A PCB, or printed circuit board, is the flat board inside nearly every electronic device that physically holds and electrically connects all the components needed to make a circuit work. It’s the green (or sometimes blue, red, or black) board you’d see if you cracked open your phone, laptop, TV remote, or car dashboard. Instead of loose wires running between components, a PCB uses thin lines of copper bonded to an insulating board to route electrical signals exactly where they need to go.
How a PCB Works
At its simplest, a PCB is a sandwich of materials. The base is a rigid, non-conductive sheet called the substrate. Bonded to one or both sides of that substrate are thin layers of copper, which are chemically etched into precise patterns of lines called traces. These traces act as the “wiring” of the circuit, carrying electricity between components. A protective layer called a solder mask covers most of the copper to prevent accidental short circuits, and that’s what gives the board its characteristic color.
Electronic components like resistors, capacitors, transistors, and microchips are soldered onto exposed copper pads on the board’s surface. Soldering creates both the electrical connection and the physical bond that holds each part in place. The result is a complete, compact circuit that can do everything from regulating a simple LED to running the processor in a smartphone.
What PCBs Are Made Of
The most common substrate material is called FR-4, a sheet of woven fiberglass soaked in epoxy resin. The “FR” stands for flame-retardant. FR-4 is popular because it’s strong, electrically insulating, and inexpensive. It has a standard thickness of about 1.57 mm and can handle temperatures up to roughly 130°C for standard versions, or around 170°C for high-temperature variants.
The copper layers are measured in ounces per square foot, which translates to physical thickness. A standard 1 oz copper layer is about 35 microns (0.035 mm) thick. Boards that need to carry more current use heavier copper, such as 2 oz (70 microns) or even thicker. For smaller, lower-power devices, thinner 0.5 oz copper (about 17.5 microns) is sometimes used.
Not all PCBs use FR-4. Flexible circuit boards use a thin, transparent polyimide film instead, which allows the board to bend and twist. High-frequency applications in aerospace or telecommunications sometimes require specialty substrates with better signal performance at frequencies above 5 GHz, since FR-4’s electrical properties degrade at microwave frequencies.
Rigid, Flexible, and Rigid-Flex Boards
Most PCBs you’ll encounter are rigid boards. They’re flat, stiff, and stay in one shape permanently. The motherboard in a desktop computer is a rigid PCB.
Flexible PCBs can bend, fold, or twist into specific shapes. Manufacturers make them by laminating a layer of polyimide film to a thin copper sheet, then etching away the copper to leave the circuit pattern. These are common in small, lightweight products where space is tight, like fitness trackers, hearing aids, or the ribbon cables inside a laptop hinge.
Rigid-flex PCBs combine both types: rigid sections for mounting larger components, connected by flexible sections that can bend around corners or fold into tight spaces. This hybrid approach eliminates the need for separate connectors and cables between boards. In aerospace applications, rigid-flex designs can reduce weight by up to 60% compared to traditional rigid assemblies. Satellites use lightweight rigid-flex boards that can be up to 75% lighter than equivalent rigid setups, which matters when every gram affects launch costs.
How Components Are Attached
There are two main ways to mount components onto a PCB. The older method, called through-hole mounting, involves pushing component leads through holes drilled in the board and soldering them on the other side. This creates very strong mechanical bonds and is still used for larger parts like big capacitors and connectors that need to withstand physical stress.
The more modern approach is surface mount technology, or SMT. Components are soldered directly onto pads on the board’s surface without any holes. SMT components are much smaller, and they can be placed on both sides of the board. This allows for significantly denser, higher-performing, and more compact designs. The shift to surface mounting is a big reason why electronics have gotten so small over the past few decades. The tradeoff is that through-hole drilling is expensive and time-consuming, and the holes pass through all layers of the board, eating into routing space on multilayer designs.
Some advanced boards go a step further, embedding components like capacitors and integrated circuits directly inside the substrate itself, freeing up even more surface area.
From Design to Physical Board
PCB design starts in specialized software. Engineers draw a schematic of the circuit, specifying every component and how they connect. The software then helps lay out the physical board, positioning components and routing copper traces between them. Automated routing tools can calculate efficient trace paths that avoid conflicts.
Once the design is finalized, the software exports it as a set of Gerber files. These are the universal file format that PCB manufacturers use to translate a digital design into a physical board. Gerber files describe each layer of the board: where the copper traces go, where holes should be drilled, where the solder mask opens up, and where text or labels appear. Every major PCB design tool can export in this format.
At the factory, the copper layers are etched using a chemical process. The areas of copper that should remain are protected (historically with ink, industrially with a photosensitive coating exposed by UV light), and then the board is bathed in a chemical like ferric chloride that dissolves the unprotected copper. What’s left is the precise pattern of traces and pads. The layers are laminated together, holes are drilled, and a solder mask and silkscreen layer are applied before the board is inspected and shipped.
Single-Layer vs. Multilayer Boards
Simple devices might use a single-layer PCB with copper on just one side. A basic TV remote or a cheap toy often has a single-layer board. Double-sided boards have copper on both sides, with small plated holes called vias connecting traces between the two layers.
More complex electronics use multilayer boards, which stack four, six, eight, or more copper layers separated by insulating substrate. A typical smartphone motherboard might have 10 or more layers. High-density interconnect (HDI) techniques use tiny microvias as small as 0.004 inches in diameter to pack more functionality into less space. More layers allow more complex routing and better signal management, but they also increase cost and manufacturing complexity.
Where PCBs Show Up
PCBs are in virtually every electronic device, but the design requirements vary enormously depending on the application. Consumer electronics like phones and laptops prioritize small size and low cost. Industrial equipment needs durability and longer service life. The electronics industry classifies boards into three reliability tiers: Class 1 covers general consumer products where cosmetic imperfections don’t matter as long as the board functions. Class 2 is for equipment like communications gear and business machines that need high performance and extended life. Class 3 is reserved for products where failure is not an option, like life support systems and aircraft flight controls.
In aerospace, PCBs must withstand extreme conditions, including vibrations that can reach 10,000 G-forces in missiles or helicopters. Medical devices bring their own challenges: boards in implants like cochlear devices need to be biocompatible, resistant to sterilization processes, and incredibly compact. A cochlear implant might use a four-layer rigid-flex board with polyimide layers just 0.005 inches thick, bent to a radius of half an inch, with components smaller than a grain of sand. The gap between a PCB in a disposable calculator and one in a pacemaker is enormous, but the fundamental technology is the same flat board with copper traces connecting components into a working circuit.