Thermal resistances at packaging interfaces are occupying a larger portion of the overall thermal resistance from a semiconductor die to ambient. Substantial improvement of the performance of thermal interface materials (TIMs) is required to achieve effective electronic cooling. This report is a brief review of the current status of TIM research. Particular focus is given to the fundamental understanding of heat conduction phenomena in polymer-based TIM composites at packaging interfaces in thin-film form. Promising nanostructured TIMs for future applications are also discussed. Increasing chip power dissipation requires lower thermal resistances at the interfaces of electronic packaging. These resistances govern the effectiveness of heat conduction from the semiconductor die to its heat sink, and eventually limit the performance of any heat sink, from conventional fin-fan structures, to advanced heat sinks such as heat pipes [1] and microchannels [2]. To reduce the interface resistances, compliant heat conductive thin layers are widely used between the semiconductor die and its cooling components. These thin layers are thermal interface materials (TIMs) (Recent reviews [3, 4]). It should be noted that the formation of the packaging interface resistances is not only due to the roughness of the contacting surfaces, but is also caused by the thermal expansion mismatch between different components. Advanced techniques (e.g. CMP) nowadays can polish copper/silicon surfaces at an atomic level, and the techniques of direct copper-to-copper and copper-to-silicon bonding are also available. However, these techniques cannot replace the use of TIMs. Thermal expansion mismatch is the primary concern along with cost. Under common working conditions, the warping action of a copper heat spreader, due to the thermal expansion mismatch, can form a gap in a size of 10 μm or more between the heat spreader and the silicon die. This gap must be filled with compliant and heat conductive materials, i.e. TIMs, to facilitate effective heat dissipation from the semiconductor die.
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