As we have already read in the basic articles on my measurements, let’s start 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.
Minimum possible layer thickness
Normally I measure down to a BLT of 25 µm and only then do the final squeezing, but this time the application of the paste already gave rise to certain doubts. That’s why I first wanted to know how far you can squeeze the paste with normal pressure and how much it 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.
Finally, I wanted to know how far you can go with a little pressure and how much a paste can still be compressed. I use the measured 135 µm at acceptable pressure and see that the data sheet is correct on the µm. But in order to bring the THERM-A-GAP GEL 60HF slightly below the minimum thickness specified in the data sheet, even 300 N per 100 mm² was necessary in the second run after I had measured all the normal BLT, which is downright insane and unfortunately had consequences. But more on that later.
The effective thermal resistances Rth, eff
Now we compare the Parker THERM-A-GAP GEL 60HF with the DOWSIL TC-5888 and DOWSIL TC-5550 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. In this context, it is also interesting to note that the effective thermal resistance only becomes better than the values of the two DOWSIL pastes above a BLT of around 215 µm. The 125 µm were no longer possible.
I have now compared the relevant layer thicknesses from 150 to 400 µm as a bar chart for Rth. And why don’t I measure and compare down to 25 µm? The explanation is simple, because it doesn’t work, because the paste has a problem with all the gradations below 150 µm BLZ!
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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