Tj = Ta + Pheat·(Rint + Rhs), with Pheat = Pel·(1−WPE). The die+solder+board resistance is baked in (≈0.05 K/W — minor next to the heatsink). Heatsink area is the length of a fixed, typical ribbed profile (100 mm wide, 28 mm fin height, 7 mm gap, 2.2 mm fin thickness; base pressed to the source, vertical, ε=0.80 anodized finish). Heat transfer uses the same formulas as the companion widget "Aluminium heatsink dissipation calculator": the Bar-Cohen & Rohsenow correlation for an array of parallel fins, fin efficiency η=tanh(mL)/(mL), and Stefan-Boltzmann radiation with a cavity-effect correction. Steady state; for a heatsink with different proportions use that widget directly.