Chemical spray offers new way to remove heat from boards

July 1, 1998
CLARKSTON, Wash. - Engineers at Isothermal Systems Research in Clarkston, Wash., are offering a new way to remove heat from VME backplane enclosures, which they say sets their approach apart from traditional conduction and convection cooling methods.

By John McHale

CLARKSTON, Wash. - Engineers at Isothermal Systems Research in Clarkston, Wash., are offering a new way to remove heat from VME backplane enclosures, which they say sets their approach apart from traditional conduction and convection cooling methods.

Isothermal experts are removing heat from VME systems with a liquid chemical spray, rather than with moving air or metal heat sinks. They claim this approach is enabling electronic systems designers to increase power density, reduce vibration, and cut weight in military and commercial VME systems.

Spray cooling removes heat more than 500 times more efficiently than air cooling, Isothermal officials say. Conventional air-cooled systems are limited to a power density of about 1 Watt per cubic inch, while spray cooling can cool systems running at more than 500 Watts per cubic inch.

The coolant benefits the military not only by reducing system size and weight, but also by eliminating the excess vibration of cold-plate cooling, says R.J. Baddeley, manager of government systems at Isothermal.

Spray cooling has the capability to reduce system size and mass by factors ranging between three and five, depending on the type of electronic system, Isothermal officials claim. Moreover, with cold-plate cooling, vibration channels through the heat sink back to the board, unlike spray cooling devices where no structural path transmits vibration, Baddeley adds.

Isothermal`s spray-cool system pumps 3M Corp. Fluorinert fluid from a reservoir to atomizers, which spray the fluid onto the boards. Then the fluid evaporates, re-condenses on the walls of the enclosure, and runs back down into the reservoir. Then the process begins again, Baddeley explains.

The Fluorinert mists the electronics, whether bare die, packaged devices, or entire circuit board assemblies, and coats surfaces with a thin liquid film that evaporates at constant temperature.

Vapor then moves the heat to an optimized heat exchanger (the enclosure wall), where the system delivers sufficient air without the drawbacks that high air velocities may cause.

Fluorinert is an inert dielectric fluid that is nontoxic, nonflammable, and will not harm the device.

If a pump breaks, a backup pump takes over, and a light on the outside of the enclosure tells the operator the pump needs fixing, Baddeley says. A chemical filter, meanwhile removes particles and oils that otherwise might clog the atomizers, Baddeley adds.

Physical damage to boards and components from the effects of mismatched coefficients of expansion or from thermal shock also does not occur because the fluid maintains the entire board at one temperature, he says. "It`s like boiling water on a stove; once the boiling point is reached the water may continue to make more bubbles but the temperature remains the same," Baddeley explains.

Liquid-cooling technology makes sense for mission-critical applications because its closed loop system keeps contaminants out of the enclosure, says Ray Alderman, executive director of the VME International Trade Association in Scottsdale, Ariz.

Fluid also has the ability to seep through tight spaces on boards and eliminate hotspots better than moving air, which may not be able to get under mezzanine cards because of component density, Alderman says.

The Isothermal technology`s only drawbacks are potential leaks if the board is not sealed properly, and possible incompatibility of the Fluorinert with the plasticizers in the board`s elastomers, Baddeley says. Elastomers are rubber components such as gaskets, and plasticizers are what make the components soft, he says.

Isothermal officials face a tough challenge to get their spray-cooling technique designed into military systems, says Donald Tilton, president of Isothermal. "The problem is that the military environmental control system infrastructure for harsh environment applications is based on the core principle that the electronic devices must be repackaged to withstand harsh environmental conditions."

Still, Isothermal is finishing up the second phase of a Small Business Innovative Research project from the Naval Air Warfare Center in Indianapolis to use the spray cooling for future avionics.

Officials of the U.S. National Security Agency and other members of the intelligence community also are using the spray cooling technology, Baddeley says.

Spray cooling enables designers to package electronic components closely together to enhance speed and performance, while reducing size and weight. Spray cooling naturally provides environmental isolation and EMI shielding necessary for many applications, Baddeley says.

Liquid cooling removes the heat more efficiently, but what do you do with the heat once it is removed, Alderman asks. If the heat leaves the enclosure in a critical military application and directly enters the environment it could create a heat bloom, which makes the application, whether it is a tank, submarine, or airplane, vulnerable to enemy sensors, he says.

Isothermal`s Silent Server and the Spray Air Transport Rack (ATR) are two spray-cooling chassis with potential military applications, company officials say.

For more information on spray cooling and Isothermal contact Baddeley by phone at 509-758-2613, by fax at 509-758-1280, by mail at 511 Third Street, Clarkston, Wash., 99403, by e-mail at Baddeley@spraycool. com or on the World Wide Web at

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