Cooling Electronics and Thermal Design
Junction-to-ambient resistance, heat sinks, interface materials, forced air, PCB copper and derating.
The thermal resistance chain
Electronic parts must stay below a maximum junction temperature Tj,max (typically 125 to 175 °C for silicon, 85 to 105 °C for a processor). The heat the device dissipates, P, flows out through a series of thermal resistances to the ambient air, and each one adds a temperature rise of P × R, exactly like the wall in Conduction: Walls, Pipes and Insulation.
Tj = Ta + P (θJC + θCS + θSA)
A power device on a heat sink
Worked example: will the MOSFET survive?
- A power MOSFET dissipates 20 W in a cabinet whose air reaches 50 °C. Datasheet θJC = 1.2 K/W, thermal pad θCS = 0.3 K/W, heat sink θSA = 1.5 K/W (natural convection).
- Tj = 50 + 20 × (1.2 + 0.3 + 1.5) = 110 °C; the case is at 86 °C and the heat sink at 80 °C. Fine against a 125 °C rating, but derate to about 110 °C for reliability.
- Required heat sink: θSA ≤ (Tj,max − Ta) ÷ P − θJC − θCS = (110 − 50) ÷ 20 − 1.2 − 0.3 = 1.5 K/W. The chosen sink just meets it, with the junction at 110 °C. The maximum power at this limit is 20 W.
- The Heat Sink and Thermal Resistance Calculator solves this chain for temperature, allowable power or required θSA.
Interface materials
Two metal surfaces touch only at their high points; the gaps are filled with air, which conducts about 10 000 times worse than aluminum (see contact resistance in Conduction: Walls, Pipes and Insulation). A thermal interface material (grease, pad, phase-change sheet, gel) fills the gaps. Typical contact resistances: dry metal on metal 2 to 5 × 10⁻⁴ m²K/W, with grease 0.2 to 0.5 × 10⁻⁴, with a gap pad 1 to 3 × 10⁻⁴ (pads are easier to use, grease is better).
For a 20 mm × 15 mm device base (300 mm²): dry contact 3.0 × 10⁻⁴ m²K/W gives 1.00 K/W and a 20 K temperature rise at 20 W; with grease (0.3 × 10⁻⁴) only 0.10 K/W and 2 K: a thin layer of grease is worth 18 K here. Use a thin even layer, flat surfaces and the right clamping pressure.
Choosing and cooling a heat sink
A heat sink’s θSA falls as airflow increases. Manufacturers publish θSA against air speed. The fins and base theory is in Fins and Extended Surfaces. The table shows typical values for the 100 × 100 mm sink of that page, and what each gives for the 20 W device:
| Cooling | θSA | Tj at 20 W | Maximum power for Tj = 110 °C |
|---|---|---|---|
| Natural convection | 1.50 K/W | 110 °C | 20 W |
| 1 m/s | 0.90 K/W | 98 °C | 25 W |
| 2 m/s | 0.60 K/W | 92 °C | 29 W |
| 3 m/s | 0.50 K/W | 90 °C | 30 W |
| 5 m/s | 0.40 K/W | 88 °C | 32 W |
Forced air cuts θSA from 1.5 to 0.5 K/W at 3 m/s, allowing about 1.5 times the power. A fan costs noise, power, dust and a reliability risk. Design natural convection for continuous-duty equipment where you can, add a fan only when you must, and control its speed by temperature.
- Spread the heat: several small devices on a heat sink or copper plate have a lower temperature than one hot spot; a thick base (3 to 10 mm) spreads the heat from a small source into the fins.
- Check the airflow path: hot exhaust air must not be drawn back in, and filters and cabinet louvres add pressure drop (see Pumps, Pump Curves and NPSH for the fan-curve and system-curve idea).
- Liquid cooling and heat pipes move heat to a remote exchanger when air cannot: heat pipes have an effective conductivity of thousands of W/m·K, and cold plates carry kilowatts.
