Compared to a traditional fanned PC, especially small form factor fanned PCs, a fanless computer doesn’t necessarily always look dramatically different. Nevertheless, there are several subtle things going on that prevent these quiet, unassuming pieces of hardware from burning up. Fanless computers often cool based on the principle of conduction – that is, something hot dissipating heat into its surroundings through physical contact. While heat conduction is a natural phenomenon, just how much heat is dissipated can be significantly influenced by design.
The basic idea is to put heat-generating components, like the computer’s CPU, in contact with a heat sink. A heat sink allows the heat created by the components to flow through it and eventually into the surrounding air. When designing a heat sink assembly, it’s important to consider what materials you’ll use and how all its parts will interface with one another.
What Materials are Used in Heat Sinks?
It stands to reason that a heat sink should be made out of a heat conductor. Much in the same way an electrical conductor provides a pathway for the flow of electrical current, a heat conductor is a material through which heat can flow readily. If something hot (such as a CPU) is in contact with one end of a heat conductor and something relatively cold (such as the air outside the PC) is in contact with the other side, heat will rapidly flow away from the heat source through the conductor. Aluminum and copper are the industry-favorite conductors used for heat sinks.
Most OnLogic heat sink assemblies consist of two main parts: an aluminum block mounted on top of the CPU, and the case lid. Heat is transferred from the CPU into the aluminum block, then the lid, and finally the surrounding air. There need to be effective heat conducting interfaces between all of these parts in order to effectively cool the system.
How to Create an Effective, Heat Conducting Interface
Creating an effective interface for heat transfer is accomplished differently on the inside and outside of the PC. Air itself is a poor heat conductor, so on the inside of the PC it’s important to eliminate as much air as possible between the CPU and heat sink. Simply mounting the heat sink on the CPU is not enough since microscopic imperfections leave air pockets at the interface point. The most common way of getting around this is adding thermal paste, which is a better conductor than air. The thermal paste fills all the gaps that would otherwise have air in them.
On the outside of the PC, heat is being dissipated directly into the air so a different approach is needed. A common design is a lid covered in specially-engineered ridges. These ridges increase the total surface area exposed to the air, effectively adding more passageways for heat to escape.
These principles can help you avoid several pitfalls when assembling or implementing a fanless PC. For example, although heat paste is a better conductor than air, it is quite a poor one compared to copper or aluminum. This means that you should only use just enough to fill the gaps where it is needed. Any more and you are creating a thermal barrier that may overheat your computer. Another common complication to avoid is placing something on top of a fanless computer. Even a single piece of paper can trap a pocket of hot air on the PCs lid, preventing the computer from cooling. You should also consider the air surrounding the PC. If the surrounding air is unable to circulate (as it would be in a confined space) or too thin (such as at high elevations) it will have less potential for carrying heat away from the PC.
At OnLogic, we work with fanless computers every day. The more you are exposed to them, the more you appreciate just how much is going on in order to keep them running at optimum temperatures.