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A PCB copper trace is a resistor: current through it dissipates I²R heat, raising its temperature above the surrounding board. The IPC-2221 standard (formerly MIL-STD-275) gives the industry-standard empirical relationship between current, trace cross-sectional area, and temperature rise: I = k×ΔT0.44×A0.725, where A is the copper cross-section in square mils (width × thickness) and k depends on whether the trace is on an outer or inner layer.
| Quantity | Formula / Value |
|---|---|
| Current (given area, rise) | I = k×ΔT0.44×A0.725 |
| Required area (given I, rise) | A = (I/(k×ΔT0.44))1/0.725 |
| k (external layer) | 0.048 |
| k (internal layer) | 0.024 |
| 1 oz copper thickness | 1.378 mils (35 µm) |
External (top/bottom) traces are exposed directly to air and can dissipate heat more easily, so they get a larger k and can carry roughly twice the current of an internal trace of the same size for the same temperature rise — internal layers are sandwiched in fibreglass with much poorer heat escape. Copper weight is usually specified in ounces per square foot (1 oz ≈ 35 µm thickness); thicker copper lets a narrower trace carry the same current.
I = k×ΔT0.44×A0.725, an empirical relationship between current (A), allowed temperature rise (°C) and copper cross-sectional area (square mils), with k=0.048 for external and 0.024 for internal layers.
External (top/bottom) traces are exposed to open air and dissipate heat more easily, using k=0.048. Internal traces are sandwiched inside the board with much poorer heat escape, using k=0.024 — roughly half the current-carrying capacity for the same size and temperature rise.
1 oz copper is about 1.378 mils (35 µm) thick per square foot. Multiply the oz rating by 1.378 to get mils, or by 35 to get micrometres, and by trace width to get cross-sectional area.
A common design target is 10–20 °C rise for general traces (well within the FR4 substrate's safe range), though high-reliability or high-ambient designs may target lower rises, and non-critical short traces can tolerate more.
The IPC-2221 formula is independent of trace length — it predicts steady-state temperature rise based only on cross-sectional area and current, assuming the heat can spread along the trace. Very short or very long traces may deviate somewhat from this idealised model.
Rearrange the formula for area: A=(I/(k×ΔT0.44))1/0.725, then divide by the copper thickness (from the oz weight) to get the required width.
Both increase cross-sectional area equally in the formula, so doubling either weight or width has a similar thermal benefit; designers usually choose based on other constraints (space available vs. cost of heavier copper).
IPC-2221 is validated over a wide but bounded current/area range and is the accepted industry standard for typical PCB design. For extreme currents (busbars, tens of amps) additional thermal analysis (or IPC-2152, an updated standard) is often used for more accuracy.
The classic IPC-2221 curves are for a nominal FR4 board; the newer IPC-2152 standard accounts for more variables (board thickness, adjacent copper, airflow) and generally predicts higher allowable currents for internal traces than the older charts.
Yes. For high currents, a wide copper pour (plane) behaves like an extremely wide "trace" and can carry much more current at low temperature rise than a routed trace — commonly used for power and ground distribution.
The formula gives the temperature RISE above ambient, not the absolute temperature. Add your expected ambient (enclosure) temperature to the calculated rise to check against the board's and components' absolute temperature limits.
Excessive heating can degrade or delaminate the FR4 substrate, discolour or lift the copper, and in extreme cases cause open-circuit failure. Staying within the IPC-2221 guidelines with reasonable margin avoids these failure modes.
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