What is PCB (printed circuit board) and how it’s made
A printed circuit board, or PCB, is inside every modern electronic device. Pretty much every gadget you use would be unthinkable without a PCB. We are so accustomed to it that we don’t even think about it. But what is it exactly? Let’s talk about that.
PCB in a few words
Early electronic components and devices were manufactured and connected using point-to-point construction. It was essentially a group of wires and components connected directly to each other. Very easily this could become a mess, everything overlapping everything. This led to many difficulties in both scaling up the production and repairing the damaged devices. You can see an example of point-to-point construction below. Imagine making your electronics projects like this today. Pretty much a nightmare!
The Austrian engineer Paul Eisler thought similarly in 1936. Recognizing the struggles, he invented the printed circuit as a part of a radio set. This led to a technique called wire-wrap, which eventually led to PCBs we work with today.
A printed circuit board, sometimes called a printed wiring board (PWB), is a sandwich-like structure of conductive and insulating material. It has two functions. The first is to attach electronic components to different spots on the board by soldering. Given that solder is metal, it serves as an electrical connection and strong mechanical adhesive. The second is supplying connections between components’ terminals. Each conductive layer has a pattern that provides an electrical connection. These layers are why you should avoid drilling your PCBs to make space for other components. It might look fine at a glance, but you can easily go through connections that aren’t visible from the outside. Internet forums are filled with people realizing this when they tried to make more space on their computer’s motherboard.
Layers of PCB
As mentioned, PCBs are made of layers. These layers are made from different materials put together to make one unit. This is why we said it has a sandwich-like structure. Let’s start from the “meat” of our sandwich, or the middle of the PCB, and get to the outside.
The substrate is the base material for the PCB. It is usually from fiberglass. This is what makes the board thick and sturdy. It retains its sturdiness and electrical insulating qualities in both humid and dry environments.
The most common type of fiberglass used in PCBs is FR4. “FR” is short for “flame retardant”, which are chemicals applied to materials to prevent or slow down a fire. FR4 is made from woven fiberglass cloth with an epoxy resin binder.
Next in our sandwich is a very thin layer of copper foil. It is applied on the PCB with heat. When talking about the number of PCB layers, we are referring to how many copper foil layers it has.
A board can be single-sided, having conducting material layered on only one side, or double-sided, having it on both. PCBs usually have between 4 and 8 layers, but they can be made with close to 100 layers. This is achieved by laminating (gluing) multiple double-sided boards together with insulating layers in between. This increases the area available for wiring as there are more conductor patterns inside the board invisible to our eyes. That is when we refer to the board’s datasheet to make sure we’re connecting everything correctly.
The more power a PCB needs to withstand, the thicker the copper foil layers will be. PCBs withstanding low power will generally have thin layers. Consequently, those handling a very high power throughput will have much bulkier layers. This value is called copper weight and is expressed in ounces of copper per square foot (oz/ft2).
The layer on top of the copper foil is the solder mask layer. PCBs get their colors from this layer. The usual color is green, but PCBs can come in many different colors. At Soldered, this color is purple.
The solder mask has an important role, and it’s not just making your board look cool. It insulates the copper traces from accidental contact with another metal or conductive bits. This insulation prevents short circuits that would lead to burnt boards, as well as soldering errors.
The silkscreen layer is applied on top of the solder mask layer. Its purpose is to help us better understand the board by adding symbols, letters, and numbers. The function of every pin and LED is usually indicated.
Like a solder mask, silkscreen can come in any color. White is most commonly used, but a good contrasting color to the solder mask’s color is generally chosen. Black silkscreen is used on a white or yellow board. It is rare to see more than one color on a PCB.
Now that we have an understanding of what PCBs are made from, let’s learn some common terminology. There are many terms or glosses to understand, but don’t get discouraged. A lot of them will make sense as you go through them .
- Annular ring – the circular copper area around the drill hole on the PCB, allows an electrical connection from one side of the hole to the other
- Array – a combination of multiple connected copies of a PCB allowing the assembly process to be completed much quicker
- Assembly – a process of placing components and accessories on a PCB resulting in a functional board
- Assembly drawing – reference depicting the assembly requirement of a PCB which includes components’ placements, construction technologies, methods, etc.
