Determination of optimal filler volume fraction for thermal interface materials in electronics cooling applications

The present study reports on an investigation aimed at the determination of the optimal filler content in thermally conductive interface materials. These materials are used in electronics cooling applications to mitigate contact resistance. The finite element method (FEM) has been used to show that a non-extreme optimal value of filler volume fraction exists in thermal interface material (TIM) pads if the deformation characteristics, in addition to thermal considerations, are taken into account. The optimal filler volume fraction is unique for each filler and matrix material combination, arrangement of filler particles in the TIM pad, and filler particle size. For the set of materials and particle shape, size and arrangement considered – spherical Al₂O₃ particles hexagonally distributed in a silicone matrix – the optimal filler volume fraction is found to be close to 0.5. Also, it is shown that a smaller particle size leads to a larger thermal conductivity and contact conductance. Furthermore, a hexagonal filler packing arrangement leads to a lower thermal conductivity but higher contact conductance as compared to square packing. However, the optimal filler volume fraction identified by this approach may not lead to enhancement in contact conductance for all contacts. The net increase in contact conductance also depends on other contact parameters including roughness of the contacting surfaces, thickness of the TIM and pressure across the contact. The optimal filler volume fraction leads to maximum enhancement in contact conductance for large surface roughness, small TIM thickness and high contact pressure.