This article will attempt to present some ideas along with a general engineering analysis of a CPU (or other component such as a graphics chip or chipset) cooling system. The object is to lay out the physics behind such a system in terms that most should easily understand. There will be a series of articles dealing with basic cooling principles and physics, as well as specific design guidelines for successful water and air-cooling.

To give a little background on myself, my degree is a Master's of Engineering with emphasis on structures and heat transfer. I worked during my graduate years designing and building cooling systems for satellites that use extreme cooling methods such as liquid nitrogen and colder methods.

My current occupation is as a process engineer for a large contract manufacturer in the printed circuit assembly business. We basically build the boards that other companies (i.e. HP, Dell, Sun, etc.) put their name on and sell to the public. It is my (along with many other engineers) duty to make sure that all the necessary processes are in place to be able to properly build the boards. If my education, current job, and hobbies are all rolled into one, what to you get? An overclocking freak that has to try everything at least once!

There is a lot of general confusion about the different modes of heat transfer and how they relate to system cooling. To fully understand how cooling works, it is imperative to understand the basic modes of heat transfer.

Conduction is the transfer of energy (heat) from a more energetic particle to that of a less energetic particle by direct interaction. This is what happens when someone sticks his or her finger on a hot iron (or for that matter an overclocked AMD processor, which may actually be hotter than the average iron!) The particles in the iron have more energy than the particles in the finger, and thus there is a net energy transfer from one to the other (ouch!).

Convection is the transfer of energy from a solid surface to that of a moving fluid. In order for heat to be transferred by convection, it must first conduct from the hotter material (either the fluid or the solid) by molecule to molecule interaction, and then the moving fluid displaces the molecules closest to the solid with other molecules as they move along.

Radiation occurs when there are two surfaces at different temperatures that emit electromagnetic waves between each other. This is basically how the sun warms the earth and everything on it. This mode of heat transfer is virtually negligible when talking about cooling your processor and motherboard; unless of course you are in a vacuum (space) or you computer is in a black case sitting out in the sun.

It is absolutely vital to understand the differences between the modes of heat transfer and when they occur in a computer system. I cannot stress this enough! There is an absolutely amazing amount of myth and misinformation circulating the internet overclocking world that could be easily put to rest if everyone could grasp a basic knowledge of the nature of heat transfer.

As power is applied to a CPU, it manifests itself in the form of heat and that heat (energy) must be moved away from the CPU or the CPU will eventually rise to an intolerable temperature and possibly burn out. To compound this problem, we as overclockers will often apply even more heat (energy, in the form of additional voltage) to the CPU in an effort to get maximum performance.

To remove the heat, we use either a heatsink/fan combo, waterblock, or a peltier combined with either of the other two aforementioned methods. Both the waterblock and the heatsink/fan systems act in basically the same way. A solid material conducts the heat away from the CPU and a fluid, either water or air, convects the heat away from the waterblock or heatsink and dumps it outside of the system.

In a peltier cooled system, the cold side of the thermal electric cooler cools the CPU by conduction but in order to do so uses additional energy. This energy must be removed from the hot side of the peltier in the same way as direct heatsink or waterblock cooling.

Remembering the definitions of conduction and convection, let's follow the path of the heat and its transfer methods from the CPU to the outside of the case. I'll only use the terms 'heatsink' and 'air' for this particular discussion, but 'waterblock' and 'water' could both be substituted instead; the physics are nearly identical.

As heat leaves the CPU, it is conducted directly into the heatsink. In order to get good conduction and thus effective heat transfer, the heatsink and the CPU must be in good contact with each other. Whenever there are multiple layers through which heat must conduct, there is what is called a contact thermal resistance. This is defined as such that where the two solids meet (i.e. the CPU slug and the heatsink) there will be a thermal contact resistance that results in an immediate temperature drop across the joint.

There is no such thing as a perfectly smooth surface and thus the two mating surfaces will not fit together flawlessly. The rougher the surfaces and the more imperfect the fit, the higher temperature drop across the interface. This is why it is important to use a quality thermal interface material between the CPU and the heatsink.

Often we overclockers even go to the extreme of lapping, or sanding down, either the CPU or the heatsink, or both, in order to get a more perfect fit. If there is a large gap or imperfect fit between the CPU and the heatsink, heat from the CPU will not be effectively transferred into the heatsink and the CPU will remain at a much higher temperature than the heatsink (bad!).

After the heat has been transferred into the heatsink, the material of the heatsink will then distribute the heat throughout itself. How well a heatsink does this is primarily a function of the thermal conductivity of the material.

Thermal conductivity is defined as the proportionality constant that when multiplied with the ratio of the temperature change to the change in distance from the zero plane, will result in the value of the energy flux (Fourier's Law). This basically means that a higher thermal conductivity constant will result in a material moving heat along its geometry more effectively. For example, the handle of a wooden spoon stuck in boiling water will not get nearly as hot as the handle of a metallic spoon because metal has a much higher thermal conductivity than wood.