Surface Preparation

If you've made it to this page, you're probably one of the few visitors who read the content of an article instead of scrambling for the charts and some unexplained results. Not only is this going to benefit you ten-fold, but you'd be surprised how many people don't do the leg work necessary to achieve excellent results. This is that leg work, and in truth, even the very best thermal paste isn't going to make the slightest difference if the mounting pressure is weak or the application is incorrect. More importantly, the entire reason for Thermal Interface Material to even exist is because of flaws in the surface and a lack of high-pressure contact between heat source and cooler.

Processor and CPU cooler surfaces are not pefectly smooth and flat surfaces, and although some surfaces appear polished to the naked eye, under a microscope the imperfections become clearly visible. As a result, when two objects are pressed together, contact is only made between a finite number of points separated by relatively large gaps. Since the actual contact area is reduced by these gaps, they create additional resistance for the transfer of thermal energy (heat). The gasses/fluids filling these gaps may largely influence the total heat flow across the surface, and then have an adverse affect on cooling performance as a result. The thermal conductivity of any interstitial material (such as thermal paste) and its pressure are the two properties governing its influence on contact conductance. In the absence of interstitial materials, as in a vacuum, the contact resistance will be much larger, since flow through the intimate contact points is dominant.

Mounting Pressure

Probably one of the most overlooked and disregarded factors involved with properly mounting the cooler onto any 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 much 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 it will conduct thermal (heat) energy.

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. Most enthusiasts shriek at the thought of using the push-pin style clips found on Intel's stock LGA775 thermal cooling solution. Although this mounting system is acceptable, 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, using a bolt-through mounting system could actually cause heat-pipe rod 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 acceptable pressure thresholds using either a push-pin style mounting system or spring-loaded clip system. 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. 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

CPU coolers primarily depend on two heat transfer methods: conduction and 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 to the absurdly rough (evidenced by the Thermalright Ultra-120 eXtreme pictured above), or the more complex (such as the OCZ Vendetta 2 pictured below). 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 now you should understand that no amount of miracle thermal paste will offer the best results, and that it's up to you ensure proper mounting pressure and to fix surface imperfections through sanding or polish. Once you've got the surfaces ready, it's time to apply that 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.

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