Abstract: High density heatsinks for microcircuit packages are formed by first mold-pressing a composite powder of free-flowing spray-dried particles of copper and at least one other denser material such as tungsten and/or molybdenum, the proportions of which are adjusted to match the thermal expansion characteristics of the microcircuit material. The pressed compacts are then heated in a sintering furnace at 1,200.degree. C. to 1,350.degree. C. in order to effect an homogeneous distribution of the melting copper throughout the structure. The process results in a readily usable component having good thermal conductivity and matched thermal expansion that requires no further machining.