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Step 1: Understanding what heat is

This is not as obvious a point as it may first seem. Heat is infact molecular vibrations in any substance. The hotter something is, the more the molecules/particles in that substance vibrate. Temperature is a measurement of these tiny vibrations. The more vigorous the vibrations, the higher the temperature. Because of the transfer of electrons within components inside computer chips, friction is generated and this energy causes the molecules within the component to vibrate (Friction itself is infact a form of heat generated by 2 or more particles rubbing together. EG. Rub you hands together for a few seconds and they will feel warmer. This is friction.)

Step 2: How heat is transferred

Heat is transferred because when a particle vibrates, it stimulates the particles beside it to vibrate. Think of this as holding a piece of rope at one end and shaking it. This causes the rope to shake a fair ammount down aswell depending on the resistance it encounters. EG if you wave it in the air, the rope will likely move all over. If the rope is on the ground, it will likely move alot less. Think of this as an example of particles stimulating neighbouring particles, which is heat transfer, and also as an example of thermal resistance, which is what makes some materials good at conducting heat, such as copper, and some materials bad at conducting heat, such as plastic.

Step 3: Thermally bonding the Heatsink and Component

Now we know the basics of heat transfer, and thermal resistance, how can we translate this into how a heatsink works?

Well lets overview the set up so far. We have a CPU or other heat generating component, and a block of metal with low thermal resistance. The problem now is how do we efficiently transfer this heat from the component to the heat sink?

The answer is thermal compound, or Thermal Interface Matrial (TIM). This is usually a paste like substance with VERY low thermal resistance, and is usually made with particles of silver. Silver has very low thermal resistance, hence why it is used in TIM's. Also, TIM is designed as a paste so that it fills in the microscopic imperfections (Or valleys) on the surface of the CPU and heatsink so that all of the surface of the CPU is thermally bonded (linked to in a way which allows heat to be transferred) to the base of the heatsink.

Our setup is almost complete. We have a Heat source (Component of some sort), the TIM, and the Heatsink. The dissipation (moving and removal) of heat from the component can now begin. The particles in the component vibrate, causing the particles in the TIM to vibrate. This i turn causes the particles in the heatsink to vibrate.

Step 4: But how does that make the CPU cooler? If more particles are vibrating, surely that means the component gets hotter?

Everytime a vibrating particle causes a neighbouring particle to vibrate, it transfers energy. We know that energy cannot be created from nothing, and in the case of a heatsink, the energy is generated from the power (Voltage and Current) being supplied to the component being cooled. When this energy is transferred, the particle which causes the neighbouring particle to vibrate loses energy. It vibrates less. Infact, this occurs to the point where the particle vibrates near enough as much as it neighbouring particle.

Within this theory lies the fundamentals to the Heatsinks operation. By spreading the heat accross a large piece of material, we can reduce the temperature of the component.

Step 5: Optimizing the heatsinks design

So we have a block of lets say copper thermally bonded to a heat producing component. How can we increase the efficiency of this block? Well for a while the block will work reasonably well (not nearly as good as todays heatsinks though) but there is a problem. Heat is only being spread accross the materials. Think of this example. You are buttering a slice of toast. Think of the butter as heat, the knife as the component, producing the heat, and the toast as our copper block. We continue spreading butter onto the toast, so what happens? More and more butter (heat) accumulates on the toast (copper). This is a problem as the component is now going to get hotter and hotter because the copper is heating up with the component.

To get rid of this heat of our material, we introduce a way of transferring this heat away from the heatsink. It may be air, or water, or some other substance such as liquid nitrogen or dry ice.

For the sake of arguement, we'll look at the most common medium for this application, air. So our job now is to transfer as much heat as possible from the copper to the surrounding air. How do we go about this? Well we know that a larger surface area being cooled is better (TIM being used to use as much of the CPU and heatsinks surface as possible to transfer heat) so we can turn the copper block into a block with fins to increase the heatsinks contact with the air surrounding the heatsink. Sorted, now the heatsink is dissipating heat into the surrounding air.

Step 6: How the heat in the air (or other medium) is moved away from the heatsink

Thankfully, physics is on our side in this. As most of you will know, hot air is less dense than cold air, because the vibrating particles cause themselves to be spaced further apart (Because they essentially push their neighbouring particles away from them). Because of this, the hot air around or copper fins rises up and away from the heatsink (EG, a hot air balloon. The balloon is filled with hot air, and the balloon rises.) Due to air pressure, cold air from underneath is sucked up into the heatsink. Of course you won't actually notice this, as it is no where near the type of suction you would get from something like a FAN.

Which brings us onto or next point.

Step 7: Increasing the flow of air to the heatsink

Quite simply, mount a fan onto our copper fins to push more more air accross the heatsink. This means that more heat can be dissipated from the heatsink and transferred into the air. This hot air then rises, and is usually sucked out of your case via an exhaust fan at the back of your case.

So there you have it. You now know how a basic heatsink works. Heatsinks today also implement heatpipes, which are hollow copper pipes filled with a liquid which evaporates and condenses to transfer heat.

I know this was a bit of a physics lecture, but I think that it really helps you to understand exactly what is going on. I hope this helps you decide on a heatsink to purchase, or helps you with designing or optimising your own custom heatsink solution.

woop

Good job - well written

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