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Chillin’ and thrillin’ with heat sinks and cold plates

(Note: An edited version of this column appeared in EETimes, April 23, 2007.)

When non-engineers hear that you're “in electronics” they often assume you simply design circuits, or just do software. They fail to realize the multidisciplinary skills it takes to put a real project together, whether it is a small product such as cell phone or a larger project such as an industrial control system. Yet almost every real design call upon other specialties, such as thermal and mechanical engineering, to be reliable and usable.

A good engineer can, at the very least, work across these disciplines simultaneously, even if not expert in them. Even if this day of super-specialization, the ability to integrate those other areas is critical to a product design which balances the conflicting marketing objectives and consequent engineering tradeoffs. For example, perhaps incurring the cost of a fan will allow use of more suitable components which, unfortunately, dissipate more heat; or maybe a little more design effort in reducing EMI at its source will allow you to go with an unshielded package, or even one without extra EMI gasketing.

Of course, each of these non-electronic areas has its own culture, tools, and techniques. For thermal management, we have heat sinks to draw heat away from the source component, as a starting effort. The heat sink can have passive convention cooling, or active forced-air cooling, usually with a fan. Minimizing the power consumed by the fan then ripples back as an electronic challenge.

But why stop with just a sink, when you can go for the plate? Using a cold plate—a metal plate with internal channels–water or other working fluid flows through to conduct heat away more effectively than convection air flow can via the heat sink. Then there are heat pipes, simple yet clever passive devices which conduct heat away from its source very effectively (but do not dissipate it). As a vivid science demo for lower-grade students, I take a heat pipe and an equivalent-size metal rod, stick both in a hot cup of coffee, and let students feel how fast the dry cool end of the pipe gets hot, compared to the plain rod!

Your thermal challenge intensifies when IC vendors put those thermal slugs under the package, to conduct heat away from the die. That seems simple enough, but it makes their problem of a hot IC into your problem of having enough surrounding PC board area, and with good airflow, to act as a heat spreader. The layout challenge really intensifies when each such IC in a tightly packed group demands several square inches of clear PC board for its own dissipation. It's the PCB equivalent of mega-mansions sited on ¼-acre lots.

When all else fails, you can go for something truly exciting such as a vortex tube chiller, which operates entirely from a high-pressure air stream. These specially shaped, tornado-like cylinders produce a cold air stream without moving parts, electricity, or refrigerant. They're a very clever application of the laws of thermodynamics and conservation of energy. If nothing else, it's fun to explain how and why they work!

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