To get you in the mood, I’ll now show you how the paste can be compressed when the test head is shut down. Although this is not particularly spectacular, I am also showing here how much paste is actually used to be on the safe side and to ensure that the entire surface really is sufficiently wetted. You can also see how fluffy, i.e. thin, the paste is:
As we have already read in the basic articles on my measurements, let’s start with the effective thermal resistance and look at the possible layer thickness (BLT, Bondline Thickness) in this context. Because that’s where it gets interesting. 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.
Minimum possible layer thickness
Normally I measure down to a BLT of 25 µm and then do the final squeeze, but the application of the paste was already very liquid, so the first thing I wanted to know was how much more you could squeeze the paste with normal pressure without destroying anything. I used the usual 60 Psi (41 N) on the measuring surface of 1 cm², which is completely sufficient and more than what a GPU cooler, for example, achieves.
We can already see that the pastes from 2024 yield much further and can be pressed quite easily on the sides. I’ll come back to this in the stress test
The effective thermal resistances Rth, eff
Now we compare the pastes from Noctua with the very stable DOWSIL TC-5550 and only look at the effective thermal resistance. Of course, we can also see here how the paste behaves under pressure and at the technically possible BLT. The paste from 2024 only scores at 25 µm, because the more stable pastes only perform better from around 30 µm.
I have now compared the layer thicknesses from 50 to 400 µm, which are relevant for PC practice, as a bar chart for Rth, because it is easier to read and compare:
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) or whether the paste turns to soup because the filling level is too low. These values are also easily comparable and meaningful, as they are always the same calibrated reference blocks. However, coarser grinding degrees or a less favorable microstructure can be just as negative a factor, which then influences the effective thermal resistance and thus also the conductivity, as too low temperatures and too high a viscosity. I have already explained in detail how to determine this value in the linked Grandlagen, so I won’t go into that here. But it is the value that can have a major influence at very low BLT, which is why the bulk resistance is more for the gallery. However, values that are too low indicate a rather high silicone content.
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