The present invention relates generally to the field of substrates for microcircuits and more directly to a ceramic composite substrate consisting of an aluminum oxide (Al.sub.2 O.sub.3) matrix containing electrically non-conductive, thermally conductive needles of aluminum nitride (AlN) uniformly distributed in the substrate in a geometry that allows for improving the heat dissipating abilities of the substrate. The methods for removal of heat from semiconductor devices whether discrete components, integrated circuits or microcircuits, are numerous. Examples of sundry methods are shown in U.S. Pat. Nos. 4,106,188; 4,069,498; 4,024,570; 3,969,754; 3,735,485; 3,723,176; 2,982,892; 2,967,984; 2,857,560; and 2,758,261.
In the present invention it is necessary to provide an electrically non-conductive substrate for a micropackage assembly having anisotropic thermal properties. That is, the substrate has improved thermal conductivity in the short transverse direction as is required for heat dissipation from the micropackage assembly and yet maintains good dielectric strength across the surface of the substrate.