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Stress Minimization during Deflection of Thermally Conductive Gap Pads
Keywords: Gap Pad, Pressure, Deflection
Highly compliant thermal gap pads are essential in applications where heat transfer is required but high pressures can not be tolerated. An assembly typically consists of a gap pad between heat generating and heat dissipating components. Compressive force is applied to the assembly using screws or clips, causing deflection of the pad as the distance between the components decreases. This deflection allows the gap pad to effectively fill both micro and macro voids between the components, which maximizes thermal transfer through the system. However, this deflection also causes stress build up to occur due to the pad’s inherent resistance to deflection. This stress can damage delicate components of the assembly and therefore should be minimized. A critical step to minimize pressure build up is to choose a gap pad that is highly compliant. There are several tests commonly used in the thermal industry as a measurement of pad compliance. Often it is not understood exactly how each test is performed, what the results mean and most importantly, which test is applicable to characterize pad performance in the application. By way of example, this paper will explain and compare common test methods used to characterize compliancy and deflection characteristics of gap pads. Examples of discussed test methods include ASTM D575 “Rubber Properties in Compression” and ASTMD 2240 “Durometer Hardness”. However, choosing a truly compliant gap pad is only one step in minimizing peak stress during assembly. By understanding how stress-strain characteristics of a pad vary with changing test conditions, a thermal engineer has the ability to change pad design characteristics and control assembly parameters in order to minimize peak pressure. In this work, stress-strain measurements were compared for several sets of assembly conditions to determine the extent which they affected the stress levels reached during the test. Assembly conditions investigated include strain rate, deflection stepping, components’ surface finish, gap pad size, shape and placement, and size relationship to mating components.
Karen Bruzda, Technology Leader
Laird Technologies
Cleveland, OH

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