The definition is a weight, not a thickness
Copper weight is the weight of foil per square foot, converted by the industry at about 35 µm per ounce. One ounce is therefore roughly 35 µm, three ounces about 105 µm and twenty ounces about 700 µm. A board carrying 3 oz or more on an inner or outer layer is sold as heavy copper.
The label matters less than what it implies. Resistance, heat spreading and etch behaviour all follow the cross-section of the conductor, and the process has to change once that cross-section stops being thin foil.
Tolerance is part of the definition
Heavy copper traces are not rectangular in section: etching undercuts the top corners, plating rounds the deposit, and thickness varies with pattern density across the panel. A ±20 % tolerance on conductor width, spacing and thickness is normal for that reason, and it is quoted on the drawing rather than discovered on the cross-section.
That single number explains most of the design difficulty. On a 105 µm layer, ±20 % is about ±21 µm of width — enough to close a fine gap that was drawn to the standard minimum.
What it changes on the board
- Feature size. 0.075 mm line and space stays available on fine-feature layers, but the minimum on a heavy layer is set by its copper weight and agreed in DFM.
- Heat path. Copper conducts heat at roughly 401 W/m·K, an order of magnitude above the laminate, so thick copper spreads heat sideways under power components.
- Flatness. Heavy copper on one side only bows the panel during lamination; the lay-up is kept symmetric where the design allows, and bow and twist are held to 0.7 %.
- Assembly. A heavy pad holds heat, so thermal reliefs and reflow profiles are set for the extra thermal mass.
Where the weight earns its cost
Heavy copper turns up where the board is part of the current path: switch-mode supplies and converters, motor drives and inverters, welding and plating equipment, solar micro-inverters and 48 V automotive hardware. The industrial control, renewable energy and automotive pages describe those applications in more detail.
Quoting a heavy copper board
Send the Gerber set with the copper weight for each layer, the current each net carries, the ambient temperature and the permitted rise, the finished thickness and the quantity. Those five inputs decide the stack-up; without them a heavy copper quote is only a guess. The published limits sit on the heavy copper PCB page and the capability page, and prototypes run in 1–3 days.
