Thermal compound, also known as thermal grease or thermal interface material, fills the microscopic gaps between a CPU and its heat sink, helping to transfer heat more efficiently from the CPU to the heat sink. It is essential for keeping the processor cool and may be the difference between a device that fails due to overheating and one that lives on for years.
There are two types of thermal paste: non-conductive and conductive. Non-conductive thermal compounds, such as ceramic and silicone-based products, are totally safe to use with electronics. They are also a good choice for beginners as they are easy to apply and clean up.
Conductive thermal compounds, such as metal-based products, on the other hand, need more focus and attention to apply correctly, and they are not recommended for novices. They also conduct electricity, which means they can cause electrical short-circuits if you spill them on the CPU pins or motherboard components accidentally. These products are typically much more expensive than the non-conductive alternatives.
The quality of a thermal compound is expressed in a number called its “thermal conductivity”, measured in Watts per meter per Kelvin (W/mK). Higher numbers mean better thermal efficiency. Most thermal compounds come in quality tiers, from consumer to high-end for overclocked systems.