- Automated optical inspection (AOI) – a machine-based technique used to check for potential errors on a PCB
- Bare board – a circuit board with no components mounted on it, just solder mask and silkscreen
- Bill of materials (BOM) – a list of all electronic components required to build a PCB
- Blind via – a via connecting the external PCB layer to one or more inner layers
- Board type – can be either single unit or panel, refers to the manufacturing method of a PCB; single unit manufacturing means PCB fabrication one by one, while panel means manufacturing multiple PCBs on a single, bigger board
- Buried resistance board – a PCB with resistors buried inside, improving the overall function and reliability
- Buried via – a via connecting the inner layers of the board, not visible from the outside
- Component – electronic components or parts, basic pieces used to build electronic devices or equipment, such as diodes, resistors, potentiometers, etc.
- Component hole – a plated hole in a PCB made for a component to be placed in
- Copper weight – thickness of copper foil on each layer of a PCB
- Cutout – removed parts of the board by design, used to adhere the board to the installation or as a PCB grounding technique
- DRC (design rule check) – software verification of PCB layout which makes sure the design does not contain errors (e.g. small drill holes, traces too thin, traces too close together, etc.)
- Drill hit – refers to places on a design where holes have been or should be drilled
- Etching – removal of non-circuit copper with an alkaline or acidic solution
- Fabrication drawing – a way to communicate a PCB design between a designer and an engineer, includes an illustration of the board, information about the drilling holes and their locations, materials, etc.
- Fingers – exposed metal pads found along the edge of a board, used when connecting two circuit boards, such as memory boards and old cartridge video games; gold fingers are most common (gold-plated copper fingers)
- First article – first manufactured board, used by designers and engineers to inspect the product for potential problems
- Footprint – a pattern that defines the area where the component will be placed during an assembly
- Functional (behavioral) test – determines how well the board’s attributes meet design demands
- Gerber file – a type of file used to control a photoplotter, a standard way of communicating board specifications with the manufacturer
- Half-cut (castellated) holes – plated holes drilled on the edge of the board resulting in a half-circle hole, common on PCBs designed to test integrated circuits
- Laminate – a method of combining different materials and multiple layers with heat, resulting in a material with greater strength and stability
- Mounting hole – a hole made to secure the PCB to its final location in a device, no mounting hole is conductive or plated
- Mouse bites – a line of tiny drilled holes clustered close together, creating a weak spot where the board can be easily broken to be separated, very similar to holes around postage stamps
- MPN (manufacturer part number) – a uniform component code recognized by all component sources
- NC drill (numeric control drill machine) – machine used by assemblers to drill holes in PCBs
- NPTH (non-plated through hole) – a hole with no plated copper, no electric connections can be made using it (e.g. mounting holes)
- Optical inspection machine – machine used for the visual inspection of PCBs
- Pad – the small surface of copper on a PCB that allows soldering the component to it, can be through-hole or SMD
- Panel – a combination of several PCB unit boards produced at the same time to improve efficiency during the manufacturing process, panels are broken apart into singular units after the manufacturing process is completed
- Panelization – grouping multiple PCBs into a panel to improve manufacturing efficiency
- Part number – identification to differentiate specific parts from one another
- Paste stencil – thin metal or plastic stencil that lies over the PCB allowing the solder paste to be applied to specified places during assembly
- PCBA (Printed circuit board assembly) – when a company solders components to the boards
- Photoplotter – a specialized device used to create images by drawing the lines and shapes to complete PCB designs
- Pick-and-place – the machine or process by which the SMD components are placed on the correct positions on the boards
- Plated-through hole (PTH) – a hole on a PCB with an annular ring, plated so that the wall of the hole is conductive all the way through
- Pogo pin – spring-loaded contact used for temporary connections when testing while programming
- Printed wiring – a process where a wire design for the PCB is produced, a design is etched into the conductive metal on a board
- Printing – a part of the PCB assembly when a circuit pattern is printed on a board
- Reference designator (Ref des) – the name of the component usually printed on the silkscreen, starts with a letter or two indicating the component’s class, followed by a number
- Reflow – melting the solder to create joints between pads and components or leads
- Registration hole – a hole drilled into the panel used to align the inner layers to the outer layers
- Schematic – a technical drawing illustrating the connections between components, often including abstract representations of components instead of real pictures
- Slot – any non-round hole on a PCB that may or may not be plated
- Solder pad – a flat tin-lead, silver, or gold plated copper pad with no holes used for components to be soldered onto with SMT
- Solder paste – a gel-like paste applied to the surface mount pads on a PCB with the help of a paste stencil, allowing components to be placed
- Surface mount device (SMD) – components designed to be soldered on the surface of PCBs, and not in a through-hole
- Surface mount technology (SMT) – a type of assembly technology that solders SMDs directly on the surface of PCBs, not running components through the through-holes, allowing better component density on the boards
- Tented via – a type of via with a solder mask covering both its pad and plated-through hole, insulating it completely, protecting the PCB against short-circuits
- Through-hole (thru-hole) – a hole passing through at least two layers of multi-layer PCB; descriptor for components with parts or pins running through a board to be soldered to another side
- Through-hole (thru-hole) technology (THT) – a type of assembly that involves inserting components’ leads into holes drilled in a PCB and soldering them to pads on the opposite side
- Trace – a continuous copper path printed on a circuit board, functioning similarly to an electric wire, connecting components on a PCB
- Tracing – width of PCB’s wires or traces
- V-score – an incomplete or partial cut through a panel or a board, allowing the boards to be broken apart into single units
- Via – a plated-through hole on a PCB used to pass signals between layers, has a conductive copper interior for electrical connection
- Wire – a conductive cable transmitting electricity or heat; a route or track on a PCB
Making a PCB
Now that we understand what a PCB is and the terminology around it, why not learn how they are made? It is a somewhat complicated process, but fret not! Here you can find a step-by-step procedure of PCB production.
