Thermal grease, also called a thermal compound, aids a heat sink in cooling a central processing unit (CPU). A CPU is the integrated circuit responsible for running a computer. The CPU is made up of millions of tiny electronic ‘switches’ jammed together into a core. Each time a switch operates, a small amount of heat is generated. With millions of these switches activating thousands of times each second, heat builds quickly. For intensive programs like computer gaming, a CPU works very hard and can potentially get very hot. Overclocking also increases thermal production.

A heat sink is a device that clamps on to the CPU, designed to draw heat away from the chip to protect it. The bottom surface of the heat sink is designed to be extremely smooth to maximize surface-to-surface contact with the CPU. But imperfections and pits in the CPU wafer allow tiny gaps that reduce the effectiveness of the heat sink. This is where thermal grease comes in. Thermal grease is a thin layer of one of a variety of compounds that thermally binds the CPU and heat sink together. Thermal grease fills any pits, gaps or imperfections that would allow air pockets between the chip and the heat sink.

Because thermal grease must conduct heat to be effective, it is made from specific materials that do this rather well. Some types of thermal grease are better at conducting, or transferring heat than others. Expensive thermal grease contains particles of silver for maximum effectiveness, while inexpensive compounds are silicon-based. Ceramic thermal grease is a mid-grade compound; one notable brand outperforms silver-based compounds according to many independent tests.

With a properly installed heat sink using thermal grease to maximize contact, heat is pulled off the chip and drawn upwards into the fins of the sink where it is dissipated by a fan. A CPU should never be operated without thermal grease and a heat sink. Temperatures can quickly rise to melt the core, making it inoperable.

CPUs sold with a heat sink and fan in a retail box include thermal grease, often in the form of a thin thermal pad. In most cases the warranty of the CPU depends on using the supplied thermal pad, heat sink and fan. If buying an original equipment manufacturer (OEM) CPU and supplying your own heat sink and fan, don’t forget to purchase thermal grease and apply it carefully per accompanying instructions.