Thermal paste
Thermal paste is a substance that increases the thermal conductivity between the surfaces of one or more objects, providing a "thermal seal" when these surfaces are not perfectly polished. In electronics, the use of thermal paste is common to aid the thermal dissipation of a component via a heat sink
Purpose
The main purpose of a thermal paste is to ensure optimal contact and avoid the presence of air between the surfaces of a component and its cooling system (often a heat sink). As these surfaces have many micro porosities (holes, bumps), air is present between the component and the heat sink. Since air is a poor thermal conductor, heat transfer is less efficient.
The application of thermal paste makes it possible to fill these imperfections with a substance whose thermal conductivity is much higher than that of air. The contact surface between the component and the heatsink is thus larger: heat transfer will be more efficient
Thermal paste is also sometimes used to hold the heatsink to the component, some pastes being very sticky. This is the case, for example, with certain heatsinks designed to cool integrated memory circuits on graphics cards, where only the thermal paste (which sometimes takes the form of a piece of double-sided adhesive tape) holds them in place
