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 two pastes raised certain doubts, while the rest behaved normally. That’s why I first wanted to know how far you can go with normal pressure and how much each paste can still be squeezed 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.
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 pastes behave under pressure and at the technically possible BLT.
I have now compared the relevant layer thicknesses from 25 to 400 µm as a bar chart for Rth.
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 great detail how to determine this value in the linked basic principles, 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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