Thermal compound is used to fill the tiny gaps between a computer’s processor and its heat sink (or water block, for those using water cooling). It improves the thermal conductivity of the connection and lowers CPU temperatures. Without it, a CPU would run much hotter and might fail.
Even brand-new heat sinks and processors don’t join together perfectly. The contact surfaces have microscopic scratches and imperfections that trap pockets of air – one of the worst thermal conductors around. Thermal paste helps bridge the gap and improves the transfer of heat to the cooling mechanism.
Most mainstream thermal greases are a combination of a bonding material and a filler. The bonding material is usually some type of silicone, urethane or acrylate while the filler is made from metals or ceramics – the choice depends on the intended use. Most consumer-grade thermal compounds use a zinc oxide based compound for the bonding layer while high performance products often include metals like silver and gold for improved conductivity.
Some newer thermal compounds are also electrically conductive. They are designed to bridge the heat sink and processor in such a way that electricity is transferred through the thermal compound rather than directly between the two components. These types of compounds require more attention and focus during installation as the slightest mistake could fry a computer.
Most traditional thermal compounds are non-conductive. They are generally easier to work with than their liquid metal counterparts and offer good performance at bargain prices. Some of them are still a bit on the dry side though and can leave nasty lumps and ridges in the gap between CPU and cooler.