The second thermal impedance required is “case-to-sink” denoted by the symbol R_{θ C-S}. This is a measure of how easily heat can be transferred from the surface (case) of the device to the surface of the heat sink. Due to irregularities in the surfaces of the TO-220 case and heat sink base, it is generally recommended to use a Thermal Interface Material (TIM or “thermal compound”) between the two surfaces to ensure that they are fully engaged from a thermal perspective. This greatly improves the transfer of heat from the TO-220 case to the heat sink, but does have a thermal impedance associated with it that must be taken into account.