Cooling through the board
Small surface-mount parts shed most of their heat down into the board’s copper. The package datasheet gives θJA for a specified board; it falls as the copper area under and around the part grows (typically from 80 to 90 K/W on a minimal pad to 40 to 50 K/W on a few square centimeters of copper). For a 1 W part in 25 °C air, that is Tj = 110 °C on the minimum pad and 70 °C on the large pour.
Heat crosses the board through thermal vias: plated holes, usually 0.3 mm. A 0.3 mm via with 25 µm copper plating through a 1.6 mm board has a resistance of L ÷ (k π d t) = 1.6 × 10⁻³ ÷ (385 × π × 0.3 × 10⁻³ × 25 × 10⁻⁶) = 176 K/W each, so an array of 9 vias is 20 K/W (filled vias and thicker plating do better). See Thermal Design on the Board for layout rules.
Enclosures and sealed boxes
Worked example: a sealed control box
- A sealed enclosure dissipates 30 W inside and has 0.5 m² of exposed surface. Natural convection plus radiation from a painted surface gives about h = 8 W/m²K.
- The surface settles ΔT = P ÷ (h A) = 30 ÷ (8 × 0.5) = 7.5 K above the surrounding air, and the air inside is warmer still (internal circulation, typically 5 to 10 K more). Add the temperature of the room and the sun on the box: a 40 °C room and a metal box in sunlight can push the inside past 70 °C.
- To keep the rise under 10 K a box needs A = 0.38 m² at this loss. Use light colors outdoors, shade, a sun shield, internal fans to spread the heat to the walls, or a heat exchanger or air conditioner for larger loads.
Temperature, reliability and derating
The failure rate of electronics rises steeply with temperature, because chemical and diffusion processes follow an Arrhenius law, rate ∝ exp(−Ea ÷ kT). With an activation energy of 0.7 eV, running at 85 °C instead of 55 °C accelerates the aging by a factor of 8.0 (the old rule that life halves for every 10 K rise would say 8). Electrolytic capacitors, whose life roughly halves per 10 K, are the classic example.
Derating means using a part below its maximum rating: a power device is rated for its junction limit at 25 °C case temperature, and the allowed power falls linearly to zero at Tj,max. At Ta = 50 °C a device that is rated 100 percent at 25 °C can carry only 75 percent (for a 125 °C limit) with zero thermal resistance to ambient, and much less in practice. Measure the temperature of the real product in its real enclosure with thermocouples or an infrared camera, at the worst-case ambient, before relying on a calculation.
Related
- Previous: Transient Heating and Cooling. Next: Insulation, Heat Loss and Troubleshooting.
- Heat Sink and Thermal Resistance Calculator.
- Thermal Design on the Board.
- Electronics Fundamentals and Power Electronics (losses to be cooled).
- Heat Transfer: the whole guide.
FAQ
What is θJA and can I rely on it?
The junction-to-ambient resistance in a standard test (a defined board in still air). It is useful for comparing packages, but your board, copper and airflow will give a different value, so treat it as a starting estimate and measure.
Should I use a fan?
Only if natural convection and a bigger sink cannot do the job. Fans add noise, power, dust and a wear-out part; use a good bearing and monitor the speed, and design so the equipment survives a fan failure for long enough to alarm.
Does orientation matter?
Yes: for natural convection, vertical fins with free air above and below work best; horizontal or enclosed fins lose a large share of their performance.
How do I measure junction temperature?
Not directly: measure the case or heat sink with a thermocouple and add P × θJC, or use the device’s on-chip temperature sensor or a temperature-sensitive electrical parameter such as the body-diode forward voltage.
References
- JEDEC JESD51 series (thermal test methods and boards); manufacturers’ application notes on thermal design.
- Lasance, C. J. M. and Simons, R. E. (eds.), Advances in Thermal Modeling of Electronic Components and Systems.
- Incropera, F. P., et al., Fundamentals of Heat and Mass Transfer, Wiley; Kraus, A. D. and Bar-Cohen, A., Thermal Analysis and Control of Electronic Equipment.
- IPC-2221 and IPC-2152 (current and temperature rise in conductors).
Last reviewed: October 2026. Thermal resistances and temperature limits are typical: use the datasheet.