Why the ten decisions matter
A heavy copper board is not a standard board with thicker foil. The copper changes plating time, etch control, mask application, panel weight and assembly profile, so every choice made at specification stage is paid for downstream. The ten below are the ones that move a quotation.
The ten decisions
- Current and temperature rise, per net. State both. A trace sized without a permitted rise has no measurable target, and the copper weight is then chosen by habit rather than by calculation.
- Copper weight per layer. Hitech Circuits builds 0.5–20 oz outside and 0.5–10 oz inside. Put the weight where the current is; a uniform heavy build is rarely necessary.
- Finished thickness. Set it with the copper included, across 0.1–6.0 mm. A 20 oz outer layer adds roughly 700 µm per side, and the laminate has to be chosen around that.
- Minimum feature size. 0.075 mm line and space stays available on fine-feature layers; the heavy layer's minimum is set by its weight. Ask for the number the layer can hold, not the number in the standard design guide.
- Hole and via strategy. The plant minimum is 0.15 mm mechanically or 0.1 mm by laser, at 15:1 aspect ratio. Where a thin layer can carry the via, keep it out of the heavy layer.
- Thermal relief and mask openings. A heavy pad holds heat. Relief spokes and mask openings are sized so the joint reaches soldering temperature and the dam survives assembly.
- Surface finish. HASL follows the copper contour, which is uneven on a heavy pad; ENIG and immersion tin give a flat surface for fine-pitch assembly, and flash gold suits connector edges. Choose the finish after the pad geometry, not before.
- Panel and array. Boards inside 580 × 1900 mm plate and etch more evenly than arrays pushed to the maximum, and copper balance across the panel decides flatness at 0.7 % bow and twist.
- Test method. Heavy boards are heavier and have taller copper, both of which affect fixture probing. Agree whether the lot runs on a fixture or a flying probe, and at what coverage.
- Tolerance and documentation. Put nominal, band and measurement method on the drawing: conductor width and thickness at ±20 % typical, verified on a cross-section coupon. Undefined tolerance becomes a discussion at incoming inspection.
What the decisions add up to
A well-specified heavy copper board usually differs from a badly specified one in three places: the copper sits on fewer layers, the minimum feature matches the weight, and the test method is agreed before tooling. Those three choices carry most of the cost difference between two quotes for the same Gerber set.
Prototypes run in 1–3 days and medium volume in 4–10 days once the stack-up is fixed. The published process limits are on the heavy copper PCB page and in the capability page; the design consequences are in the heavy copper design guide.
