No matter how highly polished a surface may seem, when seen under a microscope it will have some irregularities. When two such metal surfaces are put together, air within the irregularities prevents heat from flowing efficiently from one metal to the other. A heat sink is usually placed on a micro-controller to remove the heat generated within the CPU. However, this arrangement will not work in the way desired unless some filler material is used to replace the air in the gaps in the interface between an IC and its heat sink. Such filler materials are usually Thermal Interface Materials or TIMs and generally called as thermally conducting paste.
Thermal pastes are semi-solid materials with very high heat conducting properties. These are placed between two surfaces, usually between a CPU/GPU and its heat sink, to allow better heat conduction between the two. Irregularities on the two surfaces may trap air between them, leading to a loss in the performance of the heat sink, as air is a very poor conductor of heat. TIM, being more than 100 times a better conductor of heat compared to air, improves the performance of the heat sink. However, the heat conducting property of thermal paste is not as good as Aluminum (heat sink material) or Copper (IC material), and a thick layer of TIM might actually hinder the ability of the heat sink to conduct heat properly.
A magnified view of the cross-section of two mating surfaces shows the microscopic imperfections that can trap air as in a pocket. TIM would fill-in these areas, replacing air. If you could have two perfectly smooth and flat surfaces, thermal paste would not be required to aid in heat transfer. However, that being impossible to achieve, thermal paste is necessary to improve the flow of heat. Depending on the application, you could use one of three types of thermal pastes – Silicon based, Ceramic based or Metal based.
Silicon based TIMs are generally used in cooling kits. These are commonly known as thermal pads and stock heat sinks use them freely. However, other pastes have better heat conducting properties.
When you need a paste that can also isolate electrically – Ceramic-based TIMs are hard to beat. They consist of a thermally conductive paste with plenty of tiny ceramic particles, which prevent flow of electricity. Although not as good as the Metal based TIMs, the difference is only minimal.
Metal-based TIMs are the most popular. These have the best performance characteristics, which comes from the innumerable tiny metal particles present in the high thermally conducting paste. However, this paste has a disadvantage that it can also conduct electricity.
Apart from the above three types of TIMs, there are also thermal epoxies. Although these function in a manner similar to the thermal pastes described, they possess an additional property – they can attach the heat sink permanently. Therefore, thermal epoxies must not be used when you need to get the heat sink off an IC. In case you have to separate a heat sink from an IC bonded together with thermal epoxy, the best procedure is to place the assembly in a freezer. The cold makes the epoxy brittle and it comes off easily.