Once a design needs thick copper, the next decision is which layers carry it. Outer and inner heavy copper go through different process steps and face different limits. Make the decision before the stackup drawing is frozen. Moving heavy layers later forces impedance layers, reference planes and board thickness to be recalculated.
This article covers only where the heavy copper goes. Symmetry and thickness build-up in mixed-weight builds are covered in heavy copper multilayer mixed-weight stackup. The three copper values in HDI build-up boards (base foil, finished copper, via wall) are covered in HDI copper thickness options. That article is about microvia build-up boards; this one is about conventional rigid heavy copper boards.
| Item | Heavy copper on outer layers | Heavy copper on inner layers |
|---|---|---|
| Where the copper comes from | Base foil plus panel plating | Mainly core base foil |
| Etch deviation | Visible on the finished surface | Enclosed in dielectric |
| Main manufacturing difficulty | Undercut, mask coverage, surface finish | Resin fill, dielectric thickness, board thickness |
| Thermal path | Short; heatsink or terminals attach directly | Heat must exit through vias and planes |
| Soldering | High-current terminals solder directly | Not solderable; needs vias to the surface |
1. Outer-layer heavy copper: base foil plus plating
Outer copper is plated after drilling, along with the hole walls. Finished outer copper is base foil plus plating. A drawing that says only "outer copper 6 oz" does not state whether that is finished or base copper. Where the copper comes from is covered in thick copper: plating vs foil build-up.
Outer layers are etched before solder mask and finish. Thicker copper needs longer etching, so undercut grows and the conductor cross-section becomes a trapezoid, narrower at the top. The deviation is visible on the finished board and feeds straight into width acceptance. Compensation and acceptance are covered in heavy copper etch compensation review.
Outer heavy copper brings two follow-on issues:
- Solder mask coverage: thicker traces leave thinner mask on the trace shoulders and deeper gaps between traces. See heavy copper solder mask and finish coverage.
- Trace and space: SKPCB's minimum trace width on heavy copper boards is 6 mil, and the window tightens as copper weight rises. Fine-pitch areas on the same outer layer need a separate review.
The benefit is a short thermal path. High-current terminals can be soldered to the heavy copper, and a heatsink can sit directly on the copper surface.
2. Inner-layer heavy copper: the difficulty moves to lamination
Inner layers are etched but not panel plated, so their copper is set mostly by core base foil and is easier to control than outer copper. Etch deviation stays inside the dielectric.
The difficulty is lamination. After a heavy inner layer is etched, there is a large step between copper and cleared areas, and prepreg resin has to flow in and fill it during pressing. Thicker copper and larger cleared areas need more resin. Incomplete fill leaves voids or delamination. Inspection methods are covered in heavy copper PCB lamination and resin fill control.
To fill the step, heavy inner layers often need thicker dielectric on both sides, which raises finished board thickness. SKPCB's heavy copper board thickness range is 0.8–6.0 mm. If connectors, enclosures or press-fit parts set a hard thickness limit, check it at the stackup stage.
Heat from inner heavy copper has to reach the surface through vias and planes. Thicker boards mean longer vias. SKPCB's PTH aspect ratio limit is 10:1, so minimum hole size grows with board thickness.
3. The order of decisions
Answer these in order. Each answer limits the next.
- Which nets really need heavy copper. Only high-current paths and areas that rely on copper for heat spreading. Removing layers thickened "just in case" does more for thickness and cost than anything else.
- Where current enters and leaves. If high-current terminals, studs or soldered connectors sit on the surface, the outer layer gives the shortest path. If current is only distributed inside the board, inner layers fit better.
- Where heat has to go. If heat must reach a heatsink or enclosure, the outer layer next to that surface is more direct. If heat only needs spreading inside the board, an inner plane works.
- What else the outer layer carries. Fine-pitch ICs, BGAs or impedance traces on the outer layer conflict with outer heavy copper on trace, space and mask bridges. In that case put the heavy copper inside and keep standard copper outside.
- Whether symmetry holds. Place heavy layers symmetrically about the board centre where possible, and use dielectric thickness to compensate where not. How much asymmetry is acceptable is an engineering review item.
4. Common allocations
| Requirement | Common allocation | Review points |
|---|---|---|
| Terminals soldered directly, no fine pitch outside | Heavy copper on outer layers | Undercut, mask coverage, finish |
| BGA or impedance traces outside, current distributed inside | Standard outer copper, adjacent inner pair in heavy copper | Resin fill, dielectric, thickness |
| Soldered terminals plus fine-pitch areas | Local heavy outer areas, or inner heavy copper brought up through vias | Via count and aspect ratio |
| Several stacked power rails | Several inner heavy layers in pairs near the centre | Symmetry, lamination cycles |
These are starting points. As layer count rises, heavy layer placement has to be re-planned with impedance layers and reference planes.
5. What the stackup drawing must state
- Copper weight for every layer, marked as base or finished copper.
- Which layers are heavy copper and what each does (current, heat, soldering).
- Dielectric type and thickness on both sides of each heavy layer, or "set by fabricator for resin fill".
- Finished thickness and tolerance. SKPCB's tolerance is ±10% or ±0.005" (about 0.13 mm), whichever is greater.
- Impedance layers and targets. SKPCB's impedance control on heavy copper boards is ±10%.
- Mask and finish requirements on outer heavy copper areas (HASL, ENIG or OSP).
SKPCB engineering review confirms the stackup before production. See PCB stackup design.
FAQ
Q: Is the maximum copper weight the same for inner and outer layers?
A: SKPCB's stated maximum for both inner and outer layers is >12 oz. What a given stackup can carry depends on layer count, board thickness and trace/space, and is confirmed per project.
Q: Why is width deviation larger on outer heavy copper?
A: Finished outer copper is base foil plus plating, so total copper is higher, etching takes longer and undercut grows. The deviation is visible on the finished board.
Q: What is the main risk with inner heavy copper?
A: Resin fill during lamination. The etched step is large, and incomplete fill leaves voids or delamination. Reducing large cleared areas on heavy layers lowers the fill demand.
Q: Can a board with outer BGAs still use heavy copper?
A: Yes. The usual approach is heavy copper on inner layers with standard outer copper, so trace, space and mask bridges in the BGA area stay achievable.
Q: How does heat leave inner heavy copper?
A: Through vias and surface copper. As board thickness rises, vias must stay within the 10:1 aspect ratio, so hole size grows.