The effective thermal conductivity
I have already written about the role of interface resistance in the basics. I am still leaving it in the calculation, as it is clearly more relevant in practice. And again, as in all my other reviews, if you already have Rth, you don’t actually need λeff, i.e. the effective thermal conductivity.
We can also see how the values change over the BLT, although we can no longer expect a linear curve here due to the included area and BLT.
Of course, the whole thing is also shown again as a bar chart for the most important layer thicknesses (again only from 150 µm due to relevance):
Apart from the fact that I also have the temperatures of the heater and the water, which are of no use to us because they either adapt to the resistances or always remain constant, I have my measurement setup with temperature sensors 1 to 6 (see diagram on page 2). You can now use these values to make some very interesting considerations.
GPU simulation
Let us first take the values of T3 and T4, which show us the two temperatures at the respective contact surfaces between which the paste is located. These curves are no longer completely linear, as the interface resistance also changes slightly. And we no longer calculate with 6 points, but only with 2 absolute values for the temperature difference instead of a gradient as withTTim, whereby the sample temperature remains constant. And what is the point of all this? The behavior is similar to that of a graphics card, which has to manage without an IHS and where the delta is usually measured between the substrate and the water temperature. This can be projected quite well, because I test the temperature difference on the two surfaces between which the paste is located.
However, this measurement is quite meaningless, because a GPU cannot cope with the 135 µm or the measured 150 µm at all, because the BLT is much too large. In addition, this is almost impossible to handle sensibly in everyday life.
CPU simulation
Now I compare T3 of each of the tested products. If we normalize the values for the heater, we already have sufficient thermal resistance in the copper reference block to simulate the CPU temperature and its differences with different pads in comparison with each other and in relation to the thickness of the paste replacement. It is precisely this variable evaluation that no test on a CPU can offer, because the respective CPUs are bent differently and it is therefore not really reproducible. But the TIMA5 test does, because I can measure all distances, which is simply not possible on a single CPU.
TIMA5 control result for the bulk values
Parker states a rounded 6.2 W/m-K for the bulk thermal conductivity and I measure 6,248 W/m-K, which is an almost ideal precision landing and also proves that even such special gels cannot achieve values of 8 W/m-K and more. The minimum BLT of 135 µm is also in line with the manufacturer’s specifications. However, the very high interface resistance indicates a rather suboptimal thermal contacting at the transitions, so that together with the consistency one can draw conclusions about the composition of the paste. This is the subject of the next page.
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