It is necessary for encapsulants to have not only a suitable coefficient of thermal expansion (CTE) compatible to IC devices and a low dielectric constant to reduce the device propagation delay, but also a high thermal conductivity to dissipate large amounts of heat from power-hungry, high-speed IC and highdensity packages. Fillers such as silica have been mixed with polymers to improve their properties. Aluminum nitride (AlN) is considered as an alternative one, because it has a higher theoretical thermal conductivity of B320 W/mK 1 , a compatible CTE with silicon chips and a low dielectric constant. Commercial AlN fillers are angular in shape, because they are prepared via grinding coarse AlN powders synthesized by direct nitridation of aluminum metal and classification. The angular AlN are not expected to have high fluidity when mixed with polymers and hence low packing density. Recently, we successfully obtained single-crystalline spherical AlN fillers. Furthermore, polymer composites filled with the spherical AlN showed excellent thermal conductivity (48 W/mK) as encapsulants for dissipating the heat generated in electronic devices.

The thermal conductivity of an AlN and an alumina-particulate-filled epoxy resin was investigated as a func tion of their volume content ratios. AlN composite exhibited thermal conductivity of 7.15W/mK at 68.5 vol% filler content but that of alumina composite was lower than 2.68W/mK even if the filler content was as much as 63.8 vol%. The thermal conductivity of the composites is strongly dependnt on that of the filler. In the AlN filler properties, the particle size of the filler used affected the thermal conductivity of compo sites, and composites with higher thermal conductivity could be obtained by using filler with large particle size. The thermal conductivity of composites is governed by the number of resin layers on the surface of an AlN particle. The composite thermal conductivities decreased with increasing the oxygen content of the filler. The thermal conductivity of an AlN particle depends on the oxygen content of the particle itself and the decrease of the composite thermal conductivity with the increase of the oxygen content of the filler is due to the increase of the aluminum oxide layer thickness produced on the surface of particles. In the ther mal conductivity model, the measured thermal conductivity of AlN-ground-powder-filled composites dis agreed with the ones calculated using Bruggeman's expression. However the measured values of alumina spherical-powder-filled composites agreed with the calculated ones. It is considered that the results depend ed on the sphericity of the used powder.

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Aluminum nitride (AlN) raw powder prepared by direct nitridation of metallic aluminum powder was ground by wet ball-milling in order to obtain submicron fine powder for low-temperature sintering. The effects on the properties of the milled powders of milling conditions such as ball size, ball material, chemical species of the grinding fluid and water content of the grinding fluids were investigated. A larger grinding rate was observed using medium balls with diameters of less than 5mm and grinding fluids with higher di pole moments such as 2-propanol and acetone. It was also indicated that SiAlON balls showed the least grind ing wear, and that an increase in the water content of the grinding fluid increased the oxygen content of the milled powder. Fine AlN powder with submicron size and sharp particle size distribution was obtained by ad justing the milling conditions. This powder showed better sinterability and higher thermal conductivity than that obtained by the usual dry grinding method.

Aluminium nitride (AlN) powders with 0-1.2mass% of carbon and 5mass% of Y2O3 addition were sintered at 2023 and 2073K under a nitrogen atmosphere, and the sintered samples were characterized by their oxyg en content, composition of the secondary phase, micro structure, density, and thermal conductivity. With an increase of carbon addition, the oxygen content decreased linearly and the secondary phase changed from aluminium-rich to yttrium-rich. The microstruc ture, density and thermal conductivity of the sintered samples were also found to depend strongly on the oxy gen content. The correlation between the oxygen con tent and the sintering behavior of AlN with Y2O3 sinter ing aid has been discussed.