Improved Prediction of Compressive Forces Required in Thermal Interface Pad Applications
Author: Jeffrey Marcus Jennings Company: Harris Corporation Date Published: 10/13/2013
Abstract: As the electronics industry trends toward higher density surface mount assembly and components with more demanding thermal dissipation requirements, proper thermal management implementation is playing an increasingly vital role in the successful realization of these future packaging designs. A common approach to address these more stringent thermal requirements involves adding thermal interface pads between mechanical assemblies to ensure low thermal contact resistance. A key aspect in the proper utilization of such thermal interfacial pads is that the material be sufficiently compressed within the gap it fills. Only under the proper compression can a thermal interface pad approach its published thermal conductivity value. Ensuring the proper compression relies on knowing the force versus deflection relationship for the thermal interface pad material. Unfortunately, because of the complex interaction of the pad material and the adjacent mechanical interfaces, that relationship no longer scales linearly with area of the pad, making accurate prediction of the forces required to properly compress an arbitrarily shaped thermal interface pad difficult. Since attachment interfaces are usually defined early in the design process, determining that additional attachment points are needed between mechanical assemblies to accommodate thermal interface pad compression after printed wiring board layout is complete is simply unacceptable. Therefore a means for providing an accurate thermal interfacial pad compression force prediction is needed. The current work presents a methodology which enables the use of available stress-strain data for a known shape to predict the compressive forces for other select shapes and sizes through the use of a shape factor. This approach provides significantly better force predictions than using the area ratios per the stress definition ( ) and is considerably more efficient than testing for force versus deflection data for every possible thermal gap pad configuration.