Writing "heavy copper" on a drawing is not yet manufacturable information
The most common documentation gap in heavy copper enquiries is not a missing Gerber. It is copper weight given as the words "heavy copper", or as a single oz figure with no layer assignment. Engineering review then has to come back with the same questions every time: outer layer or inner layer? Base foil or finished copper after plating? Does the board also carry a fine-line region that has to hold at the same time?
Copper weight is not a dial you can turn on its own. Each step up in copper weight means more copper to remove during etching, so sidewall etch-back and trace-width shrinkage grow, and the smallest trace you can hold reliably gets wider. At the same time the minimum board thickness rises and the achievable layer count drops. Asking for the thickest copper and the finest lines on one board puts two extremes into the same etch step.
So this article does not explain what a heavy copper PCB is. It answers one question: given a current-carrying requirement, which copper weight tier applies, and what does the next tier up cost.
Start with the two published manufacturing windows
Sunking splits thick-copper work into two deployed capability lines with different windows:
| Item | Heavy Copper PCB | Thick Copper PCB |
|---|---|---|
| Copper thickness | 3oz–>12oz | 6oz–>12oz |
| Layers | 1–16 layers | 1–8 layers |
| Min. trace width | 6 mil | 8 mil |
| Min. trace space | per project review | 12 mil |
| Board thickness | 0.8–6.0 mm | 1.6–6.0 mm |
| Current carrying capacity | per project review | ≥100A |
| Surface finish | HASL / ENIG / OSP | HASL / ENIG / OSP |
| Base material | FR-4 TG170 / TG180 | FR-4 family |
| Impedance control | ±10% | per project review |
The way to read this table is not to pick a column. It is to read the difference between the columns. Moving from the heavy copper line to the thick copper line raises the copper floor from 3oz to 6oz, and the price is: layer ceiling drops from 16 to 8, minimum trace width relaxes from 6 mil to 8 mil, minimum space is explicitly bounded at 12 mil, and minimum board thickness rises from 0.8 mm to 1.6 mm.
Put plainly: doubling the copper floor costs you half the layer count and roughly a third of your trace-width resolution. These four parameters move together. You cannot accept the copper benefit and assume the rest holds still.
Three things every tier tightens at once
Minimum trace width. Thicker copper means longer etch time, more visible sidewall etch-back, and a larger difference between trace top and trace bottom width. That is why 6 mil belongs to the 3oz-and-up heavy copper line and 8 mil belongs to the 6oz-and-up thick copper line. The capabilities page states the same rule from the other side: outer-layer trace width is 4 mil at Standard, 3 mil at Advanced, 2 mil at R&D, with maximum outer and inner copper of 3oz / 12oz / >12oz respectively. The finest-line tier is not simultaneously the thickest-copper tier. Those two extremes are realised in separate regions, not stacked in one.
If a board genuinely carries both a high-current bus and a fine-line signal region, the fix is to separate them onto different layers, not to demand 2 mil and 12oz in the same layer. Etch compensation practice is covered in heavy copper etch compensation review and is not repeated here.
Achievable layer count. The heavy copper line reaches 16 layers, the thick copper line 8. The constraint is lamination: each thick copper layer needs more resin to fill the copper height difference, and the thicker the stack and the larger the copper steps, the harder post-lamination thickness uniformity and layer-to-layer registration become. When you need more layers *and* high current, the answer is mixed copper weight, not a uniformly thicker board.
Minimum board thickness. 0.8 mm versus 1.6 mm. Thick copper consumes thickness itself, and the resin fill requirement adds more. Thin boards and thick copper are mutually exclusive demands. An enclosure thickness limit fixed early in the project often rules out the highest copper tier before layout even starts.
How to state current-carrying requirements as design inputs
"This board carries high current" is not a design input. A reviewable input contains at least four items:
- Continuous and peak current, and how long the peak is sustained;
- Allowable temperature rise in degrees above ambient, not an absolute temperature;
- Ambient temperature and cooling conditions: forced air, heatsink, or contact with a metal enclosure;
- Which layer carries the current: outer-layer bus, inner power plane, or several layers in parallel.
