Industrial PCB Fabrication: How a Board Is Made in the Factory

From Design to Laminate: The Subtractive Process

After you send Gerber files to the factory, what happens behind the scenes? Many engineers treat the fab as a black box: a file goes in, a board comes out. But understanding how a board is actually made turns you into a better designer — you learn why trace widths, clearances, and hole sizes have limits, and why a price can suddenly jump.

Most PCBs are made by the subtractive process: we start with a panel fully covered in copper, then remove the excess and keep only the traces we want. The opposite — the additive process — deposits traces onto a bare insulator and is newer, costlier, and reserved for special applications.

The raw panel is called copper-clad laminate: an FR-4 core (glass fiber + epoxy) with copper foil bonded to each face at a standard thickness of 18µm (½ oz) or 35µm (1 oz). Every board begins as this solid sheet and ends as a fine network of traces after a sequence of chemical and mechanical steps.

Pattern Transfer: Photolithography

The first step draws the trace pattern onto the copper with a protective layer, using photolithography — the same principle as old photographic developing.

• Photoresist coating: the copper is coated with a light-sensitive layer, either a dry film laminated under heat and pressure, or a sprayed liquid resist. This resist hardens when exposed to UV light.
• Exposure: UV light is projected onto the resist through a photo-tool carrying the trace image, hardening only the exposed areas. Modern fabs skip the film and use Laser Direct Imaging (LDI), drawing the pattern directly at higher resolution.
• Developing: the panel is washed in a mild alkaline solution (sodium carbonate) that dissolves the un-hardened resist, exposing copper where we want it removed and leaving resist protecting the traces.

This step's precision sets the smallest trace width and clearance a fab can build — typically 0.1mm for standard lines.

Chemical Etching: Carving the Traces

The panel now passes through etching: a chemical that eats the bare copper and leaves the resist-protected copper behind, forming the final traces.

A key challenge is the etch factor: the chemical eats copper sideways as well as down, so traces end up slightly narrower than designed (undercut). Good fabs compensate for this in pre-processing. After etching, the resist is stripped with a strong alkaline solution, revealing the clean copper traces for the first time.

Building Multilayer Boards: Lamination

Double-sided boards move straight on, but multilayer boards need a crucial extra step — lamination — that fuses several layers into one block.

1. Inner-layer prep: each inner core is imaged and etched separately, then inspected by AOI.
2. Oxide treatment: the inner copper is chemically roughened and darkened to bond strongly with the next dielectric and prevent delamination.
3. Lay-up: inner layers are stacked with prepreg (glass cloth impregnated with semi-cured epoxy) between them, plus outer copper foils.
4. Pressing: the stack goes into a vacuum heat press at ~180°C and high pressure. The prepreg resin flows, fills the gaps, and fully cures, bonding everything into one rigid panel.

Layer alignment is critical: a shift of a fraction of a millimeter can make holes miss the pads on inner layers — which is why more layers mean higher cost and tighter registration.

Drilling and Hole Plating

We now connect layers and faces electrically through holes, in two stages.

Drilling: stacked panels are drilled by CNC machines with fine carbide bits at thousands of holes per minute, over backup material. Very small microvias are drilled by laser, since mechanical bits cannot reach diameters below ~0.15mm.

Plating: drilled hole walls are insulating FR-4, so they do not yet conduct:

• Electroless copper: the panel is immersed in a chemical bath that deposits a very thin copper layer on every surface, including hole walls — the first conductive bridge.
• Electrolytic plating: current is passed through a copper bath, building copper up to the required thickness and forming plated through-holes that link the layers reliably.

Before plating, a desmear step removes melted resin left inside the holes by drilling heat, ensuring a clean connection.

Solder Mask and Silkscreen

With the copper complete, we protect and label the board:

• Solder mask: the signature green (or any color) layer. A liquid photoimageable (LPI) mask is coated, dried, UV-exposed through a pattern, then developed to expose copper only at the pads while covering the traces. It prevents shorts during soldering and protects copper from oxidation.
• Silkscreen: the white legend carrying component names (R1, C5, U3), markings, and logos — screen-printed with epoxy ink or digitally inkjet-printed, then cured.

Design tip: leave enough space between mask openings on adjacent pads. Overlap creates solder bridges during assembly.

Surface Finish: Protecting Exposed Copper

Bare copper at the pads oxidizes quickly and loses solderability, so it is coated with a surface finish that preserves solderability and sets joint quality:

Lead-free HASL suffices for most industrial boards, but if your board carries a fine-pitch BGA, ENIG is worth the extra cost for its flatness.

Electrical Test and Final Profiling

Before shipping, the board passes two final stages:

Electrical test: verifies every net is connected (continuity) and isolated from its neighbors (isolation). A flying probe is used for low volumes (moving needles touch the pads), or a fixed bed-of-nails fixture for high volume, which is far faster.

Depaneling: boards are made on large panels holding multiple copies, then separated by V-scoring (a grooved line snapped by hand) for straight edges, or tab-routing (small breakable tabs) for complex shapes. A final visual inspection (FQC) checks dimensions and mask quality before packing.

A Board's Journey Through the Factory

Following a simple double-sided control board:

1. A 35µm copper-clad FR-4 sheet is cut to production size.
2. Dry-film resist is laminated on both faces, LDI-exposed to the trace pattern, and developed.
3. Bare copper is etched with cupric chloride; traces appear and resist is stripped.
4. Via and component holes are CNC-drilled.
5. Electroless then electrolytic copper plates the hole walls, linking both faces.
6. Green solder mask is applied and developed to expose pads; white legend is printed.
7. Pads receive a lead-free HASL finish.
8. The board is flying-probe tested, V-score depaneled, finally inspected, and shipped.

In a modern fab this whole journey takes 1-2 working days — which is what you pay for when you order five boards for a few dollars.

Summary

PCB fabrication is a precise journey from a solid copper sheet to a living network of traces: photolithography transfers the pattern, etching carves the traces, lamination fuses the layers, drilling and plating connect the dimensions, then solder mask and finish protect the copper, and finally testing and profiling. Understanding this process not only makes you appreciate the engineering behind every board — it makes your design manufacturing-friendly (DFM), so it comes out cheaper and better. In the next lesson we see how to confirm a fabricated board is actually sound, through inspection and quality control.