[myv65]

Not a week goes by that this subject doesn't raise its ugly head. . .

The statement that "copper absorbs heat better and aluminum gets rid of it better" can be construed as true, but only when one really understands how things work. First, let's get one thing straight, the statement only applies to an extremely narrow definition. Second, it mainly applies only to air cooled heat sinks.

If you look at the top performing air heat sinks, many have a copper base and aluminum pins or fins. This has little to do with manufacturing cost and everything to do with cooling effectiveness. When you need to transport heat with a high flux (heat per cross sectional area), conductivity is the only thing that really matters. The die pumps out a lot of heat from an area smaller than a dime. In order to move this heat somewhere else with a low delta-T you need high conductivity. Copper bottoms always beat aluminum (and copper water blocks always beat aluminum ones).

Once you get the heat away from the die you need to get rid of it. With water cooling, this means a short trip to water. Since convection via water is pretty efficient (anywhere from 10 to 20 times better than air), you don't need a whole lot of surface area. Since you don't need much surface area, the blocks can be small and you don't need to make the heat travel far.

With air cooling, convection becomes the bottleneck. Now you've got a choice. You can use a little copper and make thin fins/pins, and create a lot of surface area. Alternately, you can use a little more aluminum and create not-quite-so-thin fins/pins. Because copper is so much more dense, you get a lot less volume of metal for a given weight. So if you want a heat sink that weighs in under 500 grams, you'll only get so much surface area out of copper.

What it really comes down to is a comparison of conductivity versus density. If you made a ratio using these properties, a higher value would equate to higher "heat transfer efficiency". When you do this with copper and aluminum, aluminum's ratio is higher. Basically you can make pins fatter in aluminum such that the delta-T of conductivity through the pin is lower than for copper, all compliments of aluminum's lower density.

Lots of folks simplify this (read: don't have a clue how it works) and say the famous statement that I quoted above. There's a very good reason why you see hybrid air heat sinks, and it's the one I've tried to explain here. If you get all of it, you'll also know why it does not apply to water cooling.

Edit for a footnote:

If you have two heat sinks or water blocks of identical geometry, the copper one will perform better (and weigh a lot more), period. If you have an equal mass of both materials, you will get better results from aluminum in air cooling because of the facts stated above.