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Compressible Metal Thermal Interface MaterialsPosted by Bob JarrettWednesday, August 22nd, 2007 |
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The high thermal conductivity of metals and alloys drives their use in thermal solutions. In critical heat flow situations, metals are frequently used as the thermal interface material in the thermal solution.
To take advantage of the high thermal conductivity, the surface interface resistance must be overcome. Metals in intimate contact have low interface resistance because they share electrons. These free electrons conduct electricity, but also help couple the metals thermally.
Typically, the THERMAL INTERFACE MATERIAL is melted to make liquid contact (as a solder, liquid metal or phase change metal THERMAL INTERFACE MATERIAL), but mechanical pressure can be used as well.
The compressible metal THERMAL INTERFACE MATERIAL relies on pressure to plastically deform into intimate contact. At low clamping pressures, harder metals make only localized point contacts with the surfaces of the thermal interface. The remainder of the interface is an air gap with very poor heat transfer. The area of contact (A) can be estimated using the yield point (σY) of the weakest material in the interface: A ~ F/ σY, where F is the clamping force.
This simplified picture of the heat conduction across the interface can be represented as:
where is the average conductivity, Ai are areas with intimate contact and Aj are areas with an air gap, xj thick. The thermal resistance of the interface is then which is inversely related to the applied clamping force. The contact area increases as the pressure in increased and the thickness of the remaining air gaps are diminished.
The area of contact (A) can be estimated using the yield point (σY) of the weakest material in the interface: A ~ F/ σY, where F is the clamping force. The force needed for an interface is of the order of the yield strength. For example, annealed copper yields at 4800 psi—clearly too high for electronic devices. Indium metal is commonly used for this application since its yield strength is <300 psi. In addition to the larger area of intimate contact, indium yields to allow a reduction in the thickness of the air gaps in the remaining areas.
This relationship of the interface resistance as a function of pressure for a number of soft metals is shown in the figure below:
The plastic flow model for indium and tin fits the experimental data very well. For these metals the contact resistance is predictable by the area contacted by plastic deformation.
Data for aluminum is typical of stiffer materials and does not fit the simple plastic model. For this and stronger metals, the contact force is transmitted elastically by the interface, resulting in markedly higher contact resistance.
The thermal resistance of the compressible interface formed with indium far exceeds the performance of polymer and graphite pads. In applications with >50 psi clamping force, the compressible metal interfaces outperform thermal greases and PCMs without the messy installation and rework.
Posted by Bob Jarrett at 13:38 PM (August 22nd, 2007)
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