Ultra Fine Specialty Products

J.L. Johnson, L.K. Tan, R. Bollina, P. Suri, and R.M German, “Evaluation of Copper Powders for Processing Heat Sinks by Metal Injection Molding,” Powder Metall. vol. 48, no. 2, 2005, pp. 123-128.

​

Abstract

    Powder characteristics that enable metal injection molding of high conductivity copper heat sinks are investigated. Gas-atomized, water-atomized, oxide-reduced, and jet-milled copper powders are characterized in terms of particle size, packing density, mixing torque, and impurity content. The sintering kinetics and rate of oxide reduction are investigated using die pressed samples. Densities of 93 to 96% of theoretical can be achieved for all of the powders. Oxide reduction is complete by about 900°C. One of the water-atomized copper powders is injection molded to produce a demonstration heat sink. Thermal and electrical conductivities are measured and related to iron content and porosity. Overall impurity contents for typical powders are about 0.15 wt.%, resulting in thermal conductivities of 280 to 320 W/(m×K) for MIM copper.

 

Conclusions

    Oxide-reduced, water-atomized, gas-atomized, and jet-milled copper powders can all be processed to give densities near 95% of theoretical with slow heating rates in hydrogen and sufficient holds for oxide reduction. Higher green densities and higher heating rates lead to swelling especially for water-atomized powders. Reduction of copper oxides occurs primarily between 700 and 900°C and has an activation energy of about 100 kJ/mol. Pore swelling can still hinder achieving densities above 95% of theoretical, even if oxygen levels are reduced to less than 200 ppm before pore closure.

 

    Impurities are a main factor affecting electrical and thermal conductivity. The thermal conductivity dropped significantly for powders that contained 50 ppm or more iron. The iron content is likely representative of the concentration of other impurities. Typical thermal conductivities as determined by both thermal diffusivity and electrical resistivity measurements were in the 280-320 W/(m×K) range. Elimination of the remaining porosity is estimated to give a thermal conductivity of 350 W/(m×K), which is typical of commercially pure cast copper alloys containing small amounts of silicon, tin, zinc, aluminum, and phosphorus.