With those four, engineering can work back to the required cross-section and then to a "copper weight × trace width" combination. The same cross-section can be achieved with thick narrow copper or thinner wide copper, and the choice depends on whether the board has room to widen. If the space exists, widen rather than thicken, because widening does not cost you minimum trace-width capability and thickening does.
The general method for conductor cross-section versus temperature rise follows IPC-2152; this article does not restate its clause values. The copper weight and trace width combination for a specific project requires engineering review confirmation. The ≥100A figure published on the thick copper page is that product line's capability statement. It is not a universal value for any board type, and it cannot be reversed into an allowable current for a given trace width.
When to switch to mixed copper weight
If any of the following holds, a single copper weight across the whole board is not the right answer:
- The board has both a power region and a signal region, and the signal region needs traces finer than that copper tier allows;
- The required layer count exceeds the layer ceiling of that copper tier;
- Thickening the whole board would break the enclosure's board-thickness limit.
The answer then is a mixed-weight stackup: thick copper on power layers, thin copper on signal layers. Layer ordering, resin fill and registration control are covered in heavy copper multilayer mixed-weight stackup. Mixed-weight review needs one extra input over single-weight: copper weight must be stated per layer, not as a single board-level figure.
Surface finish and impedance on thick copper
All three finishes — HASL, ENIG and OSP — are available on both product lines. The selection driver is assembly and storage requirements, not copper weight (see the surface finish comparison). Worth noting: thick copper boards have larger surface height differences, so finish uniformity and solder mask coverage should be confirmed together during review.
On impedance, the heavy copper page publishes ±10%. This does not contradict the capabilities page figure of ±5% (±10% standard): ±5% belongs to the dedicated impedance-control board type, while impedance control on thick copper is quoted at ±10%. Tight impedance control on thick copper layers is a conditional case and is subject to engineering review confirmation.
What to prepare before review
- A stackup table with copper weight per layer, stating whether each figure is base foil or finished copper after plating;
- The four current-carrying items (continuous/peak current, allowable temperature rise, ambient and cooling conditions, current-carrying layer);
- Board thickness limit and outline dimension constraints;
- Minimum trace width/space in the fine-line region, and which layer it sits on;
- Surface finish and base material TG requirements;
- Acceptance class (IPC-A-600 / IPC-6012 Class 2 or Class 3) and test requirements.
Projects with complete documentation get a capability verdict in the first review. Projects missing current-carrying conditions or per-layer copper weight usually cost an extra day in round trips. Submit through the quote entry, or self-check against the capabilities page first.
FAQ
Is thicker copper always safer?
No. Each tier up simultaneously raises minimum trace width, lowers the layer ceiling and raises minimum board thickness. Move up a tier only when the current-carrying requirement genuinely cannot be met by widening traces or adding parallel layers.
How do I choose between 3oz and 6oz?
Look at layer count and trace width first. If you need more than 8 layers, or the board has trace widths between 6 and 8 mil, stay on the 3oz-and-up heavy copper line. Only go to the thick copper line when you genuinely need 6oz or more and the layer count stays within 8.
Is the published ≥100A a per-trace rating?
No. It is the current-carrying capability statement for the thick copper product line, dependent on actual copper weight, trace width, cooling and temperature-rise conditions. Allowable current for a single conductor must be reviewed against real cross-section and cooling, and cannot be derived from this figure.
Can one layer carry both 12oz copper and 3 mil traces?
Not in the same layer. On the capabilities page both the finest line (2 mil) and the heaviest copper (>12oz) appear in the R&D tier, but they are two extremes realised in separate regions, not a combination available in one area. Fine lines and thick copper belong on different layers.
How thin can a thick copper board be?
The thick copper line window is 1.6–6.0 mm; the heavy copper line is 0.8–6.0 mm. Anything below the relevant floor is a conditional case requiring engineering review confirmation.
How should a mixed-weight drawing be marked?
The stackup table must give copper weight for every layer, distinguish base foil from finished copper, and state which layers carry current versus signal. A drawing with only a board-level copper weight cannot be reviewed for mixed-weight construction.