How it's calculated
IPC-2221 gives the current a track can carry for a given temperature rise: I = k × ΔT0.44 × A0.725, where A is the cross-section in square mils and k is 0.048 for outer layers and 0.024 for inner layers, which can't shed heat as easily. The width then follows from the copper thickness: 1 oz copper is about 35 µm, or 1.378 mils. The calculation is the one used by most PCB design tools and fabricators.
Practical points
- IPC-2221 is conservative for short tracks and those near copper pours, but it's a safe starting point.
- Power tracks also need low voltage drop. A track that stays cool can still drop more voltage than a sensitive circuit can accept, so check the resistance figure.
- For high currents, use wider pours, thicker copper, or tracks on both sides joined by vias.
- RF tracks are sized for impedance, not current. A 50 Ω microstrip on 1.6 mm FR-4 is about 3 mm wide.
For cable runs off the board, use the DC voltage drop calculator, and for wire sizes, the wire gauge converter.
Questions
How wide should a 1 A PCB trace be?
About 0.3 mm on an outer layer with 1 oz copper and a 10 °C rise. Use 0.5 mm or more for margin.
How much current can a 1 mm trace carry?
About 2.4 A on an outer layer with 1 oz copper and a 10 °C temperature rise.
Why are inner layers wider?
Inner layers are surrounded by board material, which holds the heat in, so IPC-2221 allows about half the current for the same width.
What is 1 oz copper?
Copper foil weighing 1 ounce per square foot, about 35 µm thick, the standard for most PCBs.