The thermal resistance Rth– in theory and practice
Let’s start with the most important aspect, the thermal resistance Rth and a classification for our PC. The most important property of Rth is that in theory it correlates nicely linearly with the layer thickness, while the thermal conductivity describes a completely different curve and remains anything but linear. This time, however, we must also consider the behavior of the rigid graphene pad, although I will devote an extra section to this later in the section with the converted graphics card and the CPU. This time, however, we must differentiate between laboratory values and reality in terms of the actual possible contact pressures and forces.
We are normally 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. This is not possible with this rigid and solid sheet. The forces or rather pressures required for maximum performance, as I mentioned and explained on the previous page, unfortunately show a completely different picture than what we see first:
I have now compared the relevant layer thicknesses from 100 to 175 µm as a bar chart for Rth:
But watch out! Now we have to be brave. According to the manufacturer, the GeForce RTX 4080 from the test should be well below 40 N. If we divide this by the ratio of die area and test specimen and round up generously, we end up with around 10 N in the graph. In other words, only around 2.0 cm²K/W, because the pad is not compressed significantly!
Control curve of the Rth in the TIMA analysis
In the data interface, you can check the calculated values again and deselect the deviating values for the calculation. In addition, there are slight anomalies above 150 µm, where the thermal resistance no longer correlates very linearly with the layer thickness due to the lack of contact pressure. We will scrutinize this later, but you have already seen it in the curve above (hump) and of course also in the contact pressure. We can see that the almost 94 W/(m-K) specified by SHT is also achieved here in my measurement, but the values are only for the gallery. If the value at 175 µm were included, then the thermal conductivity, for example, would only be a good 36 W/(m-K), with a strong downward trend with the contact pressure.
Minimum possible layer thickness
At least I wanted to know how far you can go with a lot of pressure or force, even if it is completely unrealistic here with the sheet. At 300 N it is still approx. 100 µm (here the pad in the diagram deviates from the 41 N or 60 PSI actually used, but this is complete nonsense. At least with a sheet like this. Because by then it is already beginning to disintegrate and crumble.
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) if you could apply the optimum pressure that the sheet needs.
- 1 - Introduction and background information
- 2 - Test methods and equipment
- 3 - The "secrets" of the KryoSheet and material analysis
- 4 - Thermal resistance in theory and practice
- 5 - Thermal condiuctivity in theory and practice
- 6 - Tests with a GeForce RTX 4080 and Core i9-13900K
- 7 - Summary and conclusion
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About the author
Igor Wallossek
Editor-in-chief and name-giver of igor'sLAB as the content successor of Tom's Hardware Germany, whose license was returned in June 2019 in order to better meet the qualitative demands of web content and challenges of new media such as YouTube with its own channel.
Computer nerd since 1983, audio freak since 1979 and pretty much open to anything with a plug or battery for over 50 years.
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