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Heatsink Thermal Resistance Calculator

Find a component's junction temperature from the power it dissipates and the chain of junction-to-case, case-to-sink, and sink-to-ambient thermal resistances.

Junction temperature
130 °C
Temperature rise above ambient
105 °C
Total thermal resistance
7 °C/W

How it works

Heat flows from a component's junction out to the surrounding air through a chain of thermal resistances, exactly the way voltage drops across resistors wired in series — thermal resistance (°C per watt) plays the role of electrical resistance, and power (watts) plays the role of current. Total thermal resistance is just the sum of the three stages: junction-to-case (from the component's own datasheet), case-to-sink (the interface material, like thermal paste or a thermal pad — enter 0 if the component is soldered or bonded directly with no separate interface), and sink-to-ambient (the heatsink's own datasheet rating).

The temperature rise above ambient is then power times that total resistance, ΔT = P × (R_jc + R_cs + R_sa), and the junction temperature is just the ambient temperature plus that rise: T_j = T_ambient + ΔT. A lower total thermal resistance — a bigger heatsink, a thinner layer of better interface material — means less temperature rise for the same power.

FAQ

Where do I find these thermal resistance values?

R_jc comes from the component's own datasheet (it depends on the die and package, not anything external). R_sa comes from the heatsink's datasheet, often specified at a particular airflow — add fan cooling and the manufacturer's rated R_sa usually drops. R_cs depends on whichever interface material you're using between the case and the heatsink; thermal paste datasheets usually state it per unit area, which you'd convert using your actual contact area.

What if the component is mounted directly to the heatsink with no separate interface layer?

Set the case-to-sink resistance to 0 — the calculator simply drops that term from the total, leaving ΔT = P × (R_jc + R_sa).

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