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Heatsink and TIM Basics
February 10 2008
If you have ever opened a PC case, then one of the key components you have immediately seen is the system's CPU cooler. A system's CPU cooler is the most important cooling component in a PC since, the vast majority of the time, the component which generates the most heat inside your PC is your CPU; and in order for the CPU to function correctly that heat needs to be dispersed. Have you ever tried to turn on a PC without a CPU cooler in place? Well, please DON'T EVER TRY IT! I have actually done this before accidentally and the CPU immediately fried with a loud "POP". Newer CPUs have fail-safes in place which are meant to circumvent this problem but it is always better just to be safe and never turn on a PC without a CPU cooler properly mounted first.
Heat is actually molecular vibrations in any material and oOur fingers interpret this vibration as heat. The greater the vibration, the hotter the material is/feels. In order to cool down the material these vibrations need to be dispersed or thinned-out in some way. If you have ever played with a tuning fork, try to think of when you would strike it and let it stop vibrating on its own. If you were to touch the fork, your finger would absorb the vibrations and the humming would stop along with the vibration. This is a very very general example of what your CPU cooler does. It takes the heat (molecular vibrations) generated by your PC's CPU and disperses it in order to keep the CPU cool and prevent any heat build up which could kill your processor.
So, "Why were older heatsinks made of aluminum and newer heatsinks are made from copper?", you ask? Aluminum and copper are both materials which have a relatively low thermal resistance. Thermal resistance is a measurement used to gauge how well a material is able to transfer heat (molecular vibrations). There are materials such as silver which have a low thermal resistance that are very good and transferring heat and then there are materials, such as plastic, which are very poor at transferring heat. As you might guess, it is much more practical to use a material with a very low thermal resistance in a CPU heatsink vs one with a high thermal resistance as this allows you to draw heat away from the CPU vs insulating it and making heat issues worse. When it comes to transferring heat, Copper is better than Aluminum and Silver is better than Copper. The reason that you do not see Silver-based coolers (yet) is because silver is much more expensive than copper when used in the same quantities. Plus, the benefit of Silver over Copper is not that big; not many would want to pay $200+ in extra costs for a cooler simply to gain a 1 - 2 degree benefit.
Another major factor affecting a heatsinks ability to dissipate heat besides the material it is made of is the shape of the cooler itself. You already know how to check if an object, such as a stove burner, is hot by holding your hand close to it, right? This is an exaggerated example of heat dissipation. The heat from the burner is being dissipated by the air surrounding it as the air moves around the burner transferring some of the vibrations (heat) from the burner's materials into the air surrounding it. This in turn makes the air closest to the burner hotter to the touch. Heatsinks optimize this metal to air heat dispersion by creating as much surface area in a tight space as possible. This is where the many, thin fins on a heatsink come from. The more fins the heatsinks have, the more area there is for air to circulate and whisk away the heat from the heatsink, which in turn draws heat from the CPU. The addition of a fan on the heatsink helps to ensure that a larger amount of cooler air is on-hand to circulate around the heatsink and aid in the heat dissipation.
In order to efficiently transfer heat from the CPU to the heatsink you cannot simply strap the heatsink to the CPU and go. For heat to transfer between two objects there must be plenty of surface area connecting the two. The more area that is touching, the more heat can be transferred from one object/material to another. This is where TIM (Thermal Interface Material) comes into play. TIM is referred to in other ways among PC geeks... Heat Transfer Compound, CPU Goop, Thermal Paste, etc. The most popular, and most effective TIMs on the market are usually silver-based (remember that Silver has a low thermal resistance). This TIM is used by lightly spreading it over the surfaces which will have contact with one another. This material can then reach into very small surface cracks and material imperfections on both surfaces to ensure that heat is efficiently transferred from one object to another. The trick is to use a VERY THIN layer of TIM. If you use too much you will end up insulating the CPU vs cooling it.
Hopefully you have a better understanding of how a heatsink works and the importance of using TIM when mounting a heatsink. We will soon have a new article up outlining a method for picking the right heatsink(s) for your particular PC build; including picking after-market coolers for your video card, RAM and motherboard northbridge. While PC Apex is primarily a site for PC enthusiasts, we realize that all PC geeks start somewhere. If you have any questions regarding this topic please feel free to leave it in the discussion thread.