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In-situ die attaching techniques for power electronics applications like laser diodes, microwave emitting diodes and microprocessors operated at high cycle rates frequently must allow loss power dissipation exceeding 100 W/cm2 at operation temperatures as low as possible. In this field new packaging concepts are required using substrates with enhanced thermal performance.A promising attempt to improve the heat transfer coefficient between heat source and heat sink significantly is the development of silicon substrates structured with microwhiskers perpendicular to the surface. However, an industrial application of this new technology as patented for heat spreaders in power electronic modules makes necessary the thermal characterization of the substrates.In this work, firstly, the peculiarities of these innovative heat spreaders including the manufacturing process are highlighted and secondly, a new method for determination of 1999 IEEUCPMT lnt'l Electronics Manufacturing Technology Symposium 0-78035502499 $10.00 0 1 999 IEEE 426 their thermal qualities based on laser heating (using a 120 W rated Nd:YAG laser), surface temperature measurement (by thermovision and with miniaturized temperature sensors), and reverse modeling (using a new simulation tool for models with high-node numbers) is demonstrated. Results obtained from heat spreaders with microwhisker treatment are compared with those from reference samples with a polished surface. In this way it is found that the heat transfer coefficient can be improved by up to 100%. Based on these results prospectively possible applications and limitations for power electronics assemblies are derived.