIMs) between high power packages and a heat exchanger for thermal management. Conventional TIMs such as thermal greases, gels, solders and phase change materials (PCMs) face challenges to meet the need of these products to operate reliably at much higher temperatures. This has driven the development of new TIMs such as Transient Liquid Phase Sintering (TLPS) Conductive Adhesives. TLPS adhesives have been developed for many potential applications due to various advantages like lead free, flux-less and particularly their low temperature processability , which enables the use of heat sensitive components in the design. With all these motivations, a project was launched and completed to assess TLPS adhesives as a unique TIM for high temperature automotive applications due to its high bulk thermal conductivity and metallic joint formation at interfaces. This paper reports the evaluation of three different TLPS conductive adhesives with different formulations. All the TLPS adhesives contain lead-free eutectic SnBi fillers, together with other fillers with much higher melting point (Tm) so as to process them using low temperature profiles. Different processing profiles were applied to process different TLPS adhesives. After processing, the assemblies could meet high temperature application requirements due to the shift of Tm of SnBi alloy from 139°C to a much higher value >200°C. Three different heat exchanger materials including bare Cu, Al and AlSiC with solder wettable surface finishes were assessed. X-ray was used to check void formation and Scanning Electron Microscopy (SEM) was conducted on the cross sections to study the quality of the whole metallurgical network. Die shear force was used to verify the joint strength. Thermal resistance measurement was carried out to verify thermal performance of TLPS assemblies. Thermal cycling test (−40 °C to +135 °C) had been performed to assess the reliability performance of the assemblies. TLPS samples built on Al and Cu substrates exhibited higher thermal resistance due to higher mis-match of coefficient of thermal expansion (CTE) with that of AlN, while TLPS samples assembled on AlSiC substrates were much more reliable and showed low thermal resistance throughout the thermal cycling test.

Revisiting the block method for evaluating thermal conductivities of clay and granite

Determination of thermal conductivities of porous media using the contact method was revisited and revalidated. Thermal conductivities of granite and clay were determined in the laboratory with and without the use of thermal interface material (TIM) (Arctic Silver®) to reduce contact resistance. KD2 probe was also used with and without TIM to compare results. Thermal conductivity of dry clay sample increased from 0.68 W/mK to 0.85 W/mK while that of granite sample increased from 2.95 W/mK to 3.95 W/mK with TIM. The difference in thermal conductivities with and without TIM was significant at ( P > 0.05).

The effect of thermal contact resistance on heat management in the electronic packaging

A finite element analysis is carried out in order to investigate the role of thermal contact resistance on heat management within a simple central processing unit (CPU)/heat sink assembly. A special attention is paid in assessing the effect of surface roughness characteristics, mechanical and thermal properties of the contacting bodies, applied contact pressures and the use of thermal interface materials on the maximum temperature experienced by the CPU. Two classes of thermal interface materials: (a) phase-change materials and (b) acrylic- or silicone-based tapes are considered. The results clearly reveal that plastic deformation of micro-contacts (promoted by high contact pressures and lower micro-hardness levels) and the use of thermal interface materials which eliminate (high thermal resistance) micro-gaps can significantly lower the overall CPU/heat sink thermal contact resistance and facilitate heat management. It is also shown that the retention of asperity micro-contacts and good wetting of the mating surfaces by the thermal interface material are critical for achieving an effective removal of the heat generated by the CPU.

Optimization of Thermal Interface Materials for Electronics Cooling Applications

Thermal interface materials (TIMs) are used in electronics cooling applications to decrease the thermal contact resistance between surfaces in contact. A methodology to determine the optimal volume fraction of filler particles in TIMs for minimizing the thermal contact resistance is presented. The method uses finite element analysis to solve the coupled thermo-mechanical problem. It is shown that there exists an optimal filler volume fraction which depends not only on the distribution of the filler particles in a TIM but also on the thickness of the TIM layer, the contact pressure and the shape and the size of the filler particles. A contact resistance alleviation factor is defined to quantify the effect of these parameters on the contact conductance with the use of TIMs. For the filler and matrix materials considered-platelet-shaped boron nitride filler particles in a silicone matrix-the maximum observed enhancement in contact conductance with the use of TIMs was by a factor of as much as nine.

Thermal conductance of Cu/Cu and Cu/Si interfaces from 85 K to 300 K

The thermal conductance of copper-copper and copper-silicon interfaces has been measured between 85 K and 300 K. Bare surfaces and interface containing various thermal greases and thermally conductive elastomers have been evaluated. The use of thermal grease was found greatly to enhance thermal conductance of copper - copper interfaces at all temperatures. The use of thermal interface materials at the silicon - copper interface may or may not contribute to thermal conductance throughout the entire temperature range owing to a glass transition in the grease materials and the shear force created between silicon and copper by the different thermal contraction of the two materials as temperature is reduced.