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 a thermal conductivity up to 700 W/mK combined with a coefficient of thermal expansion (CTE) of 7-8 x 10-6/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.