The thermal resistances 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. However, we must also consider the burn-in of the phase change pad, and I will devote an extra section to this later in the section with the converted graphics card
We are interested in layer thicknesses of 200 µm and less on the CPU, with the GPU it is usually even 100 µm and less, depending on the bending. Everything else is really for the gallery. Some manufacturers also specify the pure, idealized bulk value here, but this is completely unrealistic. The Thermalright TF8 was the best paste so far, even if you need medium-high skills to apply it. And then came this pad… I can’t measure more than 250 µm due to the thickness of the layer. Double coating would be possible, but would be pointless.
I now have the relevant layer thicknesses from 200 to 50 µm as a bar chart for Rth in comparison to the pastes:
Control curve of Rth in the TIMA analysis
In the data interface, you can check the determined values again and deselect the deviating values for the determination. In addition, there are the slight anomalies of the PCM between 125 and 25 µm, where the thermal resistance no longer correlates very linearly with the layer thickness. We will scrutinize this later, but you have already seen it in the curve above (hump).
Minimum possible layer thickness
But at least I wanted to know how far you can go with proper pressure. Around 17 µm is ok, but is only finally reached after a few minutes. So this pad needs a certain amount of time for a “burn-in”, in this case the displacement. If you increase the pressure to 300 N, the pad suddenly goes down to 8 µm after a few minutes, but this is almost equivalent to self-destruction. Because if you then cool it down and heat it up again with less pressure, the surface cracks. This is probably also the reason why NVIDIA no longer uses these pads for the H100.
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 paste “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 be just as much a negative factor, which then influences the effective thermal resistance and thus also the conductivity, as too low temperatures and too high a viscosity. Anything below 20 is acceptable and below 10 is the top class and the pad shows why it is superior to all other pastes to date:
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