The PCB manufacturing process, step by step

Overview: single-sided, double-sided and multilayer

Every PCB starts as a copper-clad laminate, usually FR-4. What happens next depends on how many copper layers the design has.

• Single-sided boards have copper on one side only. They skip plated holes and lamination, so the flow is short: drill, image, etch, solder mask, finish, electrical test, profile.
• Double-sided boards have copper on both sides, connected by plated through-holes. They add drilling and hole metallisation to the flow.
• Multilayer boards have inner copper layers buried inside the laminate. The inner layers are processed first, then pressed together with prepreg into a single panel before drilling and outer-layer processing.

The steps below describe a typical multilayer flow. Simpler boards follow the same sequence with some steps left out.

The manufacturing steps

1. Design data and DFM review. The manufacturer receives Gerber files or ODB++, drill files, a stack-up and fabrication notes. A DFM review checks the data against the factory's process limits and flags anything that cannot be built as drawn.
2. CAM and panelisation. CAM engineers convert the data into production tooling: phototools or direct-imaging files, drill programs and test programs. Boards are arranged on a production panel (panelisation), with compensation for etching and registration.
3. Inner-layer imaging and etching. Photoresist is applied to the inner-layer cores and exposed with the circuit pattern. The unprotected copper is etched away and the resist stripped, leaving the inner-layer tracks and planes.
4. Inner-layer AOI. Automated optical inspection compares each etched layer with the CAM data. Opens, shorts and nicks are caught here, because they cannot be repaired once the layers are laminated.
5. Oxide treatment and lamination. The copper surfaces are chemically roughened (oxide or oxide-alternative treatment) for adhesion. Cores and prepreg sheets are stacked with outer copper foil and pressed under heat and pressure until the resin cures into one solid panel.
6. Drilling. Holes for vias and component leads are drilled, usually by CNC machines. X-ray targeting aligns the drill program with the buried inner layers.
7. Desmear and hole metallisation (PTH). Resin smear from drilling is removed so inner-layer copper is exposed in the hole wall. A thin conductive layer, typically electroless copper or a direct metallisation process, is deposited so the holes can be electroplated.
8. Outer-layer imaging and pattern plating. Resist is applied to the outer layers and imaged. Copper is electroplated onto the exposed tracks, pads and hole walls to build up the required thickness, followed by a thin etch resist such as tin.
9. Etching and strip. The imaging resist is removed, the unwanted base copper is etched away, and the tin etch resist is stripped, leaving the finished outer-layer circuit.
10. Solder mask. Liquid photoimageable solder mask is coated, imaged and cured. It insulates the copper and stops solder bridging between pads during assembly.
11. Surface finish. Exposed copper pads receive a surface finish such as ENIG, HASL, immersion silver or OSP. The finish protects the copper and determines solderability and shelf life.
12. Legend/silkscreen. Reference designators, logos and markings are printed, usually with inkjet or screen printing.
13. Electrical test. Every net is tested for opens and shorts against the netlist, with a flying probe for prototypes and small batches or a dedicated fixture for volume.
14. Profiling (routing, V-scoring). Boards are routed out of the production panel or left in a customer panel with V-score lines or breakaway tabs.
15. Final inspection to IPC-A-600 / IPC-6012, and packing. Boards are visually inspected against IPC-A-600 acceptance criteria and the performance requirements of IPC-6012 for the specified class, then packed, typically vacuum-sealed with desiccant.

Where defects and delays usually start

Most delays in PCB manufacturing do not start on the factory floor. They start with the data. When the CAM engineer finds an ambiguity, the job stops until someone answers a technical query.

Common causes:

• Missing or inconsistent files: no drill file, mismatched layer names, or a board outline on the wrong layer.
• No stack-up or an unclear one, especially for controlled-impedance designs.
• Features below the factory's process limits: tracks, gaps, annular rings or drill sizes that cannot be produced reliably.
• Conflicting notes, where the fabrication drawing says one thing and the files say another.
• Solder mask slivers and openings that do not match the pads.

On the production side, the familiar defects are registration errors between layers, plating voids in holes, under- or over-etching, and delamination. Most of these are prevented by a sound design and a clear specification rather than inspected out afterwards. That is why a DFM check before ordering pays off.

How the process differs for HDI, flex and rigid-flex

HDI boards use microvias and finer features to fit dense components such as fine-pitch BGAs. The main differences are:

• Sequential lamination: the board is built up in several pressing cycles instead of one, with layers added on each side of a core.
• Laser drilling: microvias are drilled by laser rather than mechanically, and are then plated and often copper-filled.
• Tighter imaging and registration, because every additional lamination cycle adds alignment risk.

Flex PCBs are built on polyimide film instead of glass-reinforced laminate. They use coverlay instead of (or alongside) solder mask, and need careful handling because the thin material moves during processing. Stiffeners are often added where components or connectors sit.

Rigid-flex PCBs combine both processes. Flexible layers run through the rigid sections and are laminated together with rigid material, so the stack-up, materials and bend areas have to be defined clearly from the start. These builds involve more process steps and more engineering review than a standard rigid board.

What to send for a quote

A complete data package lets the manufacturer price the job accurately and avoids questions later. Send:

• Gerber (RS-274X or X2) or ODB++ files for every copper, solder mask and legend layer
• Drill files (Excellon) with plated and non-plated holes identified
• A board outline and any panel drawing
• Stack-up, including controlled-impedance requirements if any
• Fabrication notes: material, thickness, copper weight, surface finish, solder mask colour and IPC class
• Quantity, or several quantities if you want to compare
• Any special requirements such as edge plating, via filling or test coupons

For more detail on board choices, see the PCB manufacturing service page.

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