Application on Square-base Coolers
Without question, the most dominant mounting shape for CPU coolers is the square base. The formula is very simple: the most effect method to cover a square processor and integrated heat spreader is to match it with a square cooler base. Science is simple that way, right? While I have seen some examples of glass slides being used to demonstrate the spread pattern of Thermal Interface Material under ideal conditions, I seldom find myself with two perfectly flat and non-porous surfaces.
If only it could be so simple in the real world of retail thermal cooling solutions. Since the ultimate end-goal is to have the integrated CPU heat spreader surface contact metal-to-metal with the coolers base, it’s necessary to have a perfectly flat and smooth contact surface at both ends. This usually means lapping the processor and cooler surfaces with wet-sanding paper and polishing compounds, along with a few hours of effort. Some manufacturers try to achieve this right out of the box, while others completely ignore thermal dynamics and finish their mounting surface with stone grinding.
Thermalright is probably the biggest offender in the category of poor factory surface finishes, which is ironic if you consider the loyal fan-base who adore their products without question. The image below is of a brand new Thermalright HR-01 CPU cooler (the 120 eXtreme was pictured on the first page of this article), right out of the box. You will no doubt notice that it doesn’t have a very smooth finish… which would have to be be polished before I could continue because thermal paste didn’t seem to spread very well with so many tracks running across the surface.
Since Thermalright products are generally ground flat and level, albeit far from smooth, they can accommodate a wide range of application methods. Ideally my article would have been made better if I had two perfectly flat real-world surfaces to work with, but time and project needs keep me from this goal at the moment. If I didn’t have the Noctua NH-U12P tied-up with another project, I would have just used it for this segment. For now I am working with the tools I have available to me, and not the tools I wish I had… so expect a rewrite at some point.
The first thermal paste application pattern to test was the single drop. One single drop of TIM was placed directly in the center of the mounting base surface, roughly half the size of a BB.
Once all four corners of the cooler were depressed, and a bit of extra force pressed down in a circular motion to ensure maximum contact, the entire unit was removed an photographed. My results show that the single drop pattern spread out to reach only two edges of the CPU, while falling short of the other two sides. This could be attributed to several variables: poor mounting system and pressure, uneven mounting surface on the cooler or processor, and perhaps even a warped motherboard.
Next up was the single line pattern, which used a small half-inch long application of paste down the center of the coolers mounting base. The direction of the thermal paste was perpendicular to the direction of the surface grain (which was still present after an hour of effort).
Perhaps I didn’t use enough thermal paste, because it appears as though the pattern might have worked with a slightly shorter yet thicker application of material. With some additional trail and error testing, this application pattern may prove successful for some coolers, but even after another retest using a bit more material there was still less coverage than the single drop method.
Since the one line pattern didn’t quite reach out to the edges, I figured that two lines might do the trick. Spaced apart at an interval of 1/3 the width of the processor, these two lines were slightly shorter than the single line pattern and equalled half the width of the processor in length.
One of the fears I hear about most with this method is that air may get trapped in the middle. However, one look at the image below and I can safely dismiss this potential problem. After the cooler was lifted, I discovered that the two line pattern was able to reach all four edges of the CPU. It seems that this method could have some real world application if correctly applied.
It took some additional cold tests for me to get the amounts correctly tuned. After the retest results (not pictured), the best ratio I found was using line lengths equal to the distance between the lines; or roughly 1/3 the width of the processor.
Next up was the cross method of applying thermal paste to the coolers mounting surface. Since two lines worked well, perhaps overlapping them would have an improved effect? I have not personally used this pattern in my own systems, but other web sites have claimed that it can work.
After a few test applications, the image above is the best coverage I could get. Plenty of material covers the surface, but there are clear signs that too much TIM has reached the edges. Again, the disclaimer here is that this pattern might work for your setup, and every application is different. In my tests however, I wanted more uniform coverage and the cross method took some practice before it could meet my needs.
In the image above my line length was too long, measuring 2/3 the width of the processor. With additional cold tests (not pictured) I learned that lines of the same thickness but only 1/3 in length did a better job and kept the material amounts in check.
Last but not least is my own personal method. Not to harp on the subject, but you don’t want a thick layer of thermal compound between mounting surfaces. There isn’t a magic thickness to suggest, since different surface finishes will require different amounts of material to fill them. If both surfaces were as smooth as glass though, I would need less than half the material used in the above tests to reach the same coverage.
So when I come across a very rough yet level surface, I prefer to smooth out the material using a rubber finger cover or plastic bag in directions opposite to the grain. This does an excellent job of filling in the groves and scratches, and can be used in combination to the above methods as a base. In cases where I use both methods, the “base” coat is extremely thin and the metal beneath it is still visible.
However if I choose to use just the one layer, I leave a coat just barely thick enough to hide the metal surface below it. This method doesn’t leave enough material to spread out across the surface and press out bubbles as it bleeds, so the cooler must not be placed flatly onto the processor surface to begin with. Instead this method must be carefully placed down at a slight angle as it is compressed, similar to how you might apply an adhesive sticker to avoid bubbles.
I will admit that there’s plenty of room for additional testing and improvement in this section, which I forewarned would most likely be the case for an upcoming edit. Please take this information for what it’s worth, because we did still discover a few application patterns that would never work well under any circumstance.
In our next section, careful consideration is given to the application of thermal paste on the new Heat-pipe Direct Touch cooling products.