The first thermal impedance required is “junction-to-case” denoted by the symbol R_{θ J-C}. This is a measure of how easily heat can be transferred from the junction, where the heat is being generated, out to the surface (case) of the device (TO-220 in this example). Most vendors will list this impedance in the datasheet alongside the junction-to-ambient metric. For this example, the assumed junction-to-case thermal impedance is 0.5°C/W.