Important note
Once again, the chart graphics in these first tests are still static graphics, but I will also use interactive graphics later on, similar to the fan bank at that time. However, this still has to be programmed.
The thermal resistors Rth
Let’s start with the most important aspect, the thermal resistance Rth. Since I can’t set the Apex against the reference, because itself is the reference, I use a second paste as a substitute, which I also used for calibration, the DOWSIL 340. This is a popular constant in the industry, but it is not suitable for CPUs or GPUs. However, it is very stable over the long term and has no surprises. 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.
We are interested in layer thicknesses of 200 µm and less on the CPU, while on the GPU it is usually 100 µm and less, depending on the bending. Everything else is really for the gallery. Some manufacturers also specify the pure, idealized bulk value here, but this is completely unrealistic. The Apex is very average and solid, but not a top paste for the brief benchmark moment. There is still room for improvement here.
I now have the relevant layer thicknesses from 200 to 50 µm as a bar chart for Rth comparing the pastes
Control curve of the Rth in the TIMA analysis
In the data interface, you can check the determined values again and deselect the deviating values (here everything from 25 µm downwards) for the determination. At this layer thickness, the paste already showed slight signs of disintegration or could not be compressed any further.
Minimum possible layer thickness
But at least I wanted to know how far you can go with proper pressure. I could have pressed the whole thing with 300 N, but then nothing would change. But what minimum layer thicknesses can still be achieved? The 10 µm is extremely good, but this is also due to the composition of the fillers. I don’t want to anticipate the analysis, but you can only achieve something like this with high degrees of grinding and as few slurrying zinc oxide particles as possible.
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). 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 negative a factor, which then influences the effective thermal resistance and thus also the conductivity, as too low temperatures and too high a viscosity.
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