Heatsink Thermal Resistance

Size a heatsink by calculating required RθSA (heatsink-to-ambient) and estimate Tj (junction temperature). Includes Show Work + share links.

How to Use

  1. Enter Tj(max), Ta, and Power (W).
  2. Enter package RθJC and interface RθCS (from datasheet / pad).
  3. Choose a mode: compute required RθSA or estimate Tj from a known heatsink.
  4. Open Show Work for the exact steps (base units: ℃, W, ℃/W).
Thermal Stack View
Junction → Case → Sink → Ambient (RθJC, RθCS, RθSA)
Tj
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Ta
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P
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RθSA
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Status: —
Inputs & Settings
Enter temperatures and resistances. Choose a mode to compute required heatsink RθSA or estimate junction temperature.
Common limits: 125℃, 150℃, 175℃ (check datasheet)
Use real ambient near the heatsink, not room-average
Use worst-case steady-state dissipation (W)
From component datasheet (package-dependent)
Thermal pad/grease + mounting quality affects this
Only needed for “Estimate Tj” mode (enter a heatsink spec)
Show Work (step-by-step)
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Work is shown using base units: ℃, W, and ℃/W. Temperatures entered in ℉ are converted internally.

Thermal Resistance Formulas

Core model: temperature rise is proportional to power and thermal resistance.

  • Total junction-to-ambient: RθJA = RθJC + RθCS + RθSA
  • Estimate junction temperature: Tj = Ta + P × RθJA
  • Required heatsink resistance: RθSA(required) = (Tj(max) − Ta)/P − RθJC − RθCS
Where T is temperature, P is power (W), and Rθ values are in ℃/W.

FAQ

What is RθSA?

RθSA is the heatsink-to-ambient thermal resistance (℃/W). Lower is better: it means less temperature rise per watt.

Where do I get RθJC and RθCS?

RθJC comes from the component datasheet (package-specific). RθCS depends on interface material (pad/grease), mounting pressure, flatness, and insulation requirements.

Why is my required RθSA negative?

A negative result means the input assumptions are inconsistent (for example: too little temperature headroom for the power), or RθJC + RθCS already consumes the entire thermal budget.

Does airflow change RθSA?

Yes. Heatsink specs often differ for natural convection vs forced airflow. Enter the RθSA that matches your airflow condition.

Tool Info

Last updated:

Updates may include UI improvements, unit support, and edge-case handling.