Heat Sink Thermal Resistance Calculator
Size a heat sink by finding the maximum sink-to-air thermal resistance that keeps a component below its junction temperature limit.
How it works
Tj = Ta + P · (θJC + θCS + θSA) · θSA(max) = (Tj,max − Ta) ÷ P − θJC − θCS
Heat flows from the silicon junction to the case, through the mounting interface, into the heat sink, and finally to the air — a chain of thermal resistances in series. The total allowed resistance is the temperature headroom (junction limit minus ambient) divided by the power being dissipated. Subtracting the fixed junction-to-case and interface resistances leaves the budget for the heat sink itself: pick a sink rated at or below that number. A lower θSA means a bigger or fan-cooled sink.
Worked example
A part dissipating 15 W with a 125 °C junction limit in 35 °C air has 90 °C of headroom, so the total budget is 90 ÷ 15 = 6.0 °C/W. Subtracting 0.8 °C/W junction-to-case and 0.3 °C/W interface leaves 6.0 − 1.1 = 4.9 °C/W for the heat sink.
Sources: Thermal resistance
Frequently asked questions
What if the required resistance comes out negative?
A negative or zero θSA means the device and interface alone already use up the whole temperature budget, so no passive sink can help. You need a lower-power part, a better thermal interface, forced-air cooling, or liquid cooling.
Where do I find θJC and θCS?
θJC (junction-to-case) is on the component datasheet. θCS (case-to-sink) comes from the interface material — a dry contact is worst, thermal grease is better, and a good pad or grease with clamping pressure is typically 0.1–0.5 °C/W.
Does a fan change the result?
A fan lowers the heat sink’s θSA — the same sink has a much lower rating in forced air than in still air. If your calculated budget is hard to meet passively, a fan is often the cheapest way to reach it.
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This calculator is for educational purposes. Double-check important results before acting on them.