Thermal compound is the goop that bridges your CPU package to your heat sink. It’s a grease with lots of minuscule thermally conductive particles mixed in. It doesn’t conduct quite as well as actual metal-to-metal contact, but it does much better than air. Without it, the gap where the flat surfaces of your CPU package and heat sink mate is filled with teeny-tiny bits of air – which are among the worst insulators on Earth. And that little bit of air impedes the CPU’s ability to lower its temperature, which can lead to overheating and failure.
There are several qualities to look for when picking a thermal compound:
The most important attribute is the “specific gravity” or “density,” which is usually measured on technical data sheets in g/cm3 and expressed as a number based on the weight of the product in g/cm3. The higher the density, the more dense the compound, and it will have a longer lifespan than compounds with less dense formulas.
You will also want to consider the thermal conductivity of the product, which is often stated on technical data sheets in watt per meter per kelvin (W/mK). The higher the number, the more efficient the product is at conducting heat.
Finally, you will want to consider how easy it is to use and clean up. If the thermal compound is sticky, it can be difficult to work with – and it will be prone to drips and runs. For these reasons, you will want to select a product with a moderately viscous consistency that will be stable when applying and resist drips and runs.