App note from Diodes Incorporated about thermal resistance and how to manage them in real world scenario. Link here (PDF)
There can be significant differences between the thermal characteristics stated on a device datasheet and what actually happens in a realworld application. Semiconductor manufacturers usually provide thermal resistance values for Junction to Case (RθJC) and Junction to Ambient (RθJA); although these are extremely useful parameters to estimate a device power handling capability, there can still be a disconnection between those figures and reality.