Two processes, two kinds of error
Heavy copper is built by adding copper and then taking some away. Plating deposits the thickness the current path needs; etching defines the trace. Each step has its own error.
Plating thickness depends on current density, and current density depends on how much copper is being plated at once. A panel with dense power planes and sparse signal areas plates unevenly unless the pattern is balanced or the current is adjusted. Etching then removes copper from the top and from the side, so the finished trace is trapezoidal rather than rectangular and narrower than the artwork.
Neither step is out of control; both are simply less precise at 700 µm than at 35 µm. That is what the ±20 % band describes.
What the band means in micrometres
- At 3 oz, 105 µm ±20 % leaves a conductor between about 84 µm and 126 µm.
- At 10 oz, 350 µm ±20 % spans about 280–420 µm.
- At 20 oz, 700 µm ±20 % spans about 560–840 µm.
On a power plane those differences are irrelevant. On a 0.15 mm gap between two heavy traces they are the difference between a reliable dam and a short.
Where tightening pays — and where it does not
Spend the tolerance where it changes behaviour:
- Controlled impedance. On layers that need it, impedance is held to ±10 %, and the copper thickness of the reference layer is part of that calculation.
- Fine-pitch connectors and press-fit holes. Here the copper dimension interacts with a mechanical part, and a wide band turns into assembly rejects.
- High-current shunts and current sense paths. Resistance is the product, so the cross-section tolerance is effectively an electrical tolerance.
Do not spend it on planes, pours and wide bus traces. A plane at 560 µm does the same job as one at 840 µm, and asking for a tight band on it raises cost without improving the product.
Writing the callout
State the nominal width, the tolerance and the measurement method together. A drawing that says "10 oz, ±20 %" and leaves the measurement undefined invites a dispute at incoming inspection; a drawing that adds "verified on cross-section at the coupon" does not.
Where the design genuinely needs a tighter band, say why — impedance, connector fit, current sense — so the fabricator can choose the cheapest way to reach it. The published range and process limits are on the heavy copper PCB page, and the acceptance criteria on the capability page.
