The thermal resistance Rth
Let’s start with the most important aspect, the thermal resistance Rth. The most important property of Rth is that it correlates nicely linearly with the layer thickness, while the thermal conductivity describes a completely different curve and remains anything but linear. But the experienced reader already knows all this, of course. We are interested in layer thicknesses of 200 µm and less on the CPU, and usually even 100 µm and significantly less on the GPU, depending on the bending.
We recognize that the supposed miracle pastes can score points due to the high silicone content, especially with the very low BLT values, especially the EX360 Ruby. But all of them are only mediocre when viewed as a whole, nothing more. The TF4 clearly has to take a back seat here, as you notice the low price, because bulk values aren’t everything.
I have now compared the relevant layer thicknesses from 25 to 400 µm as a bar chart for Rth. But beware, not all pastes achieve optimum values in the maximum range!
Minimum possible layer thickness
But at least I wanted to know how far you can go with a little pressure and how much a paste can still be compressed. I use the usual 60 Psi (41 N) on the measuring surface of 1 cm², which is completely sufficient and is more than what a GPU cooler, for example, achieves.
Interface Resistance
What also seems interesting is the contact resistance, in our case the interface resistance. Here you can see how well the surface of the material “clings” to the contact surfaces (IHS, heatsink). These values are also easy to compare and meaningful, as they are always the same calibrated reference blocks. Coarser degrees of grinding or a less favorable microstructure can also be a negative factor, which then influences the effective thermal resistance and thus also the conductivity, as can too low temperatures and too high a viscosity.
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