The definition, in one sentence
A printed circuit board is a flat laminate structure that holds copper conductors in defined patterns, so that components can be mounted on it and connected without discrete wiring.
That is the whole idea. Everything else in a PCB specification is a detail about how the conductors are arranged, how many layers of them there are, how they are protected, and how the board will be assembled and tested. If a supplier cannot explain a quotation in those terms, the quotation is not yet a specification.
The word printed is historical. Early boards used printed resist patterns to define an etch; modern boards use photoimaging, laser direct imaging or additive processes. The conductor is still copper, and the function is still to replace a bundle of wires with a stable, repeatable, mechanically supported interconnection.
The five layers you can actually point at
Look at the edge of a finished board with a magnifier and you can identify the layers. Understanding what each one does is the fastest route to reading a stackup drawing.
A multilayer board repeats the pattern: copper, prepreg, core, copper. The core is a cured laminate; the prepreg is partially cured resin and glass that flows and bonds during lamination. The distinction matters because prepreg is where thickness tolerance and resin flow come from, and it is the layer most often under-specified on a drawing.
| Layer | Material | What it does | Common buyer confusion |
|---|---|---|---|
| Substrate | Cured resin with woven glass reinforcement, typically FR-4 | Mechanical support and electrical insulation between copper layers | Confused with the whole board; the substrate is the structural core |
| Copper | Foil bonded to the substrate, then patterned | Conductors, planes and pads | Thickness is quoted as weight per area, not as a dimension |
| Solder mask | Photoimageable polymer, usually green | Insulation between pads and protection of the copper during assembly | Not decoration, and not always green for a technical reason |
| Legend, or silkscreen | White or coloured ink | Reference designators, polarity marks, part outlines | Placement accuracy matters for assembly, not for function |
| Surface finish | HASL, ENIG, OSP, immersion tin or silver | Solderability and protection of exposed copper | The finish is a process and cost decision, not a cosmetic one |
What a PCB is not
Four things get confused with a PCB, and the confusion costs money at quotation.
A PCB is not a PCBA. The board is bare; the assembly is the board plus components plus solder. A bare-board price and an assembly price describe different scopes, and comparing them is a category error. Our PCB versus PCBA comparison sets out the file packages for each.
A PCB is not an IC substrate. Substrates for chip packages use finer lines, different materials and different inspection standards. A board shop that builds excellent twelve-layer boards is not thereby qualified to build package substrates.
A PCB is not a wiring harness. A harness carries signals between boards; a board distributes them within itself. They are quoted by different suppliers and specified in different documents.
A PCB is not a breadboard. A breadboard proves a circuit; a PCB makes it manufacturable, reproducible and testable. The step between them is where most design errors are discovered, which is why a design review before fabrication is cheaper than a respin after it.
Six steps from files to finished board
The process is longer than six steps, but these are the six that determine whether the board meets the drawing.
1. Data preparation and engineering review
Gerber or equivalent fabrication data, drill files, and a drawing describing stackup, materials, finish and tolerances. A fabrication review checks that the data is complete, that the drill data matches the pad sizes, and that the specified construction can actually be built. Deficiencies found here cost hours; the same deficiencies found after drilling cost panels.
2. Inner-layer imaging and etching
Copper foil is laminated to the core, covered with resist, exposed to the circuit pattern, developed and etched. What remains is the conductor. Layer-to-layer registration is established here, and it is the reason a multilayer board has an alignment tolerance in the first place.
3. Lamination
Inner layers are stacked with prepreg between them, pressed under heat and pressure so the resin flows, cures and bonds. Thickness tolerance, resin content and the absence of voids are decided in this step. A stackup that specifies impossible prepreg combinations will fail here, not at design review.
4. Drilling
Mechanical drilling for through-holes, laser drilling for microvias in HDI constructions. Hole size, position, aspect ratio and the finished plating thickness afterwards all affect reliability. A finished hole is smaller than a drilled hole because plating adds copper to the wall; quoting one and specifying the other is a common error.
5. Plating, outer layers and finish
Copper is deposited in the holes and on the surface, outer layers are patterned, solder mask is applied and developed, legend is printed, and the surface finish is applied. Each of these is a wet process with its own controls, and each can change the board's dimensional behaviour.
6. Electrical test and inspection
A bare board is tested for continuity and isolation between nets, inspected visually or optically, and checked against the drawing. Flying probe and fixture test answer different questions at different volumes. What test does not do is prove that the assembled board will function; that is assembly test, and it is a separate scope.
The full sequence, with the controls at each stage, is described in our manufacturing process overview.
The four things a quotation needs
A fabricator can price a board only when four things are defined. Every quotation delay is usually one of them missing.
Quantity and required date complete the picture, but they do not change the specification; they change the price. Sending them without the four above produces a quotation that will be requoted.
- Construction: layer count, stackup, finished thickness, and whether the board is rigid, flex or rigid-flex.
- Material: laminate grade, and whether an equivalent is acceptable. FR-4 is a family, not a specification.
- Copper: finished weight on inner and outer layers, which is not the same as the starting foil.
