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How heatpipes and vapor chambers work

AMD TFE 2008 A quick technical primer
Fri Oct 31 2008, 08:50

WHAT IS A heatpipe, and more to the point, how is it different from a vapor chamber? Since much of AMD's Technical Forum and Exposition (TFE) was about heat and cooling, we thought we would bring you up to date on the technology before we went into the nuances of every different iteration.

You all know what they look like from the outside, copper tubes that go from the bottom of some very expensive and odd-looking heatsinks to the fins up top. Some have a couple, others have eight or more, but they are all basically the same bent copper tube.

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Thermaltake SpinQ heatsink with heatpipes

How they work is more of a mystery, but it really isn't all that complex. The basic heatpipe is a copper tube with the ends permanently sealed. Inside you usually have a small volume of water, sometimes other fluids, but what you get initially is all that there is, hence the sealing. The inside is under very low pressure so the water boils with very little heat input, and cools quite rapidly as well.

The water boils, moves down the tube, hits a cool spot, and then condenses. The heat is transferred by the vapor from the hot spot, usually a CPU or a GPU, to the cool spot, the fins or heatsink. Simple so far, but how does the water get back from the cool spot where it condenses to the hot spot where it is needed? If you are thinking gravity, you are only partially right, it can work via gravity, but that is a minor portion of the flow. Also, if you rely on gravity, make sure the geek installing your heatpipe doesn't angle the computer wrong or things get very ugly.

The big secret to heatpipes is that the inside is coated with a wick. No, not a little cloth bit rescued from a candle, but it works in a similar way. The water pooling in the cool spots is wicked back down to the hot spot where it is boiled off. Rinse, wick and repeat. The amount of water in the heatpipe is only enough to wet the wick fully, nothing more. If you are unclear on the concept, get a glass of water, and put one end of a napkin in it. Watch the water climb the napkin. Magic. Burn the sorcerers once you are done, 'theories' like this are dangerous thought crime.

The wick itself can be made of many different substances, many different form factors, and several combine approaches. The most common is simply copper powder. You put copper in the tube, make it stick to the inside somehow, and heat it until it becomes one with the metal tube. Instant wick.

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Copper powder in a heatpipe

Instead of sintered copper powder, you can use other materials, the form factor is more important than what it actually is. You can also use a mesh to do the same job, think of a piece of cloth woven from metal, but once again, you can use anything you want. The mesh acts just like the napkin mentioned above, and if you think about it, paper is more or less woven wood fibers.

Both methods have benefits and drawbacks. Powder is hard to apply evenly, but works really well. Then again, it may flake off if you bend the tube. Meshes stay in place and are much easier and less costly to apply, you just slide it in the tube and seal the ends. They also are much less efficient, but meshes don't flake when you bend them, and can provide mechanical support.

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Vapor chamber on an R770

All of this applies not only to heatpipes, but vapor chambers as well. What is the difference? If you look at a heatpipe, it has about the size of a drinking straw. If you flatten it to a thickness of about 5mm, you have a vapor chamber. The technology is almost entirely the same, and the line that divides one from the other changes depending on who you talk to.

So, why use one and not the other? Cost and form factor are the real reasons. If you can put a big flat plate in your device, you might want to use a vapor chamber. If you are more cost sensitive, use a heatpipe or four. They do the same job, moving heat from point A to point B in the same way, so use what you want.

The magic is in the details, coatings, fillings, wicks, micro-machining, assembly and cost. People will kill for a few cents in this market, so competition is fierce. At AMD's TFE, there were a lot of new technologies around heatpipes and vapor chambers. Now that you know the basic concept, it is time to dive into the nuances. µ

 

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