Same trace, twice the copper
One ounce of copper is about 35 µm, two ounces about 70 µm. Doubling the thickness doubles the cross-section of every trace on that layer, so resistance falls by half: a 1 oz trace sits at roughly 0.5 mΩ per square, a 2 oz trace at roughly 0.25 mΩ.
That is the whole electrical argument. Everything else — cost, minimum feature size, etch control, panel flatness — follows from the process needed to produce the thicker layer.
What the resistance buys
Take a 1 A current through a net 100 squares long. At 0.5 mΩ per square the trace dissipates about 0.5 mW per square, 50 mW in total, with a voltage drop of about 49 mV. At 2 oz the same net drops about 25 mV and dissipates 25 mW.
On a 3.3 V rail those numbers decide whether the design has margin. On a 48 V rail the voltage drop matters far less, and the temperature rise — not the drop — becomes the reason to add copper.
Cost and design rules
Copper weight is bought twice: in the foil and in the plating time needed to reach it. Heavier foil also etches differently, so the minimum line and space that a fabricator can hold widens as the weight rises. A board that needs 0.075 mm features and 2 oz everywhere is a harder build than one that keeps the heavy copper in planes and wide pours.
Three practical consequences:
- Layout first, weight second. Widen the trace to the space available, then choose the copper weight that meets the temperature-rise target.
- Keep heavy copper where the current is. A mixed stack-up with 2 oz on the power layers and 1 oz on signal layers is cheaper than a uniform 2 oz build and usually just as good.
- Check the connector, not only the trace. A thick trace feeding a fine-pitch connector moves the bottleneck rather than removing it.
Choosing between them
For most digital and low-power analogue boards, 1 oz is enough and 0.075 mm features are worth more than extra copper. For power supplies, motor stages and battery hardware, 2 oz is the usual starting point, and where the current path cannot be widened, the design moves into the heavy copper range.
If the current and the available width point above 3 oz, the heavy copper PCB page lists the published range (0.5–20 oz outer, 0.5–10 oz inner) and the design guide explains what has to change in the layout before quoting.
