CPU Cooler Preparation
It bears repeating here that no heat-sink will work effectively unless it transfers heat from the CPU. To do that, it needs to be in contact with the CPU heat spreader or die, with the greater the contact surface the greater the potential for heat transfer. One of our own writers here at Benchmark Reviews has done a lot of work in this area, and it is certainly worth the time it takes to read (and re-read) the discoveries he made during the famous 80+ thermal paste tests (I still see Newegg reviews reference the discoveries made therein).
I mention this because I still see this as a major source of misinformation – most end users will use far too much thermal interface material when switching CPU coolers. Possibly through little fault of their own – I’ve read official repair manuals stating to use the entire tube of thermal paste when replacing a CPU and heat-sink. This is, in almost every case, FAR too much – to the point of being harmful in most cases. So do yourself a favor and get acquainted with CPU Cooler Preparations and Thermal Paste Application.
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 only 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 Thermal Interface Material articles, 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”. 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.
After discussing 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:
- 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.
- 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.
- 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 Heat-Pipe Direct Touch). 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.
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 thermal cooling solutions. Although this mounting system is acceptable for casually-used computers, there is still plenty of room for improvement when overclocking.
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.