Let’s now throw the two unlucky pastes into the ring, both of which are very expensive but also highly questionable. And so we start, as we already know from the basic articles on my measurements, with the effective thermal resistance and in this context also look at the possible layer thickness (BLT, Bondline Thickness). 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, and today there is even something on top.
Minimum possible layer thickness
Normally I measure down to a BLT of 25 µm and only then do the final squeeze, but this time the application of the Quantum already gave rise to certain doubts, while the weaker Ultra behaved normally. That’s why I first wanted to know how far you can go with normal pressure and how much the Quantum can still be compressed without destroying anything. I normally use 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. And seriously, the 6 µm is so much nonsense… because this thin layer already has the word pump-out tattooed on its forehead. This paste is already on the run before the CPU or GPU make the first thermal twitches.
At the end, I wanted to know how far you can go with a little pressure and how much the Quantum can still be compressed. I increase the force to 200 N and end up at 2 µm in the style of a grinder, i.e. already within the tolerance range of measurement accuracy. But not only the thermal conductivity was almost gone, but also the “paste”. This is precisely why I will come back to ageing later on.
The effective thermal resistances Rth, eff
Now we compare the pastes with the best pastes tested so far and only look at the effective thermal resistance. Of course, we also see here how the paste behaves under pressure and at the technically possible BLT. Anyone who measures this paste as performing is doing something fundamentally wrong and had to do so.
I have now compared the relevant layer thicknesses from 25 to 400 µm as a bar chart for Rth. Here, too, both pastes are not only a clean sweep but also an outright fail.
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 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. 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.
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