Difference in thermal pastes for lapped surfaces?

mikela78

Member
Jan 15, 2004
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The specifications for AS5 on the arctic silver website mentions thicknesses of .001 and .003 inches, and implies that this thickness is best for filling in the microscopic valleys in the heatsink surface. Before I lapped my Thermalright SP-94, 947U, and 700 heatsinks the ridges were probably about .001 to .005 inches, but now they are a couple orders of magnitude smaller and it seems a mil of AS5 is more than I need.

I've been using the thermal paste provided with my Thermalright heatsinks. Would I see a benefit using AS5 if I use so little? My theory is that most of the performance difference in thermal pastes does not depend on the ability to fill in cracks as much as thermal conductivity, and thermal conductivity is not so vital if I have decreased the distance between the CPU core and heatsink by more than an order of magnitude.
 

Zepper

Elite Member
May 1, 2001
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Very little heat transfer is done conductively. Even in contacting surfaces, the great majority of heat transfer is done by radiation. So eliminating any air cavities no matter how small is the name of the game.
.bh.
 

mikela78

Member
Jan 15, 2004
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Very little heat transfer is done conductively? That surprises me, but it sounds like you know what you're talking about.

Do you mean that the heat transfer from the core to the heatsink is due to the emission and absorbtion of photons? If so, then a heatsink in a vacuum an inch away from the core and a little larger (so that the subtended angle is the same) would conduct heat just as well as a heatsink in mechanical contact with the core, and this doesn't seem like it would be the case. Besides, doesn't thermal radiation go as T^4? If T is only 320K, that's not much radiation...

With the different thermal pastes, the ones that do better contain highly conductive materials such as silver. I just think the difference between thermal pastes would decrease if the surfaces were flat enough that very little paste is used, eliminating that third layer between the core and heatsink.
 

psiu

Golden Member
Oct 1, 2003
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The thermal paste is there exactly for the reason of filling in the microscopic holes and valleys with something better conductive than air. IF you could have two perfectly smooth or perfectly matched surfaces, you would not want thermal grease, as it simply interferes with the heat conduction.

Of course, this will not happen except in a research lab, so just put the thermal grease on :D

And yes, if you can get the contact layer smoothed down enough, then try to get that stuff on as thin as possible, while ensuring complete coverage.
 

MDE

Lifer
Jul 17, 2003
13,199
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Don't worry about putting a perfect layer on, the excess will squeeze out leaving you with a perfect layer.