Understanding how PCBs are manufactured helps you design boards that are easier, cheaper, and more reliable to produce. Every design choice — from trace width to via size to surface finish — directly maps to a specific manufacturing step with its own capabilities and limitations. This guide walks through the complete PCB fabrication process from raw materials to finished board, explaining what happens at each stage and how it affects your design decisions.
Table of Contents
- Raw Material Preparation
- Inner Layer Processing
- Lamination
- Drilling
- Electroless and Electrolytic Plating
- Outer Layer Patterning
- Surface Finish, Mask, and Silkscreen
- Frequently Asked Questions
Raw Material Preparation
PCB manufacturing starts with copper-clad laminate — sheets of FR-4 with copper foil bonded to one or both sides:
- FR-4 core: Glass fibre cloth impregnated with epoxy resin, pressed and cured into rigid sheets. Standard thickness: 0.1-3.2mm
- Copper foil: Electrodeposited copper, typically 1oz (35µm) or 2oz (70µm) thick, bonded to the laminate surface
- Prepreg: Uncured (B-stage) glass fibre cloth used as adhesive between layers during lamination. Provides the dielectric between copper layers
The laminate manufacturer (Shengyi, Isola, Rogers) controls the material properties (Dk, Df, Tg) that determine your board’s electrical and thermal performance.
Inner Layer Processing
For multi-layer boards (4+ layers), inner layers are processed first:
- Cleaning: Copper surface is cleaned and micro-etched for photoresist adhesion
- Photoresist application: Dry film photoresist laminated onto the copper surface
- Exposure: UV light shines through your Gerber artwork film. Exposed resist hardens (for positive process)
- Development: Unexposed resist is washed away, revealing the copper pattern to be kept
- Etching: Chemical etchant (cupric chloride or ammonium persulfate) removes exposed copper. The photoresist protects the circuit pattern
- Strip: Remaining photoresist is chemically removed, leaving bare copper traces on the laminate
- AOI: Automated optical inspection verifies the inner layer pattern against the Gerber data
The etching process is subtractive — it removes copper to create the pattern. This is why minimum trace width and spacing are limited by the etch factor: thicker copper (2oz) requires wider minimum features than thinner copper (1oz).
Lamination
Inner layers are stacked with prepreg sheets and outer copper foils, then pressed under heat and pressure:
- Stack the layers in the correct order with registration pins for alignment
- Place in a hydraulic press at 175-185°C and 300-400 PSI for 1-2 hours
- The prepreg resin melts, flows around the inner layer copper features, and cures
- Result: a solid multi-layer sandwich with copper patterns on inner layers and blank copper on outer surfaces
Lamination registration accuracy is typically ±0.075mm — this is why inner layer pad sizes need adequate annular ring margins. If an inner layer via pad is too small, the drill can miss it.
Drilling
After lamination, through-holes and vias are drilled:
- CNC drilling: Computer-controlled drill machines bore holes at positions specified in your drill file
- Drill bits: Tungsten carbide bits, 0.15-6.35mm diameter. Standard PCB drill bits: 0.2-0.5mm for vias, 0.8-1.0mm for component holes
- Speed: Modern machines drill at 150,000-300,000 RPM, achieving 200-500 holes per minute
- Accuracy: ±0.05mm position accuracy for standard, ±0.025mm for advanced
- Aspect ratio: Maximum hole depth to diameter ratio of 8:1 (standard) to 12:1 (advanced). A 1.6mm board can have minimum 0.2mm drill (8:1 ratio)
After drilling, the holes are deburred and cleaned (desmear process) to remove resin smear from the hole walls, ensuring copper plating adheres to the inner layer copper pads.
Electroless and Electrolytic Plating
- Electroless copper: A thin layer of copper (0.5-1µm) is chemically deposited on the hole walls and board surface. This creates a conductive seed layer on the non-conductive hole walls
- Electrolytic copper: The board is placed in an electroplating bath. Electric current deposits copper onto all conductive surfaces, building up to 20-25µm in the holes and on the surface. This is what creates the electrical connection between layers through the plated via
Plating uniformity determines via reliability. The plating must be thick enough in the hole centre (typically 20µm minimum) to handle thermal stress during soldering. Poor plating causes barrel cracks that create intermittent connections.
Outer Layer Patterning
Outer layer circuits are created using a pattern-plating process (different from inner layers):
- Apply photoresist to the outer copper surface
- Expose and develop to create the circuit pattern (resist covers areas to be REMOVED)
- Electroplate additional copper (25µm) and tin/tin-lead onto the exposed circuit pattern
- Strip the photoresist, exposing the copper to be removed
- Etch the exposed copper. The tin plating protects the circuit pattern from the etchant
- Strip the tin plating, revealing the final copper circuit
Surface Finish, Mask, and Silkscreen
- Solder mask application: Liquid photoimageable solder mask is applied to both surfaces. UV exposure through the mask artwork defines pad openings. Development removes uncured mask from pad areas. Final thermal cure at 150°C
- Surface finish: HASL (dip in molten solder and level with air knives), ENIG (electroless nickel + immersion gold), or OSP (organic coating). Applied to exposed copper pads
- Silkscreen: Component markings printed using inkjet or screen printing. UV-cured for durability
- Profiling: Board outline routed using CNC router or V-score machine. Separates individual boards from the panel
- Electrical test: Flying probe or bed-of-nails test verifies all connections and isolation per the netlist
- Final inspection: Visual inspection, dimension check, and packaging for shipment
Frequently Asked Questions
How long does the entire process take?
A standard 2-layer board takes 2-3 days in the factory. A 4-layer board takes 3-5 days. 6+ layers take 5-8 days. These are pure manufacturing times — add shipping and queue time for the total lead time you experience as a customer.
What causes most PCB manufacturing defects?
The top three defect sources are: (1) Drilling — misaligned or broken drills cause missing or offset holes, (2) Etching — under-etching causes shorts, over-etching causes opens, (3) Plating — insufficient hole plating causes via failures. Modern factories achieve defect rates below 50 ppm (parts per million).
Can I visit a PCB factory in India?
Yes, many Indian manufacturers (PCBPower in Ahmedabad, Rush PCB in Delhi, Shogini in Bangalore) welcome factory visits. Seeing the process firsthand helps you understand manufacturing constraints and design better boards. Contact the manufacturer to schedule a tour.
Why does my fabricator ask for minimum annular ring?
The annular ring is the copper ring around a drilled hole. Drilling has position tolerance (±0.05mm) and the lamination has registration tolerance (±0.075mm). The annular ring must be large enough that even with worst-case tolerance stacking, copper completely surrounds the hole. Minimum 0.1mm annular ring for standard, 0.075mm for advanced.
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