Thermal grease is applied to the surface of a central processing unit (CPU), the main chip of a computer, before attaching a heat sink. A heat sink helps lower the temperature of the chip by dispersing heat. The grease creates maximum surface-to-surface conductivity to draw heat away from the CPU and into the heat sink. Without it, imperfections in the surface of the CPU wafer and heat sink would allow air gaps, reducing the effectiveness of the heat sink, leading to overheating, errors, and possible failure.
There are different types of thermal grease, each with its own level of thermal conductivity. People who put an extra workload on the CPU, such as gamers and overclockers, might be more interested in getting a highly efficient compound.
Thermal tape is the least messy and least expensive of interfaces; however, it also the least effective. It is not recommended for most computers.
Thermal grease pads are often included in retail CPU packages that include a heat sink and fan. The pad is commonly gray-colored with protective wrapping that should not be touched or smeared once the wrapping is removed. It melts with heat, conforming to the surfaces of the CPU and heat sink, then stiffens when cool. The pads are sufficient solutions but not considered outstanding. Anyone who is considering replacing the thermal compound in the CPU should read the computer's warranty. Some manufacturers require customers to use the provided materials to keep the CPU warranty from being voided.
Silicon and zinc-based grease is a white paste that usually comes in a tube. It is not heat conductive itself, but does fill potential gaps between the surfaces, providing a satisfactory interface.
Ceramic-based thermal compounds include particles of materials such as aluminum oxide. Though ceramic paste is normally considered mid-grade, one popular brand has outperformed silver-based greases in several independent tests. This formula contains 5 sub-micron particle shapes to better fill the microscopic valleys in the CPU chip for greater thermal transfer. Maximum performance is reached after several cycles of normal use — using the system as needed, and then turning it off to cool. Once this period is completed, the computer can be left on all the time if the user wants to do so. The advantage of ceramic thermal grease over silver-based grease is that ceramic grease is not electrically conductive.
Silver-based thermal grease contains highly conductive metal particles. It is excellent for conducting heat but can also conduct electricity. If accidentally applied to CPU pins or motherboard circuitry, it can cause an electrical short, so caution is advised when applying metal-based compounds. Used correctly, however, it is generally considered one of the best thermal compound options.
Regardless of the type of thermal grease used, users should follow the manufacturer’s directions carefully. Different types should not be mixed. Whenever a heat sink is removed from a CPU, both surfaces must be thoroughly cleaned and new grease applied before the parts are reinstalled. Rubbing alcohol can be used to safely remove most thermal compounds.