Mounting Pressure

Probably one of the most overlooked and disregarded factors involved with properly mounting the cooler onto the processor is the amount of contact pressure applied between the mating surfaces. Compression will often times reduce the amount of thermal compound needed between the cooler and processor, and allow a larger metal to metal contact area which is more efficient than having fluid weaken the thermal conductance. The greater the contact pressure between elements, the better the contact conductance which will allow the thermal transfer of heat energy to occur.

Unfortunately, it is often times not possible to get optimal pressure onto the CPU simply because of poor mounting designs used by the cooler manufacturers. Enthusiasts shriek at the thought of using the push-pin style clips found on Intel's stock LGA 775 thermal cooling solution. Although this mounting system is not at all bad, as we discovered with our top five performers in the Best CPU Cooler Performance - Q1 2008, there is still plenty of room for improvement.

Generally speaking, you do not want an excessive amount of pressure onto the processor as damage may result. In some cases, such as Heat-pipe Direct Touch technology, the exposed copper rod has been pressed into the metal mounting base and then leveled flat by a grinder. Because of the copper rod walls are made considerably thinner by this process, any other mounting system besides push-pins may cause warping. Improper installation not withstanding, it is more ideal to have a very strong mounting system such as those which use a back plate behind the motherboard and a spring-loaded fastening system for tightening. The Noctua NH-U12P is an excellent example of such a design.

In all of the tests which follow, it is important to note that our experiments focus on the spread pattern of thermal paste under normal pressure using the push-pin style mounting system. We recognize this mounting system as a minumum pressure solution, and a starting point for improvement. In most situations your results will be different than our own, since higher compression would result in a larger spread pattern and less thermal paste used.

It's more efficient to have the metal surfaces make maximum contact with one another while allowing thermal material to fill in the voids, and you'll need excellent compression to achieve this goal. The lesson learned here is that high compression between the two contact surfaces is better, so long as the elements can handle the added pressure without damaging the components.

Surface Finish Impact

If you're a painter, it's important to understand your canvas or else you won't know which paint to use. The same is true for enthusiasts wanting to overclock, or even just get the most out of their equipment. CPU coolers primarily depend on two heat transfer methods: conduction and radiation (heat-pipes also add convection). This being the case, we'll concentrate our attention towards the topic of conduction as it relates to the mating surfaces between a heat source (the processor) and cooler.

Because of their density, metals are the best conductors of thermal energy. As density decreases so does conduction, which relegates fluids to be naturally less conductive. So ideally the less fluid between metals, the better heat will transfer between them. Even less conductive than fluid is air, which then also means that you want even less of this between surfaces than fluid. Ultimately, the perfectly flat and well-polished surface is going to be preferred over the rougher and less even surface which required more TIM (fluid) to fill the gaps.

This is important to keep in mind, as the mounting surface of your average processor is relatively flat and smooth but not perfect. Even more important is the surface of your particular CPU cooler, which might range from a polished mirror finish (such as the Noctua NH-U12P), to the absurdly rough (evidenced by the Thermalright Ultra-120 eXtreme pictured above), or the more complex (such as the OCZ Vendetta 2). Surfaces with a mirror finish can always be shined up a little brighter, and rough surfaces can be wet-sanded (lapped) down smooth and later polished, but Heat-pipe Direct Touch coolers require some extra attention.

To sum up this topic of surface finish and its impact on cooling, science teaches us that a smooth flat mating surface is the most ideal for CPU coolers. It is critically important to remove the presence of air from between the surfaces, and that using only enough Thermal Interface Material to fill-in the rough surface pits is going to provide the best results. In a perfect environment, your processor would mate together with the cooler and compress metal on metal with no thermal paste at all; but we don't live in perfect world and current manufacturing technology cannot provide for this ideal environment.

So it's up to you to fix these imperfections through sanding or polish, and apply the magic amount of TIM to your project. But how much is enough, and what shape or pattern should be used? Read on to find out.

Benchmark Reviews invites you to share your comments and suggestions for this topic in our Discussion Forum.

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