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PCB trace width

Find the copper width needed to carry a current without the trace getting hotter than you allow. Internal layers need roughly twice the width of external ones for the same current.

Above ambient. 10 °C is a conservative default, 20 °C is common for power traces.

1 oz is the default on almost every low cost fab.

Layer

Top or bottom, open to air

Trace width

0.78mm

30.8 mils. At a 20 °C rise the same trace needs 0.51 mm, so allowing the trace to run a little warmer saves a useful amount of board space.

Width in mils

30.8mils

At 20 °C rise

0.51mm

Copper thickness

1.378mils

Cross section

42.4mils²

  • Note

    IPC-2221 assumes still air and no neighbours

    The curve is fitted to bare copper on a board sitting in still air with nothing else heating it. A trace running past a regulator, under a heatsink, or inside a sealed enclosure runs hotter than this. Solder mask, adjacent copper pours, and vias all change the answer too, in both directions.

Why this matters

A trace that is too narrow does not fail immediately, which is what makes it dangerous. It runs warm, the board works, and it passes every bench test. Then the enclosure goes on, the ambient temperature rises, the load peaks for longer than it did on the bench, and the trace lifts off the laminate. The other half of the mistake is forgetting which layer the trace is on: the same current on an internal layer is buried in an insulator and needs about twice the copper.

Assumptions and sources

  • IPC-2221, which is a curve fitted to measured data rather than a physical derivation. It is deliberately conservative.
  • Still air, bare copper, no adjacent heat sources, and nothing else on the board contributing to the temperature rise.
  • Copper thickness is taken as 1.378 mils per ounce, which is the nominal figure. Real plating varies, and outer layers on a plated board often come out thicker than nominal.
  • Temperature rise is above ambient, not an absolute temperature. A 10 °C rise in a 60 °C enclosure is a 70 °C trace.
  • The result is for a straight trace. Vias, corners, and thermal reliefs are separate constraints and none of them are modelled here.

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