Work out junction temperature instantly with clear inputs, formula shown and shareable results.
Junction temperature is ambient plus power times total thermal resistance. Reliability degrades exponentially with junction temperature — the common rule is that each 10 degree rise halves expected life — so designing to the absolute maximum is never appropriate.
Junction temperature
Tj = Tambient + P x Rja; P_max = (Tj_max - Tambient) / Rja
At least 10-15 degrees below the rated maximum at worst-case ambient, and more if the part is safety-related or expected to last many years.
Most wear-out mechanisms — electromigration, dielectric breakdown, solder fatigue — follow Arrhenius kinetics with activation energies that give roughly that doubling rate.