Today's personal computers (PCs) and workstations are experiencing a rapid growth of accelerated clocking and computational speeds. PC clock speeds have progressed from the Intel Corporation 486™ microprocessor speeds of 60 and 90 MHz, to the present Pentium III™ clock speeds in excess of 600 MHz. Moore's Law generally predicts a doubling of computing power and circuit complexity every year and a half or so.
The downside corollary to Moore's Law, however, is that with doubling the number of devices in an integrated circuit (IC) consequently raises the amount of heat generated requiring dissipation. As an integrated circuit drives current between transistors it consumes power, producing waste heat that eventually transfers outward through the chip from the surface of the die. Generally, a PC chip designed for commercial use can withstand up to 150° C. Exceeding that limit, however, will cause the chip to make errors in its calculations, or perhaps fail completely.
Current solutions in heat dissipation for chips include heat spreaders, heat sinks and fans. Heat spreaders, which generally are made of a tungsten-copper alloy and are placed directly over a chip, have the effect of increasing the chip's surface area, allowing more heat to be vented upward. Similarly, heat sinks spread the heat upward through fins or folds, which are vertical ridges or columns that allow heat to be conducted in three dimensions—length, width, and height, as opposed to the two-dimensional length and width of heat spreaders.
Fans within a computer housing can further aid heat dissipation from the chip or heat sink surface by convection. The amount of heat a fan dissipates away from a chip depends on the volume of air the fan moves, the ambient temperature, and the difference between the chip temperature and the ambient temperature.
The miniaturizing of integrated circuits have generally allowed for a reduction of operating voltages, resulting in lower heat production. However, chip shrinkage also means that heat-generating devices are packed closer together. Thus, the “power density” or the amount of heat concentrated at particular spots across the chip may begin to climb. As a consequence, heat is generated faster than it can be dissipated as higher clock speeds are demanded.
For example, PCs with a 486™ microprocessor drew generally 12 to 15 watts, primarily concentrated in the processor. A power supply with an embedded fan was typically sufficient to circulate air and cool the inside of the PC chassis, while a passive heat sink could cool the processor. On the other hand, Pentium™ processors from Intel Corporation consumed about 25 watts, thus requiring more cooling means than the passive heat sink for the processor alone. Similarly, the Pentium II™ processor consumes about 40 watts, while future processors like the 64-bit Merced™ may consume up to 65 watts. Other transistor-laden components must also contend with increasing heat generation: add-on cards; chipsets; graphics chips; and high-performance dynamic random access memory (DRAM).
There is, consequently, a need for improved heat dissipation from integrated circuits.