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Optimized CPU Cooling with Top-Down Heatsinks

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Additional Cooling Performance Considerations

Up to this point we’ve spent a lot of time discussing which type of cooler can yield the best results, but there are several important factors to consider beyond the heatsink and its positioning on the motherboard:

Contact Surface Preparation

Processor and CPU cooler surfaces are not perfectly 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.

Thermal Paste Application

The entire reason for using Thermal Interface Material is to compensate for flaws in the surface and a lack of high-pressure contact between heat source and cooler, so the sections above are more critical to good performance than the application of TIM itself. This section offers a condensed version of our Best Thermal Paste Application Methods article.

After publishing our 80-way Thermal Interface Material Performance Test, many enthusiasts argued that by spreading out the TIM with a latex glove (or finger cover) was not the best way to distribute the interface material. Most answers from both the professional reviewer industry as well as enthusiast community claim that you should use a single drop “about the size of a pea”. Well, we tried that advice, and it turns out that maybe the community isn’t as keen as they thought. The example image below is of a few frozen peas beside a small BB size drop of thermal paste. The image beside it is of the same cooler two hours later after we completed testing. If there was ever any real advice that applies to every situation, it would be that thermal paste isn’t meant to separate the two surfaces but rather fill the microscopic pits where metal to metal contact isn’t possible.

TIM_Before_Spread.jpg TIM_After_Spread.jpg

When we discussed this topic with real industry experts who are much more informed of the process, they offered some specific advice that didn’t appear to be a “one size fits all” answer:

  1. CPU Cooling products which operate below the ambient room temperature (some Peltier and Thermo-electric coolers for example) should not use silicon-based materials because condensation may occur and accelerate compound separation.
  2. All “white” style TIM’s exhibit compound breakdown over time due to their thin viscosity and ceramic base (usually beryllium oxide, aluminum nitride and oxide, zinc oxide, and silicon dioxide). These interface materials should not be used from older “stale” stock without first mixing the material very well.
  3. Thicker carbon and metal-based (usually aluminum-oxide) TIM’s may benefit from several thermal cycles to establish a “cure” period which allows expanding and contracting surfaces to smooth out any inconsistencies and further level the material.

The more we researched this subject, the more we discovered that because there are so many different cooling solutions on the market it becomes impossible to give generalized advice to specific situations. Despite this, there is one single principle that holds true in every condition: Under perfect conditions the contact surfaces between the processor and cooler would be perfectly flat and not contain any microscopic pits, which would allow direct contact of metal on metal without any need for Thermal Interface Material. But since we don’t have perfectly flat surfaces, Thermal Material must fill the tiny imperfections. Still, there’s one rule to recognize: less is more.

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 or the more complex (such as the direct contact heat-pipe design). 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 direct contact heat-pipe 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.

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 cooling solutions. 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 direct contact heat-pipe 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 lesson learned here is that high compression between the two contact surfaces is better, so long as the hardware can handle the added pressure without damaging the components.

Conclusion

Optimized cooling requires attention to many details, from the style of heat-pipe heatsink (top-down ‘C’ design or front-back ‘U’ design), to the contact surface, ideal thermal interface material between the contact metals, and the proper contact pressure. This article should help you determine what is best for your system configuration, and reach the most stable overclock possible. We wish you success!

COMMENT QUESTION: Would you consider a top-down heatsink such as the Cooler Master GeminII S524 to cool your overclocked computer system?

NewEgg.com

 


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3 comments

  1. Bill Bright

    As a long time electronics technician, I was very pleased to see your comment,

    “The entire CPU cooler is designed to reuse the fan’s downward airflow to cool surrounding hardware components, which is very beneficial for both high overclocks and overall system stability.”

    I am constantly debating the value of aftermarket coolers with some who believe the OEM coolers of today are still junk. I see first time (and even experienced) home builders automatically toss their OEM coolers for just about anything that isn’t OEM. And very often, they are the cheapest side-firing coolers they can find. I think this is a mistake. Today’s OEM coolers are much better than those of 10 years ago. But also, as you noted, the components surrounding the CPU socket also take advantage of the “expected” downward firing OEM cooler. And unless you address additional case cooling requirements, automatically replacing the OEM cooler can actually hurt performance, system cooling and system stability. I contend it is the case’s responsibility to provide an adequate supply of cool air flowing through the case. The CPU cooler needs to then toss the CPU’s heat into that flow. So with proper case cooling, today’s OEM coolers are fully capable of, and do provide adequate CPU cooling, even with mild to moderate overclocking.

  2. JackNaylorPE

    One thing that can still be said about OEM coolers, they ain’t quite there yet. With no OC applied, if you fire up a CPU stress test such as OCCT, it will shut down since CPU temps break the self protection features inherent to the test which, by default, is 85C. This is getting better as Intel is now getting back to providing better thermal heat transfer again, but even w/ DC, users are still seeing a lot to be gained by delidding, so there’s still much room for improvement.

    I would not put an OC on a user’s build w/ stock cooler still as I have to worry about what and how he’s going to test it. When you break 85C at stock settings in 68F room temperature, you have to be worried what he’s going to run, how he’s going to run it and when he’s going to run it … as in the dog days of August when indoor temps reach 95F.

  3. JackNaylorPE

    As for the revised article and the question put forth “Would you consider a top-down heatsink such as the Cooler Master GeminII S524 to cool your overclocked computer system?” I lean to water cooling, and not CLC type, but on user builds I certainly would, but before doing so would be anxious to see how comparably cost / size / rpm coolers of each type compare.

    Clamping mechanisms also I think need more attention with respect to the presence of stops or other mechanisms to prevent over tightening. I have had 3 rebuilds in recent years, all w/ Hyper 212’s where MoBo was cracked (AsRock), circuit traces cut (sharp burr on bottom of washer – Asus board), or traces possibly broken (Asus) by what appears to have been the user over tightening the mounting mechanism.

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