- Features and acceptance: minimum line and space, hole sizes, surface finish, impedance targets, outline tolerance, and the inspection standard the board will be judged against.
Rigid, flex and rigid-flex share a definition
The definition does not change when the board bends. What changes is the construction, and therefore what a buyer has to specify.
A rigid board uses glass-reinforced laminate as its core, which is stiff and dimensionally stable. A flexible board uses a thin polyimide film with copper bonded to it, with or without an adhesive layer, and its performance depends on bend radius, bend count and the direction of the copper relative to the bend. A rigid-flex board combines both, with a flexible section laminated between rigid outer material, which introduces a bond interface that must survive both assembly heat and repeated flexing.
Three consequences for a quotation. Flex and rigid-flex work is quoted on a different cost base, because the material is more expensive and the process includes coverlay lamination and often laser cutting. The design rules are different, particularly for bend area copper and for plated holes near a flex transition. And the qualification question is different: a rigid board is judged on thermal cycling and insulation, while a flex board is also judged on bend endurance, which is a test that has to be specified.
A buyer who requests a quotation for a flexible board using a rigid-board specification will receive a price and a board, and possibly neither the endurance nor the tolerance the application needed.
How a board is judged
Every board is accepted against something, and it is worth knowing what that something is before the first shipment rather than after.
The common reference is a class system that defines acceptance criteria for different product end uses: consumer and general industrial work typically accepts a middle class, while higher-reliability applications specify the stricter class with its tighter criteria for annular ring, plating thickness, mask registration and repair. The difference is not a different process; it is a different acceptance threshold on the same process, and it changes yield and therefore price.
Two practical points follow. Specify the class you need, because a board built to a stricter class and accepted against a looser one is money spent without evidence. And ask what records accompany the shipment, since a certificate of conformance without test data answers a different question than an inspection report with measured values.
Our testing and inspection page sets out what bare-board test covers and where its scope ends.
What actually drives cost
Buyers often assume cost tracks board area. It does, weakly. The stronger drivers are these.
Two of these are design decisions the buyer controls and one is a purchasing decision. The rest are consequences.
| Driver | Effect | Why |
|---|---|---|
| Layer count | Each pair of layers adds material and two lamination cycles | Compounding, not linear |
| Hole count and aspect ratio | Drilling time and plating difficulty | Deep, small holes are the hardest combination |
| Minimum line and space | Yield falls as features shrink | Defect density rises faster than feature size falls |
| Surface finish | Gold and nickel content, process steps | ENIG costs more than HASL and is not always needed |
| Panel utilisation | Boards per production panel | A layout that wastes panel area pays for the waste |
| Quantity | Setup cost amortised over units | The first board is expensive; the thousandth is not |
| Tolerance and inspection | Tighter acceptance means more rejects | Specification drives yield, and yield drives price |
Ten terms worth knowing before the meeting
A glossary is not expertise, but it changes the quality of a first supplier conversation from description to specification. Our fabrication file checklist covers the documents that carry these terms in practice.
- Gerber: the file format that describes each copper and mask layer as geometry.
- Drill file: hole positions and diameters, in a separate data set from the Gerbers.
- Stackup: the ordered sequence of copper, prepreg and core, with thicknesses.
- Prepreg: resin-impregnated glass cloth, partially cured, that bonds layers during lamination.
- Via: a plated hole connecting layers. A buried via connects inner layers only; a blind via reaches an outer layer without passing through.
- Annular ring: the copper width around a drilled hole. Too little and the hole breaks out during drilling or thermal cycling.
- Solder mask dam: the mask web between adjacent pads. Too narrow and it lifts; too wide and it blocks solder.
- Surface finish: the coating on exposed copper that keeps it solderable.
- Impedance: the controlled electrical characteristic of a trace, set by width, dielectric thickness and material.
- Panel: the production format a fabricator actually builds, containing one or more boards.
Where this leads
A definition is only useful if it changes a decision. In practice the decisions are: how many layers, which laminate, which finish, and what acceptance standard. Those four answers, written down, are a request for quotation.
The next step in this series is the material question, because FR-4 is a family rather than a single product; the PCB basics index collects the pages that follow. If you already have files and want a fabrication review, send the package and we will tell you what is missing before it becomes a cost.
Questions, answered
Before your next build
Is a PCB the same as a printed wiring board?
Functionally yes. Printed wiring board is the older term and still appears on drawings, particularly in aerospace and defence documentation. The distinction is historical rather than technical.
How many layers can a PCB have?
Consumer boards are commonly 2 to 8 layers; industrial and telecom designs use 12 to 24; high-reliability and high-speed computing boards go considerably higher. The practical limit is set by yield and by the fabricator's lamination and registration capability, not by the concept.
What is FR-4?
A family of glass-reinforced epoxy laminates that meet a flammability specification. It is not a single material, which is why two quotations for FR-4 can differ in dielectric performance, thermal rating and price.
Do I need a stackup drawing?
If the board has controlled impedance, more than four layers, or a thickness tolerance tighter than the default, yes. Without one, the fabricator chooses a construction, and you inherit the consequences.