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The heat is always on

The heat is always on

Getting the heat out of electronics packages is always near the top of the list of what design engineers see as their greatest technology concerns. But unless someone can repeal the Laws of Thermodynamics, it's a problem that will always be around.

Mike Rolla, a project lead engineer for developing AlphaServers at Compaq Computer (Marlborough, MA) notes that in designing, "I work on lower-end products and want the most bang for the buck" in terms of thermal management. "We are always battling cost and acoustics, and balancing cooling and quiet if on a desktop. We still go with aluminum heat sinks if we can, as the cheapest." He adds that his designers then look at progressively more costly solutions-heat sinks with copper bases, adding a fan, and finally fluid- or vapor-based systems. "It's a balancing act," he concludes, "working on a system level to keep a CPU at a given temp, trading, say, product space, ambient temperature, voltages, and fan speeds, with acceptable acoustics-to find the sweet spot."

The MAX Clip keeps constant pressure between an electronic component and extruded heat sink fo rconsistent heat dissipation -- with clamping force ranging from 5 to 18 lbs, depending on clip shape. The small ridge in the extrusion prevents the clip from falling out aft it is snapped into position.

Fortunately for Rolla and other designers, the range of solutions to beat the heat, or rather get rid of it, is widening. Designers can start with the uncomplicated, including new heat sinks like the ones from Intricast (Santa Clara, CA) that attach simply with plastic clips rather than needing springs or metal clips using special tools.

Aavid Thermalloy's (Concord, NH) MAX Clip System(TM)has patented S-shaped clips. These snap into various extruded rail shapes and hold them against, say, power semiconductor modules that have no attachment provisions. Close inspection of the extruded rail shapes reveals a small ridge running the length of the extrusion channel, preventing the clip's curve from slipping out. The clip maintains a constant, predetermined pressure, without chance of over tightening or loosening due to vibration or thermal cycling.

Cool it down. Chris Soule, an engineer and Aavid's technical marketing manager, sees several technologies gaining wider acceptance in solving designers' thermal problems. Currently, the company is making a heat pipe system for getting the heat out of laptop computer microprocessors.

Also gaining acceptance, he notes, is active liquid cooling of microelectronics, especially in power and stationary telecom applications. Soule says, "Air is up against the wall in the ability to remove heat, as well as the noise of moving air. There is at least a 10:1 volume and area reduction going to liquid systems." Such devices employ micro-channels typically carrying a 50/50 water/glycol solution. Coupled with micro-fin heatsinks (on the order of tens of thousandths of an inch fin thickness), these systems are seeing use in applications from welders to inverters on electric powered automobiles and people movers, as well as in locomotives.

More effective heat dissipation using liquids rather than air, particularly for power applications (top), may become more widespread as power densities climb. Currently heat-pipe (phase-change) systems are being used for laptop microprocessor cooling (bottom).

Soule feels a problem in going to liquid cooling is acceptance, especially with "mixing liquid cooling with high-voltage electronics." Also, maintenance and installation are concerns, he adds, "One issue, is can you get a pump as reliable as a fan." Other options include heat pipes and thermosiphons (a loop like a heat pipe without the phase change).

A trend driving more compact power electronics, and thus upping power and heat density is the need to make equipment smaller, says Soule. With careful thermal management, such smaller robots and cables can be positioned closer to an assembly line, without the need for large, bulky water-cooled cables.

Finally, Soule notes that materials and electronic packaging improvements are in the offing to better shed heat. These include the development of composite metallics or ceramics with the thermal performance and CTE (coefficient of thermal expansion) of silicon. This would allow greater thermal compatibility and packaging of microchip dies directly on a heat sink, with less interference of intervening layers that would increase thermal resistance.

Cool Technologies

Heat dissipation device Benefit
Easy-to-attach Labor cost/time saving; minimal attachment
Heat sinks Provisions on electronic device
Heat pipes Fluid heat vaporizzation (change of state) adds to cooling effect
Liquid cooling Greater efficiency compared to air, quiet; small system size
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