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Characterization of thermal interface materials for thermal simulation

: Schacht, R.; May, D.; Wunderle, B.; Wittler, O.; Gollhardt, A.; Michel, B.; Reichl, H.

International Microelectronics and Packaging Society -IMAPS-, Deutschland:
Deutsche IMAPS Konferenz 2005. CD-ROM : 10. - 11. Oktober 2005, München
München: IMAPS-Deutschland, 2005
Paper 12
Deutsche IMAPS Konferenz <2005, München>
Fraunhofer IZM ()
thermal interface material; thermal interface resistance; thermal material characterisation

Thermal management of electronic products relies on the effective dissipation of heat. This can be achieved by the optimisation of the system design with the help of simulation methods. The precision of these models relies also on the used material data. For the determination of this data an experimental set-up for a static measurement is presented, which evaluate thermal conductivity of thermal interface materials (e.g adhesive, solder, foils, or pastes). It is common practice for high power applications to assemble power devices as bare dies in chip on board technique or to use flip chip assemblies with a directly mounted copper or aluminium heatsink. The thermal resistance of the thermal interface material is the bottleneck of the thermal heat flow from the active device junction to the cooler. All the more important is the fact of strongly localised hot spots, as heat spreading requires better thermal interface materials as just heat-transfer. If possible, it is useful and common to reduce the thickness of the thermal interface material. But for thinner thermal interface materials the thermal interface resistance (between the silicon and the thermal interface material as well as between the thermal interface material and the cooler material) is no longer negligible. Therefore it is essential to account for this effect in the measurement set-up. Common measurement techniques place the thermal interface material between two copper plates, heating the plate above and cooling the plate below. In this set-up the heat flow through the thermal interface material is supposed to equal the electrical power loss. The thickness of the thermal interface material is some millimetres. This approach does not account for size effects and interface resistances occurring in real set-ups. Therefore a more accurate measurement set-up is constructed and presented. This set-up contains in situ measurement of applied force, the measurement is using interfaces as in real power assembly (same interface resistance) and is universal useable for application of all marketgoing thermal interface materials. A range of different interface materials is characterised with varying thickness. Thus the thermal conductivity of the materials and the interface resistance is characterised. It is shown that the interface material can become a key issue for the thermal management of dynamically loaded devices. On this example a model with statically determined material data is validated and discussed in simulation and experiment. The paper gives an overview over the set-up and the measurement technique and discusses experimental and simulation results.