I = k · ΔT0.44 · A0.725, where A is the conductor
cross-section in mil² and k is 0.048 for external layers or 0.024 for internal layers,
which run cooler because the board carries heat away less readily. Solving for area and dividing by
copper thickness gives the width. Resistance uses copper resistivity 1.72×10⁻⁸ Ω·m corrected for
temperature at 0.393 %/°C.
| 0.5 oz/ft² | 17.5 µm · 0.69 mil | Inner layers on thin stackups |
|---|---|---|
| 1 oz/ft² | 35 µm · 1.38 mil | The default almost everywhere |
| 2 oz/ft² | 70 µm · 2.76 mil | Power boards; needs wider spacing to etch cleanly |
| 3 oz/ft² | 105 µm · 4.13 mil | Heavy copper, specialist and expensive |
| Typical fab minimum width | 4–6 mil · 0.10–0.15 mm | Below this most low-cost fabs surcharge or refuse |
| Typical fab minimum spacing | 4–6 mil · 0.10–0.15 mm | Etching cannot hold finer reliably at 1 oz |
Copper weight is quoted as the weight of copper spread over one square foot, which is why the unit looks like a mass. Heavier copper needs wider gaps because the etchant undercuts more.