[This is an opportunity to practice my English. Please correct me should you find any mistake.]
Thermal grease (also called thermal gel, thermal compound, thermal paste, heat paste, heat sink paste or heat sink compound) is a viscous fluid
substance, originally with properties akin to grease
, which increases the thermal conductivity
of a thermal interface by filling microscopic air-gaps present due to the imperfectly flat and smooth surfaces of the components; the compound has far greater thermal conductivity than air (but far less than metal). In electronics
, it is often used to aid a component's thermal dissipation via a heat sink
As an example, I hereby mention heat sinks a little. I think this will give a brief view and ease the way to understand what thermal grease is. There are heat sinks in the picture below (one on the left in the picture made by aluminum and a fan, one is made in aluminum fins and six hot pipes with a fan inside, which has been already installed on the motherboard, one embedded in video card Sapphire HD 7750 Ultimate). The thermal grease in the picture is ST700, made by Balance Stars Co LTD. There three CPUs as well made by AMD.
The thermal grease is used between chip and heat sink to conduct the dissipated heat by the chip when it is working, especially at high loads, or too much heat may gathered and temperature increased greatly which will damage the chip or make it self protection shutdown. The thermal grease is a must because both the heat sink base and CPU cap (IHS) are not smooth when magnified to some extent. Look the figure below. Because the air is a poor thermal conductor, we have to use some paste (thermal grease) to fill the chink between the heat sink and CPU cap to drive air away.
There are different kinds of thermal grease such as ceramic-based, metal-based, carbon-based and liquid-metal based. The most common form of thermal grease these days is ceramic-based which is included with every retail AMD and Intel processor you buy. If you look on the bottom of the heat-sink fan you will see a black pad. That is the thermal grease. When you have a setup like this you do not have to apply any thermal grease to the processor as it will bind from the heat-sink. The problem is the heat sink included in retail processor package generally does not work very well to dissipate heat when the chip working at high load (e.g. when playing games). At this occasion, except a lot of noise generated, the temperature of the processor also increases greatly in a short duration. Other occasions are at over-clock and high load, the processor's temperature may increase sharply. The best solution to solve this high temperature problem is to use powerful heat sink with better thermal grease.
Coating thermal grease is easy, just as mentioned in picture below. One thing to remember is to coat as thin as possible, because the heat transfer ability of thermal grease is far behind metal (generally copper for the IHS of CPU or heat sink base), even you are using the best thermal grease.