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How to Size a Heat Sink for a Component
Manesh Jayawardhana
CIO & Co-founder
A regulator dissipating 12 W, a 125°C junction limit, and a 45°C ambient inside the enclosure. The arithmetic gives 6.67 °C/W of allowable thermal resistance, so you order a 6 °C/W heat sink and it runs hot anyway.
The heat sink isn’t the whole path. Two other resistances sit between the silicon and the fins, and together they often account for a third of the budget.
Thermal resistance in series
Heat flows from junction to ambient through several stages, and thermal resistances in series simply add:
R_total = R_junction-to-case + R_case-to-sink + R_sink-to-ambient
The total allowable resistance comes from the temperature rise you can afford divided by the power:
R_total = (T_junction − T_ambient) ÷ P
For our example: (125 − 45) ÷ 12 = 6.67 °C/W.
Now subtract what you don’t control. Junction-to-case is a datasheet figure — say 1.2 °C/W. Case-to-sink is the mounting interface: thermal paste, a pad, or a mica washer, typically 0.3 to 1.5 °C/W depending on what’s between the surfaces and how well it’s clamped.
With 1.2 and 0.8, the heat sink itself must be 4.67 °C/W or better — noticeably more demanding than the 6.67 the first calculation suggested.
Two things people get wrong
Designing to the maximum junction temperature. The datasheet’s 125°C is an absolute limit, not a target. Semiconductor lifetime falls sharply with temperature, and derating 25 to 30 degrees below the maximum is normal practice. Designing to the limit means any real-world variation — a hot day, a blocked vent, a dusty sink — pushes the part outside its rating.
Ignoring the interface. A degree per watt of interface resistance sounds trivial and is a significant share of a tight budget. A dry mounting, an uneven surface, or over-thick paste can double it.
| Term | Where It Comes From | Typical |
|---|---|---|
| Junction to case | Datasheet | 0.5–3 °C/W |
| Case to sink | Paste, pad, washer | 0.3–1.5 °C/W |
| Sink to ambient | Heat sink rating | What you’re choosing |
Why people get stuck here
- Only the heat sink counted. The other two terms are invisible unless you look for them.
- Airflow conditions unstated. Heat sink ratings are quoted in still air or at a specified airflow, and the difference is large.
- Ambient taken as room temperature. Inside an enclosure, ambient is whatever the enclosure reaches, which can be 20 degrees above the room.
- Peak power used instead of actual. Sizing for a worst case that never occurs wastes space and money; sizing for average when peaks are sustained doesn’t work.
Common mistakes to avoid
- Using room temperature as ambient for a component inside a sealed box.
- Applying too much thermal paste, which increases resistance rather than reducing it — a thin, complete layer is the goal.
- Choosing a heat sink rated with forced air and installing it in natural convection.
- Mounting a sink with its fins horizontal, which cripples convection.
- Ignoring the enclosure’s own thermal path, which ultimately has to shed all the heat.
How to do it with Heat Sink Calculator
The Heat Sink Calculator works through the full resistance chain rather than just the final term.
- Enter the power actually dissipated, not the device’s rated maximum.
- Set a junction temperature below the datasheet limit — derate deliberately.
- Enter the ambient temperature inside the enclosure, not the room.
- Subtract junction-to-case and interface resistances to get the required sink figure.
- Choose a sink rated below that value, in the airflow condition you actually have.
Other electronics calculators are in the tools directory.
Frequently asked questions
Does thermal paste really matter?
Yes. Interface resistance of around a degree per watt is a significant share of a tight thermal budget, and a dry or uneven mounting can double it. Thin and complete beats thick.
Should I design to the maximum junction temperature?
No. Semiconductor life falls sharply with temperature, and real conditions vary. Designing 25 to 30 degrees below the maximum is standard practice.
Is natural convection enough?
It depends on the sink’s still-air rating, which is what manufacturers quote for natural convection. Forced air changes the figure dramatically — always check which condition a rating refers to.
Final thought
Add every term in the chain before choosing a sink. The heat sink is usually the part you shop for and rarely the only part that matters.