Heatsink Theory Guide

Heatsink Theory Guide - Introduction

Ed. note: This article was originally published in Atomic: Maximum Power Computing by Asher Moses. IntroductionAround three years ago, a chunk of aluminium with a small fan was all that was required to cool your processor. This was mainly because overclocking was not as popular or as easily done as it is today, and the heat older CPUs produced was also quite low. Now three years later, much has changed. Overclocking has risen tremendously in popularity with whole publications devoted to teaching consumers how to squeeze as much performance out of their processors as possible. This obviously resulted in a demand for more effective heatsink designs to not only cater for the overclocking community, but just to cope with the heat produced by today's CPUs. This in turn created a huge market niche, with new heatsinks popping up everyday, each with a unique and innovative design. While most of these heatsinks have been made out of either aluminium or copper, some companies have gone to extremes and produced heatsinks made out of silver and in some instances gold. With so many heatsinks available, it is tough to make a decision on which to purchase.

Heatsink Theory Guide - Materials

MaterialsThe basic purpose of a heatsink is to remove heat from the core of your CPU. Therefore, metals with good heat transfer properties are ideal for use in heatsinks. Heat transfer or thermal conductivity is measured in W/cm-K (watts per centimetre per degree Kelvin). Thermal conductivity relates to how well a metal can absorb and transfer heat within itself. Different metals conduct heat differently because thermal conductivity is strongly driven by the interaction of electrons throughout the material. In the case of (most) metals, electrons are shared as members of a collective 'whole' in which they float rather freely from on atom to the next. The more easily these electrons can move through the atomics, the better that metal will conduct heat. The small table below compares the thermal conductivity of a few different metals:While diamond and silver conduct heat the best, most manufacturers have opted to make their heatsinks mainly out of aluminium. This is because aluminium is much easier to obtain, not to mention significantly cheaper. Can you imagine how much you would have to pay for a heatsink made out of pure diamond? Copper has also become a very popular material among heatsink manufacturers because it features good thermal conductivity properties and costs a fraction of what you'd be paying for silver, gold or diamond. However, as copper is much harder to machine than aluminium, a lot of manufacturers have stayed away from it to keep production costs down.Another thing manufacturers look at when designing a heatsink is the thermal convection properties of a metal. While thermal conductivity refers to how well a metal absorbs and transfers heat within itself, thermal convection refers to how well a metal transfers the heat to a liquid or gas (for example, air). Air unfortunately does a rather poor job of conducting heat, which is why we usually see fans attached to the heatsink. When an object such as a fan is used to force the air across the heatsink, it is called forced convection.The final important thing to look at when deciding on a metal for a heatsink is its ability to emit radiation. Radiation is the emission and propagation of energy in the form of rays or waves. In the case of a heatsink, the energy that is going to be emitted is heat. Black objects tend to emit energy very well. This is why we have seen many black coloured heatsinks in the past. That doesn't mean you should go out and paint your heatsink, as this will greatly reduce its effectiveness because the paint will act as an insulator.Heatsinks get their colour from a process called anodizing, where their surface is coated electrolytically with a protective or decorative oxide. While a black heatsink will emit large amounts of radiation, most of the heat will be dissipated using convection. The performance difference between an anodized black heatsink and an identical heatsink of a different colour will be negligible. Other than looking cooler. And who wants to compromise performance for aesthetics?

Heatsink Theory Guide - Design

DesignThough the material used is a key factor in determining the performance of a heatsink, another very important aspect is the design. When I say design, I mean the heatsinks' shape, size, fins, pins and so on. Firstly, the larger the heatsink's surface area, the more heat that will be transferred.Surface area is basically the amount of the heatsink's surface that is exposed to the air. This is why, rather than making a heatsink one solid block, manufacturers often include fins or pins on their heatsinks to increase its surface area without increasing the size of the heatsink itself. The surface of each fin is often ribbed as well, increasing surface area even further.The base of the heatsink should be flat to allow heat to easily transfer from the CPU, to the heatsink and into the fins where the heat is dissipated. The heatsink's fins should also be aligned vertically so air can flow freely through the heatsink.To make them more attractive to the consumer, manufacturers also tend to anodize their heatsinks. As mentioned earlier, the colour of the heatsink can also determine how well it radiates heat, however; there will be no real-world performance difference. Make sure you do not purchase a heatsink purely based on the way it looks. Keep in mind that sometimes units are not scientifically designed for maximum performance, but rather designed with aesthetics mainly in mind. This can be very off-putting to the unsuspecting consumer who will most probably purchase a heatsink because it looks better. A lot of the time the simplest looking heatsink can perform the best.

