Work out thermal resistance junction to case instantly with clear inputs, formula shown and shareable results.
Thermal resistance is temperature difference divided by power. Junction-to-case is fixed by the package and is the part the designer cannot change; case-to-ambient is the heatsink and airflow, which is where all the design freedom lies.
Thermal resistance
Rjc = (Tj - Tc) / P; Rja = Rjc + Rcs + Rsa
Case-to-ambient. Rjc is a package property, so once it and the power are fixed, only better thermal interface material, heatsink and airflow help.
At 85 W even 0.3 C/W of Rjc costs 25 degrees of junction temperature that no external cooling can recover.