Why trace width matters
A copper trace on a printed circuit board has resistance, so current heats it. Too narrow, and the trace runs hot, drops voltage or even fuses open. The IPC-2221 standard gives an empirical formula linking current, temperature rise and cross-section.
The IPC-2221 formula
I = k × ΔT^0.44 × A^0.725
- I: current in amps;
- ΔT: allowed temperature rise above ambient in °C;
- A: cross-section in square mils;
- k = 0.048 for outer layers and 0.024 for inner layers, which cool less well.
Solving for the area and dividing by the copper thickness gives the width. One ounce per square foot of copper is about 1.378 mil (35 µm) thick.
Example
2 A, a 10 °C rise, 1 oz copper on an outer layer:
- A = (2 ÷ (0.048 × 10^0.44))^(1/0.725) ≈ 42.4 mil²;
- width = 42.4 ÷ 1.378 ≈ 30.8 mil, or 0.78 mm;
- over 2 in of trace, resistance is about 32 mΩ, the voltage drop about 64 mV and the loss about 0.13 W.
On an inner layer, the same current needs a trace about 2.6 times wider.
Choosing the temperature rise
- 10 °C is a common, conservative choice.
- 20 to 30 °C is acceptable for many designs if nearby parts tolerate it.
- Add the rise to the board’s highest ambient temperature and stay under the laminate’s limit (around 130 °C for standard FR-4).
Practical tips
- Voltage drop often matters more than heating: at low voltages, a few tens of millivolts can matter. Check it for power rails.
- Use 2 oz copper, wider pours or several layers in parallel for high currents.
- Vias also have a current limit; use several in parallel for power connections.
- IPC-2221 charts are based on old data and are conservative for most boards. The newer IPC-2152 standard accounts for board thickness, copper planes and other effects, and usually allows narrower traces.
- Above about 35 A, the IPC-2221 curves are extrapolated; use copper bars, bus bars or thermal simulation.