Heatsink Theory Guide - Manufacturing Methods

Manufacturing MethodsHeatsinks can be classified in terms of manufacturing methods and their final form shapes. The most common types of air-cooled heatsinks include stampings, extrusions, forging, bonded/fabricated fins, die castings and folded fins. Following is an explanation of each technique. Stampings - Copper or aluminium sheet metals are stamped into desired shapes. They offer a low cost solution to low density thermal problems. They are suitable for high volume production, because advanced tooling with high speed stamping would lower costs. Additional labour-saving options, such as clips and interface materials, can be factory applied to help to reduce assembly costs. Extrusions - This is the most popular method of manufacturing heatsinks. Extrusion is the process by which long straight metal parts can be produced. It is done by squeezing metal in a closed cavity through a tool, known as a die, using either a mechanical or hydraulic press. After the metal has cooled, it is cut to the size you want the heatsink to be, based on motherboard standard sizes. Forging - This is the process whereby metal is heated and shaped by plastic deformation by suitably applying compressive force. Usually the compressive force is in the form of hammer blows using a power hammer or a press. Heatsink manufacturers rarely use this method but it is used occasionally, and is definitely worth mentioning. Bonded/Fabricated Fins - Most air cooled heatsinks are convection limited, and the overall thermal performance of an air cooled heatsink can often be improved significantly if more surface area can be exposed to the air stream. These high performance heatsinks utilise thermally conductive aluminium-filled epoxy to bond planar fins onto a grooved extrusion base plate. Die Castings - Die casting is where the metal is injected into the mould under high pressure of 1,450-30,500psi. This results in a more uniform part, generally good surface finish and good dimensional accuracy, as good as 0.2% of casting dimension. In other words, you can make more complex shapes using die casting than you can using other methods such as extrusion. Folded Fins - Corrugated sheet metal in either aluminium or copper increases surface area, hence the volumetric performance. The heatsink is then attached to either a base plate or directly to the heating surface via epoxying or brazing. It is not suitable for high profile heatsinks on account of the availability and fin efficiency.

Heatsink Theory Guide - Fans & Thermal Interface Material

FansAlmost all of today's desktop CPUs (with the only exception being VIA's C3) require a fan attached to a heatsink to keep them sufficiently cooled. When choosing a fan to go with your heatsink, there are a few important things you should look at. The first is the type of fan. Two types of fans that usually come with heatsinks are ball bearing fans and sleeve bearing fans. Most people prefer ball bearing fans rather than sleeve bearing fans because they spin faster, thus increasing cooling performance. Sleeve bearing fans have also been known to fail when they get old, which would be a disaster, especially for owners of AMD processors, since seconds after the fan stops working , the processor will be fried. A good way to tell if your fan is of good quality is to carefully listen to the sound if makes while it's running. you should just hear the sound of the air flowing, which can sometimes be rather loud depending on the speed of your fan. You should not hear a buzzing sound and if you do, chances are your fan's motor is poor. If you would like high performance and do not mind a high noise level, I recommend a 7,000RPM ball bearing fan. However, if you would like a relatively quiet PC, you should be looking at a fan that rotates at around 4,000RPM.Thermal Interface MaterialTo have the best possible heat transfer, the base of your heatsink has to be flat and free of air gaps. While a heatsink's base may often seem flat, there are always tiny bumps and dents that you cannot see and no matter how much you lap your heatsink, you will never get it perfectly flat.To combat this problem, a TIM (Thermal Interface Material), such as a thermal compound or thermal pad, is used. A thermal pad usually comes preinstalled on a heatsink and is made out of graphite or some sort of polymer. while no installation is required and they are all that is need for most users, the performance they offer is far inferior to that of a thermal compound.A thermal compound is a paste that is applied to the CPU or the heatsink. Most thermal compounds consist of silicon, silver and metal oxide. This is because these materials provide especially high thermal conductivity.

Heatsink Theory Guide - Testing the Theory

Testing the TheoryWhile all this may look good on paper, it is useless if there is not a significant performance difference. To show you just how important choosing the right heatsink/fan unit is, I compared the Globalwin CAK38 to a stock heatsink/fan unit that is usually bundled with AMD processors. The CAK38 features all the right components for a high performance heatsink. It is made out of pure copper, features many thin fins for increased surface area and is cooled by a very effective 7,000RPM fan. The AMD stock heatsink on the other hand is a block of aluminium with fewer; thicker fins and is cooled by a measly 4,800RPM fan. Both coolers were tested on an AMD 1.2GHz Athlon Thunderbird processor and the temperatures was measured using a Senfu Thermometer Probe.To make things fair, I tested the stock AMD heatsink with a 7,000RPM fan as well as its stock fan. To make sure the processors were running at full load, I ran an hour of Quake 3 Arena loops with Prime95 running in the background.As you can see from the test results, choosing the right heatsink is very important. The Globalwin CAK38 was able to outperform the stock AMD heatsink by 10°C when running with stock fans, and by 5°C when both heatsinks had 7,000RPM fans installed. This is a huge difference and it is clear to see why many users do not even consider using stock heatsink/fan units with their processors. This applies even more so to overclockers. Hopefully armed with the knowledge given to you in this article, you will be able to make a much more educated decision when purchasing your next heatsink/fan combo.

Heatsink Theory Guide - Conclusion

ConclusionWith processors rapidly evolving and therefore the requirements for more effective cooling solutions, a lot of you may be asking, 'Will there be a time when an air cooled heatsink will not be sufficient cooling for a processor?'. Well, as long as alternatives such as water cooling and peltier cooling remain expensive and difficult to install, air cooling will continue to be the preferable choice among most users for some time to come.That said, as processors get faster and hotter, we will most likely see more heatsinks made out of better heat conductors, such as silver, because aluminium and maybe even copper may not be able to cope with the heat output of future processors and even chipsets.In an ideal world, as processors evolve they should somehow generate less heat, thus allowing heatsink manufacturers to focus mainly on producing a cost effective, low noise computing solution. Thermal management is also a big issue for processor manufacturers during research and development. But, as they say in latin, 'Tempus Omnia Revelat' (time reveals all). So I guess we'll just have to sit back, wait and see.