Designers should take into account mechanical stresses at the interfaces caused by differing material coefficients of thermal expansion. The idea is to eliminate any air pockets between the two surfaces. The most common interface material is thermal grease, typically made from zinc oxide in a silicon or petroleum base. There are also pastes available with thermal conductors such as aluminum oxide and aluminum nitride. Pads and foils are less messy to apply and can be cut to match the component footprint.
Some pads are available with adhesive surfaces to allow permanent attachment.
Aluminum oxide and aluminum nitride are used in thermal pads, as are sheets made from graphite, indium, and aluminum. Thermal epoxies create rigid, permanent bonds. They are typically supplied as two-part systems comprising a hardener and a resin filled with silver, aluminum, aluminum oxide, or aluminum nitride. Because they are permanent, epoxies should be used only in areas that will not require future disassembly. Rigid bonds can also be achieved using solder.