Thermal paste, also known as heatsink paste, and thermal compound paste, is a conductor of heat energy. Its purpose is to fill the microscopic air gaps and voids on the metal surfaces, thus enhancing the transfer of the heat between the interfacing surfaces.
There are many uses of pastes ranging from simple electronics usage to high-end computer processor applications. They have different thermal conductivity, and some are electrically conductive, whilst others are not.
Thermal compound is readily available at places such Maplin, PC World, and RS and arrives in small tubes. Usually, replacement involves the removal of dried-out paste using isopropyl alcohol, or acetone such as nail polish remover, and application of an extremely thin layer of the paste between the mating interfaces.
How to Apply
Air gaps prevent the conduction of heat energy because air is a poor conductor of heat. Therefore, the purpose of thermal paste is to fill these microscopic air gaps on the surfaces, which has the effect of increasing thermal conductivity. When this happens, the heatsink can draw away more heat from the component.
Ideally, you want to apply a very thin layer of paste on the metal surfaces before fixing the heat sink to the component. The thinner the layer, the better, keeping in mind that too much can actually behave as an insulator and reduce thermal conduction.
Manufacturers usually provide a spreading spatula and there are many specific spread methods. If you do not have a spatula, then a sharp straight edge such as a blade usually helps to spread the paste thinly and evenly.
Types and Alternatives
The general rule of thumb is that white coloured pastes are ceramic based and non-conductive. They are usually utilised in electronics for mounting power transistors to heatsinks, however you must always check the documentation first to make sure.
You can also buy metal-based thermal compounds, which are a little more expensive and grey in colour. They usually contain metal particles because metal is an even better conductor of heat energy. Silver, aluminium, and zinc oxide particles increase thermal conductivity; however, these compounds are also electrically conductive, which means that special care is required when applying them. If you were to apply some on the CPU pins then it can cause all sorts of hardware malfunctions such as computer resets and lock-ups.
Working in IT, I have often had to correct the work of the so-called “engineers”, and usually find compound inside the CPU socket, on the CPU pins, and even on the motherboard. Many engineers are simply not aware that these compounds conduct electricity.
The conductive type of paste is not for mounting power transistors either. Usually power transistors have a layer of mica between the mating surfaces to prevent electrical conduction. If you used a conducting type of compound here, then the consequences can be disastrous.
Thermal paste is relatively inexpensive, depending upon where you buy it. The 31-gram tube of thermal paste pictured above came from eBay for one pound, and it comes all the way from China.