Heat Sink & Junction Temp

Thermal resistance in series

Heat flows from the silicon junction to ambient through a chain of thermal resistances, each adding a temperature rise of $P\theta$:

Turn it around to size a heat sink: for a junction limit $T_{j,\max }$ the sink must provide

A smaller ${\theta }_{SA}$ means a bigger or fan-cooled sink. If the required value comes out negative, no passive sink is enough — reduce $P$, add airflow, or lower ambient. Always design with margin below $T_{j,\max }$.