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December 2005
library  >  PAPERS  >  Theoretical/General

Thermal resistance of interface materials as a function of pressure



a very large factor in reducing thermal interface resistance is the application of pressure. even though much effort in recent years has been devoted to improving the thermal conductivity of interface materials, the thermal conductivity of a material is only a piece of the heat transfer puzzle.

 

as shown in figure 1, the thermal resistance created by the material itself (rm) for a given thickness is constant as a function of pressure; whereas the contact thermal resistances at the interfaces (rc1 + rc2) are very dependent on pressure. at low pressures of t) of a typical interface material as described in figure 1.

 

 

 

therefore the total thermal resistance (rt) is the sum of three resistances:

 

 

rt = rc1 + rm + rc2
rt = total thermal resistance
rc1 = contact thermal resistance of surface 1
rm = thermal resistance of the material itself (thickness÷thermal conductivity for a given area)1
rc2 = contact thermal resistance of surface 2

 

 

pressure acts to mate the interface material to its surfaces, minimizing the amount of air remaining at the interface, therefore decreasing the thermal resistance. air, the number one enemy to reducing contact resistance, is truly a thermal insulator with a thermal conductivity of 0.027w/m°c. increasing pressure not only reduces the thermal contact resistance, but can reduce rm of soft materials by reducing the material thickness.

 

the amount of pressure required to reduce thermal resistance is a function of the interface material's compressibility and surface properties. the surface properties of the mating surfaces are also a factor.

 

in today's electronic systems the use of excessive pressure can create stresses detrimental to a system. for example, delicate leads and solder joints can be broken and circuit boards can lose their planarity from excessive pressure. in most cases pressure over 50 psi is damaging. often 10 psi is all that can be tolerated. therefore, the need for very soft, compliant materials with high thermal conductivities are essential for low total thermal resistance at low pressures. figure 2 shows the thermal resistance versus pressure for insulating interface materials with a thermal resistance of 0.20°c-in2/w or less at 50 psi.


unfortunately the pressure used to acquire reported thermal resistance values is often not stated by vendors. the values reported are often measured at pressures well in excess of 100 psi, much higher than would be practical in real applications. an international standard for characterizing interface materials is definitely needed.

 

*1 sq. in. = 6.45 sq. cm. 1 psi = 0.0069 mpa

 

1. steinberg, dave s., cooling techniques for electronic equipment p.48, 1980.


 

carol a. latham
thermagon, inc.
cleveland, ohio

 

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