Understanding the benefit of adding a vapor chamber to a heatsink

[jt783]

I am currently researching heatsinks for IC cooling. I understand that a vapor chamber will help spread the heat out through the base of the device. But when considering the R-value of the system, doesnt adding a vapor chamber only increase the R-value of the system? I would understand removing some of the base to replace with the vapor chamber, but many resources only add them as an addition.

If a vapor chamber is added to the bottom, wont the TIM material's R-value also increase the R of the system.

Will any heatsink perform be better if a vapor chamber is placed on the bottom?

 

[IRstuff]

I think the short answer is not necessarily. I'm assuming that by "vapor chamber" you are referring to something that allows boiling to occur on the hot side and condensation to occur on the cold side. In cases where such phenomenon could occur, which requires a certain temperature profile, then the answer is probably. The operation requires a sufficiently large delta temperature and a fluid that can be boiled and condensed within those temperatures.

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[moltenmetal]

I think the word you're looking for here is "heat pipe". Heat pipes are commonly used in electronics cooling applications to do a couple things: 1) convey heat away from a chip surface at a lower delta T than would be possible with a conductor like solid copper, and 2) to move heat from the heat generation location to a more distant heat removal location (i.e. a heat sink near a laptop exhaust fan) far more effectively (i.e. at lower delta T) than is possible with any solid conductor.

The boiling/condensing heat transfer possible in a heat pipe has the benefit of occurring at closer to a single temperature- not truly a single temperature because the system is closed and the boiling fluid rides its saturation curve. Accordingly, it outperformas any solid conoductive material you can buy.

Note that heat pipes work best, or most easily, when the boiler (heated portion) is below the condenser (heat removal heatsink). However, the use of a sintered material inside the heatpipe to act as a wick can actually make it possible for them to work in other configurations, i.e. side by side such as in a laptop.

 

[Compositepro]

Heat pipes can transfer up to 50,000 times the heat flow rate compared to an equivalent piece of solid copper. However, it still represents a resistance to heat flow. If the vapor chamber extends up through the fins, then it will greatly increase the heat flow rate. If this chamber is simply a gap between the top and bottom surface of the heat sink, then it will impede the flow of heat across this gap compared to no gap. The only benefit to such a vapor chamber would be as a heat spreader. If there are hot spots in contact with a large area heat sink the vapor chamber will greatly aid in transferring heat horizontally to cooler areas where it can then transfer to more fins.

 

[IRstuff]

vapor chambers are distinct from heat pipes. HPs provide essentially point extraction of heat, while vapor chambers are more for larger areas and, as stated above, for heat spreading.

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