Each device has different thermal conductivity paths, but basically, you should calculate the thermal resistance from a semiconductor chip in a package to ambient air. Use this information when considering thermal dissipation for a system design. Power devices, in particular, have considerable loss, which significantly affects the system reliability and life time. Extreme care should be exercised when considering thermal design. Allow sufficient margins relative to the maximum allowable junction temperature for each device (120°C, 150°C, etc).
Thermal Design and Attachment of a Thermal Fin: Power MOSFET Application Notes (PDF:1,061KB)