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Cathar · 08-27-2003, 05:32 PM

A few points to make on the thermal paste and base-flatness.

1) The White Water and Cascade blocks that I used for testing were both lapped by me at the same time. I actually lapped each block twice. I did not use the block that was sent to BillA.

2) The raw thermal conductivity of a thermal paste is NOT the way to predict the thermal conductivity through a thermal interface. Using this conductivity assumes a 100% molecular interaction between both surfaces and all molecules of the thermal paste.

Here's an example. Copper has a thermal conductivity of ~395W/mK, far better than the ~9W/mK of AS3, so why not just lap flat a piece of copper and stick that in between the CPU and the heat sink?

The answer, of course, is spreadability. However, and I posit, that it is very wrong to assume that even AS3 will give you anything like a 100% efficient thermal contact patch. We know this to be true because AS3 "settles" over time, sometimes picking up 2-3C for a ~100W CPU over a week or so. However, is it 100% efficient by the time it's finished settling? A better question perhaps is "is the silver micronised down to the point of individual molecules?".

I still believe Bill's value of ~0.08-0.10C/W for the thermal interface layer. Yes, Bill's thermal probe was about 1mm below the surface, but the heat flows through the copper in a fairly linear gradient when talking about distances this small, and the graph I saw was plotting the gradients either side of the measurement points and extrapolating from there. There were some small anomalies, hence the "uncertainty", but to date it is the best evidence I've seen to quantify the size of the thermal barrier, unless someone has some better real testing to show.

08-27-2003, 05:32 PM	#70
Cathar Thermophile Join Date: Sep 2002 Location: Melbourne, Australia Posts: 2,538	A few points to make on the thermal paste and base-flatness. 1) The White Water and Cascade blocks that I used for testing were both lapped by me at the same time. I actually lapped each block twice. I did not use the block that was sent to BillA. 2) The raw thermal conductivity of a thermal paste is NOT the way to predict the thermal conductivity through a thermal interface. Using this conductivity assumes a 100% molecular interaction between both surfaces and all molecules of the thermal paste. Here's an example. Copper has a thermal conductivity of ~395W/mK, far better than the ~9W/mK of AS3, so why not just lap flat a piece of copper and stick that in between the CPU and the heat sink? The answer, of course, is spreadability. However, and I posit, that it is very wrong to assume that even AS3 will give you anything like a 100% efficient thermal contact patch. We know this to be true because AS3 "settles" over time, sometimes picking up 2-3C for a ~100W CPU over a week or so. However, is it 100% efficient by the time it's finished settling? A better question perhaps is "is the silver micronised down to the point of individual molecules?". I still believe Bill's value of ~0.08-0.10C/W for the thermal interface layer. Yes, Bill's thermal probe was about 1mm below the surface, but the heat flows through the copper in a fairly linear gradient when talking about distances this small, and the graph I saw was plotting the gradients either side of the measurement points and extrapolating from there. There were some small anomalies, hence the "uncertainty", but to date it is the best evidence I've seen to quantify the size of the thermal barrier, unless someone has some better real testing to show.