PCB fabrication turns a sheet of copper-clad laminate into a tested board in about two dozen steps, and the order carries as much weight as any single one of them. A panel is imaged, etched, laminated, drilled, plated, masked, finished, and tested, with inspection between the stages rather than only at the end. Most yield loss traces back to one step drifting out of tolerance and the next step covering for it.
Material Selection
Three inputs decide what the rest of the process is able to do.
The substrate is usually FR-4, the glass-cloth epoxy laminate that covers most general electronics: good insulation, good mechanical strength, low cost. Where the signal is fast, PTFE and similar low-loss laminates take over, because a lower dielectric constant and a lower loss factor are what keep a high-frequency edge intact.
The copper foil is the conductive layer, usually 1 oz (about 35 µm) or 2 oz, sized from the current the circuit has to carry. Roughness and adhesion matter as much as weight: a treatment side that is too rough costs etching precision, and adhesion that is too low lets the foil lift during lamination or reflow.
The consumables decide how the process runs: dry film carries the pattern, ink becomes the solder mask and the legend, the chemistry does the etching and the plating, and the drill bits make the holes. Each has to match the process window, not just the part number.
Manufacturing Process
The sequence below is the standard multilayer flow. Panel plating and pattern plating differ in detail from shop to shop; the steps and their order do not.
1. Inner Layer Fabrication
- Material cutting: sheets are cut to panel size on a CNC router, with square edges and a tolerance tight enough that the registration marks land where the exposure tool expects them.
- Dry film application: the photoresist goes on by hot roll or vacuum lamination, and has to bond without trapping air, because a bubble becomes a pinhole defect later.
- Exposure: UV light through the artwork drives a photochemical reaction in the film over the circuit areas, leaving those areas insoluble in the developer.
- Development: the panel goes through developer, which washes away the unexposed film and leaves the copper bare where the etch has to work.
- Etching: the exposed copper is dissolved away, leaving only the traces. Time, temperature, and chemistry concentration decide whether the result is a clean trace or an undercut one, which is why this step gets its own process controls.
- Stripping: the remaining film comes off in a chemical bath, and the inner layer is ready for inspection.
2. Lamination
- Browning: the inner layer gets an oxide treatment that gives the copper a micro-roughened surface for the prepreg to grip. Skip it, and the bond fails at the first thermal cycle.
- Stack-up: inner layers, prepreg, and outer copper are stacked in the order the drawing specifies, with registration pins holding the alignment.
- Lamination: a vacuum press cures the prepreg under heat and pressure. The profile matters: too little pressure leaves voids, too much squeezes the dielectric thinner than the impedance model assumed.
3. Drilling and Plating
- Drilling: CNC routers cut the vias and mounting holes. Speed and feed rate decide whether the hole wall is smooth or smeared, and smear has to be removed before plating or the connection is unreliable. The drill schedule sets the rest.
- Electroless copper: a thin conductive layer is deposited chemically in the hole, because there is no current path through a bare dielectric wall.
- Panel plating: electroplating thickens the copper across the whole panel, bringing the hole wall and the surface up to the specified weight.
4. Outer Layer Circuits
- Imaging: dry film, exposure, and development repeat what happened on the inner layers, this time on the outer copper.
- Pattern plating: copper is built up on the exposed traces, and a tin resist is plated over it. This is the step that lets the etch run: the tin stops the etchant, so the field copper goes and the traces stay.
- Etch and strip: the field copper is etched away, then the tin resist comes off, leaving the finished outer circuitry.
5. Solder Mask and Surface Finish
- Solder mask: ink goes over the whole panel, then exposure and development open the pads that have to be soldered and leave the rest covered. The mask stops solder bridging and protects the copper underneath.
- Legend: reference designators, part numbers, and version codes go on by screen or inkjet printing, so that assembly and repair can find what they are looking for.
- Surface finish: the choice depends on what the pad has to survive. ENIG (electroless nickel, immersion gold) gives a flat surface for fine-pitch and BGA work, at a cost. HASL is cheaper and more forgiving on larger pitches, but the pads come out domed. OSP protects the copper for a defined storage window and costs the least. The trade-offs between them are covered in surface finish selection.
Inspection and Testing
- Optical inspection: AOI compares the imaged panel against the design data and flags opens, shorts, lifted copper, and mask defects. On fine-pitch work it catches what the eye cannot, and it logs the position so the defect can be traced back to a step.
- Electrical test: a bare board is checked for opens and shorts on a flying probe or on a fixture built for that panel, which is the test that catches a broken trace or a bridge before components go on. A controlled-impedance board gets a TDR reading from the coupon laminated alongside it, because impedance belongs to the stackup and cannot be measured on the finished trace afterwards. The full test sequence runs from bare board to assembled unit.
- Reliability testing: thermal shock cycles the board between temperature extremes to stress the plated holes, damp heat exposes it to high humidity to bring out corrosion and leakage, and vibration stands in for shipping and service. A board that passes electrical test and fails thermal shock was built with a marginal hole wall.
Cutting and Packaging
Finished panels are routed or laser-cut to the outline, cleaned, and packed. Two steps close the job:
- Board separation: routing gives the final outline; laser cutting handles shapes a router cannot reach. Both leave an edge that has to be clean enough not to shed laminate dust in the assembly line.
- Packaging: boards ship in anti-static or vacuum packaging with desiccant, because an OSP or immersion finish degrades in humid air, and a board that oxidized in transit fails at the printer rather than at the fabricator.
Summary
The steps are fixed, but the parameters at each one decide the yield. Material selection sets the ceiling, imaging and etching set the trace geometry, lamination sets the dielectric thickness that impedance depends on, drilling sets the reliability of every connection, and the finish decides how long the board can sit on a shelf before assembly. Control each one against a number, and the process stops being a sequence of steps and becomes something a fabricator can repeat.
