The sequence
Fabrication is a fixed order of operations. A six-layer board passes through these steps:
- Data preparation and DFM. Gerbers, drill file and stack-up are checked, the panel is designed, and the process route is fixed.
- Inner-layer imaging. Dry film resist is laminated, exposed through the artwork and developed; the copper it protects stays.
- Inner-layer etching. The unprotected copper is removed, then the resist is stripped. The inner circuits now exist.
- AOI and lamination. Inner layers are inspected, then stacked with prepreg between them and pressed under heat and pressure until the resin flows and cures.
- Drilling. Holes are drilled through the laminate: 0.15 mm mechanical minimum, 0.1 mm by laser for microvias.
- Plating. A thin copper layer is deposited in the holes (and on the surface, depending on the process) to connect the layers electrically.
- Outer-layer imaging and etching. The outer circuits are formed, then the panel is etched and stripped.
- Solder mask and legend. Mask is applied, exposed and cured; legend is printed and cured.
- Surface finish. HASL, ENIG, immersion silver or tin, OSP or another finish is applied to the exposed copper.
- Routing, test and inspection. Boards are separated, electrically tested, inspected and packed.
Where the process can go wrong
Each step has one characteristic failure, and most of them are decided before the panel is even started.
- Registration. Layers have to line up. Drill-to-copper and layer-to-layer misregistration are why annular ring is specified and inspected.
- Etch control. Over-etching narrows traces and widens gaps; under-etching leaves copper where it should not be. Fine features live or die here.
- Plating distribution. Holes plate less in the centre of a panel than at the edges unless the current and pattern are managed; heavy copper makes it worse.
- Resin flow. Too little prepreg flow leaves voids; too much thins the dielectric and shifts impedance.
- Finish thickness. A finish that is too thin oxidises in storage; too thick cracks or bridges fine pitch.
What the customer controls
Three inputs decide most of the outcome: the stack-up (materials, copper weights, dielectric thickness), the feature set (line width, hole sizes, annular ring) and the acceptance class. Everything else is process control on the fabricator's side.
A board specified inside the published capability — 1–38 layers, 0.075 mm line and space, 0.15 mm mechanical holes, 0.1–6.0 mm thickness — is a routine build. A board that asks for all of them at once is a different conversation, which is what DFM is for.
Where to look next
The process limits are on the capability page; the panel that carries the boards is described in the panelisation note; the terms used above are defined in the fabrication glossary. For thickness extremes, see the thin board and thick board notes.
