UK What a printed circuit board is
A printed circuit board, or PCB, is the flat board inside almost every electronic device. It does two jobs: it holds the components in place mechanically, and it connects them electrically through thin copper tracks bonded to an insulating base. The tracks replace the hand-wired connections of early electronics, so the same circuit can be built thousands of times with identical results.
The base is usually FR-4, a flame-retardant laminate of woven glass fibre and epoxy resin. Polyimide is used for flexible boards, and aluminium-backed boards for parts that run hot, such as LED lighting.
A bare board with no parts fitted is the PCB, sometimes called a printed wiring board (PWB). Once the components are soldered on, it becomes a printed circuit board assembly (PCBA). In everyday speech, both are just called circuit boards.
The layers of a PCB
Even a simple PCB is a stack of materials, each with its own job. Computer boards repeat the copper and insulating layers many times, with signal layers sandwiched between solid copper planes for power and ground.
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Substrate. The rigid core, usually FR-4, which gives the board its strength and insulates one copper layer from the next.
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Copper. Foil etched into tracks, pads and planes. Thickness is quoted in ounces per square foot, typically 1 oz (about 35 µm), with heavier copper for high-current boards.
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Vias. Plated holes that join tracks on different layers. Blind and buried vias connect only some layers, which saves space on dense boards.
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Solder mask. The coloured coating, usually green, that covers the copper except where parts are soldered. It prevents solder bridges and protects the tracks from oxidation.
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Silkscreen. The printed labels that mark component positions, reference numbers and polarity.
Types of printed circuit board
PCBs are grouped by how many copper layers they have and by how rigid they are. IPC-6012 sets the qualification and performance requirements for rigid boards, and IPC-6013 does the same for flexible and rigid-flex boards.
| Type | Construction | Typical use |
|---|---|---|
| Single-sided | One copper layer on one face | Power supplies, simple controls and low-cost electronics |
| Double-sided | Copper on both faces, joined by plated vias | Interface boards, sensors and I/O modules |
| Multilayer | Four or more copper layers laminated together | Motherboards, single board computers and network cards |
| HDI | Microvias and very fine tracks across many layers | Processor modules, phones and compact embedded boards |
| Flex and rigid-flex | Polyimide layers that bend, alone or bonded to rigid sections | Display connections, cameras and tight enclosures |
| Metal-core | Copper circuit on an aluminium or copper base | LED backlights and power electronics that need to shed heat |
A board with a modern processor and DDR5 memory typically needs at least 6 to 10 layers, with the track widths and layer stack calculated to give controlled impedance for high-speed signals such as PCIe, USB and Ethernet.
How components attach to a PCB
Most parts are now surface-mounted. In surface-mount technology (SMT), solder paste is printed onto the pads, a machine places the components and the whole board passes through a reflow oven. Processors and chipsets often use ball grid array (BGA) packages, with hundreds of solder balls hidden under the chip.
Through-hole technology (THT) pushes component leads through drilled holes and solders them on the far side. It is slower to build but stronger, so it is still used for connectors, large capacitors and anything that takes mechanical strain.
IPC-A-610 sets the acceptance criteria for soldered assemblies in three classes: Class 1 for general electronics, Class 2 for dedicated service equipment, and Class 3 for high-reliability products that must keep working in demanding conditions.
PCBs in industrial computers
Every computer is built around a PCB. A motherboard is a large multilayer board that carries the processor, memory slots, chipset and connectors. Our industrial motherboards use the same standard form factors as desktop boards, while single board computers fit a complete system, processor and memory included, onto one compact board.
Boards for industrial use differ less in the copper than in what surrounds it: higher-temperature laminates, components rated for wider temperature ranges, more mounting points against vibration and a longer production life. A board validated for the industrial temperature range has been tested with every component on it, not just the bare PCB.
Protecting a PCB on a harsh site
Bare PCBs are vulnerable to moisture, conductive dust, salt and condensation, which cause corrosion and leakage between tracks. On demanding sites, a few measures make the difference.
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Conformal coating. A protective film over the finished assembly, qualified to IPC-CC-830, for humid, salty or dusty air.
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Sealed enclosures. An IP65 or higher rating keeps water and dust away from the board altogether.
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Fanless cooling. Cooling through the case avoids drawing dirty air across the components.
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Careful handling. ESD protection during assembly and servicing, because static can damage parts without visible signs.
Quick answers on printed circuit boards
PCBs: common questions
What does PCB stand for?
Printed circuit board. In electronics, PCB means the board that carries and connects components. The same letters also stand for polychlorinated biphenyls, a banned industrial chemical, which is unrelated.
What is a PCB made of?
Usually FR-4 and copper. The core is woven glass fibre bonded with epoxy resin, with copper foil for the tracks, a solder mask on top and a printed silkscreen. Flexible boards use polyimide instead of FR-4.
Is a PCB the same as a motherboard?
A motherboard is one kind of PCB. Every motherboard is a printed circuit board, but most PCBs are much simpler, from a single-sided board in a power supply to a small sensor board.
What is the difference between a PCB and a PCBA?
Components. A PCB is the bare board. A PCBA, or printed circuit board assembly, is the same board with all its components soldered in place and ready to work.
Can PCBs be recycled?
Yes, through e-waste recycling. Specialist recyclers recover copper, gold, silver and palladium from boards. In the UK and EU, the WEEE regulations require electronic equipment to be collected and treated rather than sent to landfill.
We supply industrial motherboards and single board computers as standard with memory, storage and the operating system fitted and tested. Send us your environment and I/O needs, and we will recommend the board and the enclosure to suit them.