A variety of methods are known for dissipating heat generated by semiconductor devices. Such methods typically entail the use of a thermally-conductive substrate, such as beryllium oxide (BeO), or a substrate modified to promote its heat-conduction capability. Significant disadvantages with the use of BeO substrates include the relatively high cost of manufacture and the toxic nature of BeO. In the case of a substrate modified to promote its thermal conductivity, thermal management is usually achieved by dissipating heat primarily in the vertical, or "z" direction, beneath the semiconductor device. For example, heat-generating semiconductor chips, such as power flip chips, are often mounted to alumina substrates that conduct and dissipate heat in the vertical direction away from the chip. These designs are limited in their ability to dissipate heat laterally, i.e., in the "x" and "y" directions, because the thermal conductivity of alumina is low compared to metals and metal-containing materials.
Another approach to dissipating heat from power semiconductor devices entails the use of ultra-thick thick films (UTTF), as taught in U.S. Pat. No. 5,527,627 to Lautzenhiser et al. Whereas conventional thick films are generally limited to thickness of up to about 0.001 inch (about 25 micrometers), Lautzenhiser et al. teach an ultra-thick thick film conductor having a thickness of about 0.005 inch (about 125 micrometers) or more, and therefore more capable of conducting heat laterally, in the x and y directions, from a heat-generating semiconductor device. However, the use of ultra-thick thick-film conductors is not always practical, as is the case when the heat-generating device is a flip chip, and particularly power flip chips that operate at power levels of 5 watts and higher.