Eli5: How do heat sinks work?

Question

Curious how they work. Why do they look like lots of layers of fins.

In: 6

Answer 1

The purpose of a heatsink is to transfer heat away from an item and get rid of it into the atmosphere.

The problem we have here is that transferring heat from a small piece of metal into the surrounding air is actually quite tricky, as it takes a lot of time for the heat to transfer from the item into the air.

If you consider something like a computer processor that only had a surface area of a few centimetres squared, it can’t generate heat quicker than it can transfer it into the air.

This is all dependent on the surface area of the item that is radiating the heat. If one square centimeter of surface area can radiate a certain amount of heat per minute, then two square centimeters can radiate twice that amount.

The other important detail is that metals is a very good thermal conductor – heat up one end of a bit of metal and it will quickly distribute that heat over the full piece.

A heat sink combines those two details to help cool something down. Firstly, the fact that the heat sink is made of thermally conductive metal means that the heat produced by the item it is attached to will quickly get absorbed into the heat sink and distributed throughout its body.
Next, the finned design of the heat sink means the metal now has an absolutely huge surface area exposed to the air around it – the surface area of each fin added together. While each square centimeter of surface area can still only radiate a small amount of heat, if you have enough square centimeters all radiating small amounts of heat at the same time, that can add up to quite a significant amount.

The one other detail to remember is that sticking on a heat sink and transferring all of the heat an item is producing into the surrounding air will only really result in creating a bubble of hot air around your heat sink, at which point the air cannot absorb any more heat and it all stops working. So what we want to do is remove the hot air and replace it with cold air instead – by pointing a fan through the fins of the heat sink, this creates a lot of air movement and helps remove the heat more effectively.