AMeCh STG-4 in test – Korean thermal paste with free vinegar as price/performance winner?

1 - In search of the vinegar dealer and a data sheet 2 - What's inside and why I'm even annoyed 3 - Individual test in detail 4 - 1:1 Comparison between the STG-4 and other pastes 5 - Summary and conclusion

When something smells like vinegar (but shouldn’t)

Let’s start with the vinegar smell first. Because my nose is actually still swollen and, due to illness, is also a good, closed bulwark against bad smells. You also know that before I actually test the pastes, I first lightly compress them to 400 µm and then heat them thoroughly to 100 °C. This is because every paste has to cool down before it can be used. Every paste has to be able to withstand this and I also remove any possible moisture influences and inclusions.

The paste has a normal consistency, but left a slightly elastic impression when spread. What surprised me was the very good adhesion on the smooth test pieces and the rather abrupt tearing off during the spreading test. This alone indicated that it could not be one of the usual silicone oils. Something was different and this is exactly what raises the tension curve of the tester enormously. Pulling it off went well and easily, but then the “this far but no further” effect set in. The term rubbery could also describe it quite well. In principle, however, this can even be advantageous.

In terms of particles, everything is actually in the green zone. Below the surface, you can still find somewhat larger particles up to approx. 10 µm, but these are more than rare. A relatively good filling has been used here. That makes me feel a little milder.

Well, and then came the vinegar note in the odor finder. A vinegar smell in a thermal paste usually indicates that the paste also contains acetoxy silicone. There are silicon atoms that are connected to acetoxy groups (acetoxy groups contain acetic acid). When this silicone comes into contact with moisture in the air, a chemical reaction takes place in which the acetoxy groups are split and acetic acid is released. Heat can accelerate this process and we know this, for example, as joint silicone in the sanitary sector.

This is a common phenomenon with very cheap silicone-based heat-conducting pastes, where acetic acid is released during curing, which in turn causes the characteristic vinegar smell. This release of acetic acid is the key to starting the cross-linking reaction, which causes the silicone to change from a paste-like to a solid, or at least slightly rubbery, form. Acetic acid therefore helps to harden or solidify the silicone and improve its mechanical properties. But is this really necessary for a heat-conducting paste? Let’s take a look at the increase in BLT, at around 70 °C the strong smell of vinegar begins…

And what about the consequences? The paste could start to cure faster, especially if it is used in an environment with humidity or heat. This can reduce thermal efficiency as the paste loses its flexibility. Acetic acid can, in rare cases, cause slight corrosion in contact with certain metals (such as copper or aluminum), but this could be problematic in the long term, especially in sensitive electronic devices. Such a vinegar smell could therefore also indicate an inferior quality of the silicone of the thermal paste, as higher quality products often have no such side effects. The safety data sheet is correct with regard to the aluminum oxide it contains and the silicone is also correct:

The very high carbon content is actually exemplary of what is suspected. I am not a chemist, but at least as I understand it, the acetoxy silicone consists mainly of a polysiloxane chain (silicon-oxygen chains) to which organic groups such as methyl groups (CH₃) are bonded. During curing, acetic acid, which originates from the acetoxy groups, is split off and the silicone cross-links to form a solid, elastic structure. Incidentally, after just one cycle, this resulted in a fairly firm bond between the test specimens, which could only be loosened again with a little effort. The stuff was like a viscoelastic rubber layer and no longer really spreadable.

On the next two pages you will find the individual test and the 1:1 comparison with the other pastes tested so far. At the end there will be a short summary, but we are not there yet.