Thermal interface materials

In all cooled appliances, the heat from the device s heat sources must first arrive via thermal conduction at the surfaces exposed to the cooling fluid before it can be transferred to the coolant. For example, as shown in Fig. 2.2, it must be conducted from the chip through the lid to the heat sink before it can be discharged to the ambient air. As can be seen, thermal interface materials (TIMs) may be used to facilitate this process. In many cases a heat spreader in the form of a flat plate with high thermal conductivity may be placed between the chip and the lid. 

In some applications, low modulus materials are desirable. In high-power semiconductor components, for example, heat is conducted away from a sUicon die (which has a coefficient of thermal expansion (CTE) of 2.49 x 10 K ) to a copper heat sink (CTE = 16.5 X 10 K ) via a thermal interface material, or TIM, which is often... 

Fig. 14.1 Schematic diagram of a microprocessor assembly showing different levels of packaging and thermal management solution using a heat sink. TIM thermal interface material, BGA ball grid array...

Only up to 10% of heat can be removed with the cathode airflow since the water balance must be maintained. To remove the waste heat from the system additional heat sinks are needed. It was established that heat can be transferred very efficiently at the stack sides. Commercially available heat sinks were mounted at the sides using electrically isolating thermal interface materials. To add as little extra weight and volume as possible to the system, high performance pin fin coolers were used. The heat sinks are made of pure aluminum with a thermal conductivity of above 220 W m" by means of impact extrusion. 

Thermal interface materials are slurries of thermally conductive particles, usually diamond or metallic oxides, suspended in liquid or low modulus adhesive. They are designed to be flexible or slightly fluid to maintain good thermal contact during temperature cycling. 

Care must be taken so that the thermal interface material does not conduct stress to the die. 

Yu, A, Ramesh, P., Itkls, M. E., Belq arova, E., and Haddon, R C. (2007], Graphite nanoplatelet-epoxy composite thermal interface materials. J. Phys. Chem. C, 21, pp. 7565-7569. 

Liu, Y Zhang, L.-Q. Wang, W.-C. Yu, H.-T. Lu, Y.-L., Preparation of Polydimethylsiloxane-Coated a-Alumina Filler with Cold Plasma for Elastomer Thermal Interface Materials. J.Appl. Polym. Sci. 2012,123, 2875-2882. Fang, C. Shao, L. Zhao, Y Wang, J. Wu, H., A Gold Nanocrystal/ Poly(dimethylsiloxane) Composite for Plasmonic Heating on Microfluidic Chips. Adv Mater. 2012,24,94-98. 

In order to predict temperature rises in electronic components, a thermal model needs to be created, which shows all of the dissipating elements and the entire heat path. Starting at the junction of each dissipating semiconductor, the model needs to include all layers in the thermal path the die attach, substrate (if used and its attachment), package, thermal interface materials (if used), heat sink and circuit card assembly (CCA). The heat dissipated in each component, physical layout, and availability of cooling air are required to calculate the temperature-rise predictions. 

The thermal resistances for many discrete components, both active and passive, have been characterized, and their values published in data sheets. Table 3.2 lists the properties of various thermal interface materials. For custom components, a thermal model needs to be created. 

DeSorgo, M., Thermal Interface Materials, in ElectronicsCooUng, 1996. Comair-Rotron, Comair-Rotron Product Data, http //www.comair-rotron.com/, accessed September 26, 2003. 

Rightley Michael J., Emerson John A., Wong C. Channy, Huber Dale L., and Jakaboski Blake. Advancement in thermal interface materials for future high-performance electronic applications Part 1, 2007, http //prod.sandia.gov/techlib/access-control. cgi/2007/070417.pdf. Accessed on January 2016. 

Frequently, thermal management solutions are built into the LGA stack. The heatsink, if constructed properly, can be used as the pressure plate. A thermal interface material such as graphite sheet or other thin thermal transfer material can be used between the heatsink and the IC package to enhance heat dissipation. 

CNTs are also widely used in actuators [168-171], The addition of CNTs to PANI fibers increased the electromechanical actuation because the CNTs improved the mechanical, electronic and electrochemical properties of the PANI fibers [172-174], Composites based on CNTs are studied for a variety of sensor applications [175-177], For example, polypyrrole or PANI deposited on single-walled CNT networks that can be used as solid state pH sensors [178], A DNA sensor was created fi om a composite of polypyrrole and CNTs fimctionalized with carboxylic groups to covalently immobilize DNA onto CNTs [179], In general, the presence of CNTs tends to increase the overall and selectivity of biosensors, The thermal transport properties of polymer composites can be improved with the addition of CNTs due to the excellent thermal conductivity of CNTs, Such composite are quite attractive for usages as printed circuit boards, connectors, thermal interface materials, heat sinks, lids, housings, etc, [92,180],... 

A thermal interface material is typically made from a polymer matrix and a highly thermally conductive filler. Thermal interface materials find three application areas in a CPU package (12) ... 

P.A. Koning, F. Hua, and C.L. Deppisch, Polymer with solder precoated fillers for thermal interface materials, US Patent 7 036 573, assigned to Intel Corporation (Santa Clara, CA), May 2,2006. 

It is clear that CNTs have many advantages over other carbon materials in terms of electrical and thermal properties. These properties offer CNTs great potential for wide applications in field emission, conducting plastics, thermal conductors, energy storage, conductive adhesives, thermal interface materials, structural materials, fibers, catalyst supports, biological applications, and ceramics and so on [35]. 

Dow Corning. Thermal interface materials, 2014. 

The thermal transport properties of polymer composites can be significantly improved with the addition of CNTs due to the excellent thermal conductivity of CNTs. Such composites are quite promising for usages as printed circuit boards, connectors, thermal interface materials, heat sinks, lids, housings, and so on. °... 

Demonstrate thermal interface materials that can withstand the high temperature and radiation while providing the thermal conductivity or isolation required... 

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