Processor speeds and densities are climbing, and that means the heat they generate is going to rise. Putting memory on the CPU also means that certain portions of the die will generate lots of heat, making it extremely difficult for conventional and even fanned heat sinks to reduce hot spots.
Thermacore Inc., Lancaster, Pa., is throwing cold water on that problem, coming out with a base for heat sinks that uses water to disperse heat evenly.
The Therma-Base line puts the heat sink atop a base that has a small amount of water inside. The water vaporizes when the chip heats up, condensing on the side of the base attached to the heat sink.
"High-end Xeon processors dissipate about 40 W, and the Hewlett-Packard and Alpha processors, as indicators of the future, dissipate 80 W to 100 W or more," said Dale Mehl, computer-industry manager at Thermacore. "When you put a large amount of cache RAM on the die with the processor, all the transistors dissipating that 40 W or more are shrunk to one side or the center, so a lot of heat is generated from a very small source. Thermal spreading is a challenge, so we're using water to distribute the heat evenly and avoid hot spots above those small heat sources."
Water was picked because it takes a lot of energy to turn water from a liquid to a vapor, and a lot of heat is expended when that cycle is reversed. In the heat sink, the water will vaporize above the hot spots, then vapors will condense on any cooler wall, such as the side that touches the conventional finned heat sink. This means heat will be spread evenly.
Reliability is critical, so Thermacore is using copper plates that are welded together. Copper doesn't show corrosion or other traits that could hamper performance or create leaks. In more than 17 years in life-cycle tests, water hasn't leaked out. That's well beyond the lifetime for most fanned heat sinks, which are often used to cool today's hot processors.
An additional benefit of the technique is that the finned heat sink can be shorter, a big factor in low-profile products. Currently, only a few chips need the capability provided by Therma-Base. But that's changing rapidly as densities and clock speeds rise.
"We anticipate that, for the next year or two, this will be only for custom OEM applications. But we've put together a high-volume manufacturing process in anticipation of [future needs]," Mehl said. "We can produce these at relatively low costs, following the curves of the heat sinks now used on about half the Pentium II mobile computers." He noted that the company's 3-mm heat pipes for notebooks cost $6.50 three-and-a-half years ago, "and now they're less than $1."
In quantities of 10,000, heat sinks that dissipate 80 to 100 W will cost $30 to $60. As volumes for the new technology rise to 10 million units, that product will be priced at $6 to $12.