Secret file thermal paste: Increasing layer thickness at high temperatures, ageing and differences in quality and their influence on benchmarks

1 - Important foreword, introduction and test setup 2 - Why cyclical tests are so important 3 - Higher temperatures produce a thicker bondline thickness! 4 - Thermal expansion, viscosity, shear forces and conclusion

Myth 2: The thickness of the thermal paste layer (BLT) decreases as the temperature rises

It is usually assumed that every layer of paste becomes thinner and thinner under pressure and with increasing temperatures. However, this is fundamentally wrong. The bondline thickness (BLT) of a thermal paste, as long as it contains silicone, for example, will increase to a greater or lesser extent with increasing temperature, which is (not only) due to the viscoelastic behavior of the material. This also distinguishes these pastes from the phase transition material PTM. I have also dedicated the next chapter to this special behavior of the pastes, after I have now measured the whole thing for you. Because the details are really interesting and almost nobody knows them. I even managed to surprise one or two specialists, including the suppliers of thermal pastes themselves. Only the silicone oil suppliers will nod knowingly.

For the next investigation, I’ll take a different approach and also dispense with the extreme, repetitive cycles. However, as before, I use a constant pressure of 60 PSI, but in contrast to the first test, I use different and increasing temperatures from 30 °C to 100 °C in relatively fine 5-degree increments. This measurement also takes at least 45 minutes to 1 hour. I am now deliberately only measuring the behavior in the first cycle, because these values can be used not only to observe how the layer thickness expands, but also to draw conclusions about water residues, for example, because the paste was not properly degassed in the vacuum.

The extreme example of this was my first test of the Cooler Master Cryofuze Violet, where, in addition to the missing Al2O3, there was also far too much water in the paste. The amount of thermally conductive particles in the paste and the quality and consistency of the silicone oil also play a role.

Cooler Master Cryo Fuze Violet, missing corundum and too much water – The manufacturer reacts

But enough of the digressions, let’s take a look at the sobering measurement result and be amazed! The two curves meet at around 60 °C, at which point the Thermal Hero Quantum literally drifts apart! Depending on the test setup, however, such a curve also contributes to the fact that not only the values of different reviews differ so much, but that especially the reviewers who found the Hero so good usually had lower temperatures in the paste. This is because the CPU temperature read out is by no means the paste temperature and so in the chain of all thermal resistances between die and water, despite similar CPU temperatures, different paste temperatures and therefore also BLT can occur from build to build! This makes such tests hardly comparable with each other as long as no uniform conditions can be guaranteed.

The Thermal Hero Quantum, which is only filled to a medium level, can be squeezed to an astonishing 15 µm at 30 °C and 60 PSI, while the Thermalright TF8 only manages around 25 µm when cold. However, if you slowly heat the pastes up to 100 °C, you can see that the Thermal Hero Quantum increases the layer by almost 127 percent (!), more than doubling it! This is good evidence of bubble formation, possibly also water residues and a low-quality silicone oil that is already outgassing. I will describe the exact processes in detail on the next page, because we urgently need to talk about this.

The Thermalright TF8 starts with 25 µm, but ends at 100 °C with only 31 µm BLT, which means an increase in layer thickness of only 24 percent. So instead of 2.3 times more, we only have just under a quarter more. This already says much more about the quality of a paste than a single test run with CPU and stress test program over 30 minutes. The blue curve indicates that the Thermal Hero Quantum is core scrap, even if some testers praise it to the skies. This is nothing and will be nothing!

Instead, I now have another 6-hour special of the Quantum, which shows us that the delta between the maximum and minimum BLT remains almost the same, but shifts downwards in relation. In the end, however, this also means that the paste is already starting to dissolve after just a few hours.

This would certainly refute this myth and all the accompanying symptoms.