Thermal-interface materials should transfer heat efficiently, but not transfer stress. Higher-performance materials containing high-conductivity graphite are becoming available with thermal-interface resistance values approaching 0.06Ω-cm2°C/W. This puts them in the range of soldered connections, but without the associated residual stresses from the assembly process (Fig. 7).
Jim Wilson, a senior principal mechanical engineer at Raytheon Space & Airborne Systems, together with Bruce Guenin, a principal research scientist at Sun Microsystems, have carefully studied the electronics cooling challenge. They conclude that the need to minimize thermal resistances between the coolant and the heat source is ripe for innovation.
BIGGER ROLES FOR SMALLER TECH
A number of researchers are working to develop simpler MEMS-based cooling approaches. These designers are also using nanomaterials such as carbon nanotubes and thermionic wires to make heat management more cost effective, particularly at the chip and board level.
Purdue University researchers came up with a prototype MEMS micropump cooling device that's small enough to fit on a chip. Its microchannels, which circulate a coolant liquid, can be totally integrated on-chip. Pumping action is created by electrohydrodynamics, which uses the interactions of ions and electric fields to cause fluid to flow.
Earlier this year, Fujitsu announced it will use carbon nanotubes in heatsinks for high-frequency power amplifiers in next-generation communications basestations. The nanotubes replace the traditional metal bumps used in conventional "face-up" or "flip-chip" packaging that connect the amplifiers to the pc boards they're mounted on. These methods suffer from amplification inductance and are inadequate for dissipating the heat of high-power transistors.
Fujitsu's technology replaces the metal bumps with bundles of vertically oriented carbon nanotubes, grown in a proprietary process using an iron catalyst coating. Thermal conductivity levels of 1400 W/m-K have been achieved, compared to 400 W/m-K for copper material. According to the company, this results in heat-dissipation levels equal to conventional methods, but with half the inductance. In turn, it will yield at least a 2-dB increase in the amplification of 5-GHz and higher frequencies.
At NASA's Jet Propulsion Laboratory, researchers proposed cooling arrays of nanowires coated with cesium for thermionic cooling. The proposed devices could be highly miniaturized, enabling heat removal from previously inaccessible IC locations for high IC clock and power levels.
Only the highest-energy electrons are thermionically emitted in thermionic cooling. Those electrons are collected to prevent their return to the emitting electrode. Electron collection is made possible by applying an appropriate positive bias potential to another electrode placed near the emitting electrode.
Nano materials are promising for future heat management. Estimates of carbon nanotube thermal conductivity run as high as 6600 mW/m-K, and values over 3000 mW/m-K have been measured, says Carl Zweben. Yet carbon-nanotube interface characteristics need to be studied, and their application as thermal-interface materials must be practical.
Thermal-management experts agree that we're on the verge of dramatic improvements in reducing heat levels and cost-effectively removing what's left. Heat management no longer can be treated as a design afterthought. It has to be grounded in the very beginning of a device and system design and layout.