Jakubowska M. Technology of High Thermal Conductivity Materials Based on Cubic Boron Nitride (c-BN) for Thick Film Structures

The investigation of new materials of high thermal conductivity becomes a serious need because of increasing integration of electronic devices together with their increasing miniaturization. Thick film materials (pastes) based on cubic boron nitride micropowder, bismuth-boro-silicate glass and ethylcellulose organic vehicle were elaborated. This technology is a novel world solution. The best results were achieved when the pastes consisted of 67.5 wt. % of c-BN micropowders and 7.5 wt. % and 25% of organic vehicle. The pastes were elaborated in two versions: airfireable when deposited on alumina, LTCC foil, silicon and for firing in inert atmosphere (nitrogen) when deposited on metal substrates. The parameters of the firing process was also optimized in respect of layers properties. The layers obtained from these materials were dense, with 5% open porosity and 15% of closed porosity, and good adhesion to the substrates. This was confirmed by SEM, X-ray analysis, TV-picture analysis, Ramann spectroscopy,IR and EPR. The layers exhibit dielectric properties e = 32, tgd = 0.12 and resistivity 13x103 MW and high thermal conductivity in the range 80-150 W/mK for the layers of 100 µm thick depending on the kind of powder and the glass content. The synthesis of c-BN micropowders was also elaborated. The micropowders of cubic boron nitride were synthesized in high pressure/high temperature technique. The synthesis was carried out at temperatures about 1450 oC and under pressures about 4-5 GPa via phase transformation in hBN (hexagonal boron nitride) precursors of different types in presence of different sorts of "cathalysts" additionally to nitrogen rich Li3N. The cubic structure of the powders was confirmed by X-ray and IR analysis, Ramann spectroscopy and EPR. The composition of glass and organic vehicle was also elaborated. The c-BN layers should show good compatibility with other thick film layers situated over and under c-BN layer in multilayers, especially conductive and resistive ones. Elaboration of these compatible resistive and conductive pastes was necessary to broaden the range of applications of c-BN layers. The conductive pastes: silver, palladium-silver, platinum, platinum silver and gold were elaborated. The pastes are lead and cadmium free fulfilling the rules of EC.