Results and conclusion
A drop in the middle, a cross, spread it with finger, or use an old credit card to smooth it perfectly? Even simple things like applying a thermal paste under your cooler can be a bit tricky sometimes. Commonly used methods are seemingly different, and if you also consider batching, there comes a question of how to it to achieve the best results.
Introduction and test procedures
As we all know, even the best cooler is ineffective without a thermal paste. There are so many combinations of how to create a thermal bridge between a heatspreader of a chip and a cooler that we thought it definitely deserves a closer look.
We have chosen and compared four methods in our tests. One of the most recommended and used is a drop in the middle of IHS, which is spread by the pressure of a cooler. This pressure is also used with the cross method that can better reach corners in some cases. These corners usually remain dry when using combination of the drop method and less paste.
It is also very popular to use finger and a foil to spread the paste on the whole surface of IHS. However, this method is often criticized by more experienced users because the layer of the paste is not perfectly smooth and that can lead to creation of air gaps. That is why people who want to avoid this use some applicator (or simply something with a sharp edge, like an old credit card or a razor blade) to precisely smooth the layer.
When you have decided which technique to use, it is time to think about the amount of paste to be used. We worked with the range of 0.01 – 0.15 ml and we used 0.1 ml, 0.066 ml, and 0.033 ml as reference values for every method.
The paste which was used in our experiment is Noctua NT-H1. We’ve chosen it because it is a part of the accessories of popular coolers Noctua and also because it has an acceptable consistency for equal spreading, despite its higher viscosity. Some pastes tend to tear, which makes it harder to create a smooth consistent layer.
The choice of the cooler was Scythe Fuma. Because of its suitable base, it was possible to see whether the air gaps reduce effectiveness of the heat transfer.
The heating element was overclocked Intel Core i7-5930K@4.5 GHz (on Gigabyte X99 UD4) with increased Vcore to 1.275 V. We simulated the burn in IntelBurnTest (7500 MB). The waste heat was around 250 W.
After the initial burn in, the tests took 300 seconds. We used high-flow industrial fans Noctua iPPC NF-F12 2000, so five minutes should be more than enough for temperatures to stabilize. It was crucial to watch if the CPU was working with the same performance the whole time. Even the smallest offset (GLOPS) could affect the final results. We repeated every test twice (new installation included) to make sure that measured values match. The intake air temperature was 21 – 21.3 °C, ensured in our air-conditioned lab.
Before we get to the comparing, we need to add that results can differ with other pastes and configurations. The reason is differences in viscosity, pressure, and contact areas of coolers. Coolers with solid block base can spread the heat differently than coolers that put heatpipes directly on the CPU’s heatspreader.
A drop in the middle, a cross, spread it with finger, or use an old credit card to smooth it perfectly? Even simple things like applying a thermal paste under your cooler can be a bit tricky sometimes. Commonly used methods are seemingly different, and if you also consider batching, there comes a question of how to it to achieve the best results.
Results and conclusion
It seems that the most effective technique is to create a smooth layer on the whole area of IHS. The lowest temperature was achieved with 0.1 ml of paste. However, we consider this a waste of material because differences between 0.1 ml, 0.0333 ml, and 0.0666 ml were negligible.
With the finger and foil method, the values go a bit higher, and again they tend to rise with less paste. Even though the contact above the chip, where the heat exchange is most intensive, is just as good as with the previous method.
The problem is that most of the coolers of a better quality (our Scythe Fuma included) have a convex base. That means that if you use less paste, the layer is too thin on the sides of IHS to provide optimal contact. The heat exchange on sides is not as intense as near the chip, but it can still limit the potential of your cooler.
The previous claim can be supported by the fact that the cross method was a bit more effective than the drop method when we used 0.033 ml. The cross could cover more area than the drop. However, when we used more paste, the situation changed. The cross put too much into corners and left too little for the middle. The drop seems like a more flexible choice in this case because it spreads better when enough paste is used.
It was also very interesting to watch both extremes (purple lines in the chart): too much paste (0.15 ml) and too little paste (0.01 ml). Well, you do not need to worry about cooling performance of your cooler when you use more paste. It does not take the role of a thermal insulator, or at least it is not so significant that it would be worth mentioning. The excess paste is pushed over the edges of the heatspreader. So, too much paste is not a disaster, especially if the cooler can put enough pressure on IHS, but it is still a wasteful solution. Also, there is a chance that the excess paste can get to your socket, which can cause a serious trouble.
The other extreme experiment, 0.01 ml, ended worse. In fact, these were the worst results. The drop method could not cover even a half of the IHS and the effort to create a smooth layer was pointless because there were too many cracks in it. So, the rise of the temperatures was inevitable. We also tried to put a straight line across IHS, which seemed reasonable due to vertical orientation of a chip, but it fared no better than a drop.
TL; DR: All in all, the best solution for coolers with a solid block base is to use the cross method with 0.03 ml of paste. Nothing can go wrong with the drop either. The final effect will be almost the same as with a perfectly smooth layer. Too much paste is not affecting cooling performance that much, but you should avoid using too little paste. The optimal drop should look like the one in the picture in the first chapter.
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