Fast hot pressing of diamond composites for electronics' cooling applications

For microelectronic circuits, the main type of failure is thermal fatigue, owing to the different thermal expansion coefficients of semiconductor chips and packaging materials. Therefore, the effective thermal management is a key issue for packaging of high performance semiconductors. The ideal thermal management material working as heat sink and heat spreader should have a high thermal conductivity combined with a reduced and tailorable thermal expansion. To meet these market demands copper composites reinforced with diamond particles were fabricated by a powder metallurgical method (powder mixing with subsequent pressure assisted consolidation). In order to design the interfacial behaviour between copper and the reinforcement different alloying elements, chromium or boron, were added to the copper matrix. The produced composites exhibit thermal conductivities up to 700 W/mK combined with a coefficient of thermal expansion (CTE) of 7- 8 x 10(exp -6)/K. The copper composites with good interfacial bonding show only small decrease in thermal conductivity and a relatively stable CTE after the thermal cycling test. Fast pressing techniques, e.g. Spark Plasma Sintering (SPS), offer advantages regarding shorter processing times, higher efficiency and often better properties, compared to traditional hot pressing. But, temperature gradients in SPS must be evaluated carefully to optimise processing in larger size specimens, where the presence of temperature gradients within the sample may be detrimental for the properties of the final product. The homogeneity of the thermal diffusivity across the diameter of the sintered samples was investigated locally and depends strongly on the used punch-die-sample set-up during the field assisted sintering of copper diamond composites.