Estimate junction temperature without over-trusting RθJA.
A first-pass thermal estimate is useful for screening, but thermal resistance is tied to a specified test structure. The real PCB, package interface, airflow, heat sink, and time scale determine the actual heat path.
Steady-state estimates
Use the ambient path only when the PCB and airflow resemble the datasheet condition. Use the case path when case temperature and the junction-to-case thermal resistance are meaningful for the package orientation and cooling arrangement.
Transient operation
For pulses, use the transient thermal impedance curve or a validated RC thermal network. Average power alone can hide a short junction-temperature peak. Repetitive pulses must also account for incomplete cooling between events.
Measurement closes the loop
Case or board thermocouples do not directly report junction temperature. Correlate measurements with electrical temperature-sensitive parameters or a validated thermal model when the margin is small.