A thick copper drawing often carries one line: "outer copper 6 oz". It does not say where the copper comes from. The fabricator can use heavy base foil, start from thinner foil and plate it up, or, most often, combine the two. Each route gives a different conductor shape, hole wall copper and copper distribution across the panel.
SKPCB builds copper above 12 oz and has 16 oz and 18 oz in volume production. The heavier the copper, the more the route matters.
The two routes side by side
| Item | Route A: heavy base foil | Route B: thin foil + plating build-up |
|---|---|---|
| Copper source | Heavy foil on the core or outer lamination | Thin foil, built up by pattern or panel plating |
| Layers | Inner and outer | Mainly outer (inner cores are normally not plated) |
| Etch depth | Full thickness | Base foil only |
| Conductor profile | Trapezoid; undercut grows with thickness | Sidewalls follow the plating resist, straighter |
| Thickness distribution | Close to uniform across the panel | Varies with pattern density and current distribution |
| Hole wall copper | Needs separate metallization | Grows together with surface copper |
| Minimum spacing | Wider, limited by full-depth etching | Tighter, limited by resist height |
Route A: heavy base foil
The foil reaches target thickness before lamination, so surface copper is nearly uniform. Inner-layer heavy copper can only take this route, because the core is sealed inside the board after lamination and cannot be plated.
The cost shows up in etching. The etchant has to cut through the full copper thickness while it also attacks the sidewalls, so the cross-section becomes a trapezoid, narrow on top and wide at the base. Undercut grows with thickness, so trace width and spacing must grow too. Heavy inner copper also leaves large gaps that resin has to fill during lamination. See heavy copper etch compensation review and heavy copper lamination resin fill control.
Route B: thin foil plus plating build-up
The outer layer starts from thinner foil. After drilling and hole metallization, plating builds up the traces and the hole walls together. Etching only removes the thin base foil, so undercut is small and spacing can be tighter.
Plated copper is not uniform. Current crowds onto isolated traces, board edges and sparse areas, which plate thicker; dense areas and large planes plate thinner. In pattern plating, copper that grows above the resist height spreads sideways into a "mushroom" profile and closes the gap.
Hole wall copper growing with the surface is this route's advantage, but aspect ratio limits it. SKPCB's maximum through-hole aspect ratio is 10:1, with board thickness from 0.2 to 6.0 mm. The deeper and narrower the hole, the harder it is for copper at the hole center to keep up with the surface. Inspection methods are covered in PCB electroplating quality inspection and release.
Production usually combines both
Finished outer copper is normally base foil plus plated copper. The fabricator sets the ratio from target thickness, minimum spacing, hole copper requirement and board thickness. You rarely need to dictate the route, but you must state the finished requirement clearly. Otherwise the fabricator applies its default split and raises an engineering query (EQ).
How to specify copper on the drawing
| Must state | Example | If missing |
|---|---|---|
| Copper definition | "finished copper" or "base copper" | 6 oz read as base copper; finished copper is heavier and spacing fails |
| Per-layer requirement | List each layer, inner and outer separately | Inner and outer layers treated as the same thickness |
| Minimum or nominal | "min xx µm" or "nominal xx µm" | No pass/fail criterion at inspection |
| Hole copper | Average and minimum hole wall copper, stated separately | Only surface copper is verified |
| Measurement method | Microsection, with measurement locations | Both sides measure different spots and disagree |
| Critical spacing | Mark areas where minimum spacing must hold | Fabricator widens spacing to suit copper weight |
For how copper weight maps to trace and space limits, see PCB copper thickness options and design rules.
Choosing a route
- Heavy inner copper: foil is the only option. Widen trace and space for undercut and allow for resin fill.
- Tight outer spacing: raise the plating share and start from thinner foil to avoid full-depth etching.
- High hole copper requirement: keep enough plating share and confirm the aspect ratio stays within 10:1.
- Uniform copper needed (for example, high-current planes): raise the foil share to reduce plating distribution effects.
Summary
"6 oz" is only an outcome. The copper route sets the conductor profile, the spacing floor, the hole copper and the thickness distribution. A drawing that states finished copper, per-layer values, hole copper and the measurement method lets the fabricator pick the right combination. To review a heavy copper stackup, see the heavy copper PCB page or request a quote.
FAQ
Q: Is the copper on a thick copper PCB plated?
A: Partly. Inner heavy copper comes from heavy foil. Finished outer copper is usually foil plus plating, in a ratio the fabricator sets from spacing, hole copper and thickness requirements.
Q: Why do thick copper boards need wider trace and space?
A: Etching heavy foil cuts through the full thickness, undercut grows with it, and the conductor ends up trapezoidal, so trace and space must increase.
Q: What are the downsides of plating build-up?
A: Plated copper is uneven: isolated traces and board edges plate thicker, dense areas thinner. Copper that grows above the resist spreads sideways and closes the gap.
Q: Can inner layers be plated up?
A: Normally no. Inner cores are sealed in the board after lamination, so inner heavy copper has to come from heavy foil.
Q: Is "6 oz" on the drawing enough?
A: No. State whether it is finished or base copper, whether inner and outer layers differ, minimum or nominal, plus hole copper and the measurement method.
Q: What is the heaviest copper SKPCB builds?
A: Above 12 oz, with 16 oz and 18 oz in volume production.