A designer will first prepare the layout of the PCB on a Computer-Aided Design or CAD system. There is various software for this, but we prefer and recommend working with KiCad . The design will be saved in the Gerber format, which is the de-facto standard. The Gerber file contains a 3D model of the PCB and defines the copper foil, solder mask, and silkscreen layers.
Meanwhile, the copper foil is laminated on a flat sheet of the substrate. Protective aluminum is then applied. This arrangement of layers is then sent to drilling according to the Gerber file prepared prior. The registration holes are drilled first. After that, the machine will drill holes to attach components on the board for every PCB on the panel. Mounting holes for each PCB will also be drilled. After all the drilling, the boards are thoroughly cleaned so no residue is left behind.
The next step is the most important, and that is the production of the copper traces. This is achieved by a chemical process called etching. The copper foil is covered with a resistive mask that has the same pattern as the circuit we want the PCB to have. The unnecessary copper, or the copper that won’t be a part of the circuit, is then removed. This can be done with either an alkaline or acidic solution. For this example, let’s go with alkaline. The whole arrangement of layers with the resistive mask is dipped in an alkaline solution at 60 to 120 degrees Celsius (140 to 248 degrees Fahrenheit). The area that’s not covered with the resistive mask will dissolve. Once this is complete, the resistive mask is washed off to reveal the wanted copper traces.
The boards are then sent to an inspection so their quality can be checked. Since the copper traces are very thin, an operator will use an optical inspection machine to check the boards. They do this by taking pictures of the copper traces and comparing them to the design files. If the board has broken or short-circuited traces, it is then thrown away. If everything is alright, the board proceeds to the next step – solder mask application.
The insulating solder mask will protect the board from exposure to dust and oxidation. The copper traces are hidden beneath the colorful solder mask layer. If not careful, the mask will block the conductive holes needed for components to be properly connected. This can be simply avoided. First, the edge area of the holes is covered with a chemically resistant mask. The solder mask is then applied over the whole PCB. The board then goes through the UV process which removes both the chemically resistant and solder mask from the edges. This leaves the edge area of the holes exposed – it is now just the wanted conductive copper.
Finally, a silkscreen is printed. It is simply just a layer of ink identifying the PCB components, symbols, markings, logos, etc. It doesn’t provide any other functionality apart from the information.
The PCB is now complete. What started as a simple copper plate is now a functional circuit board. You can now either solder the components using liquid tin or ship the bare boards to vendors and companies.
Assembling a PCB
After making the PCB, let’s take a look at how you would assemble it into a functional board. To assemble a PCB, you will almost always use some SMD components. They reduce the cost and increase the quality of the final product compared to THT components.
Unlike THT, SMD components don’t require holes on the board. They will be placed on solder pads. There are two ways to apply solder paste to pads on a PCB. The first way is with a laser-cut stainless steel or nickel stencil. The stencil with the pattern and locations of the solder pads is put on top of the panel. Then, the solder paste is poured into it. A specialized squeegee is used to evenly distribute the paste onto the boards. The other way to add solder paste to PCBs is with a jet-printing mechanism. This is very similar to how an inkjet printer operates.
After the paste is added, the boards are placed on a conveyor belt of a pick-and-place machine. Components are placed on the machine on reels or plastic tubes. The machine’s camera will first determine the coordinates of the reference points on the panel to determine PCB’s solder pads. Using a pre-written program, the machine will pick the components from the reels or plastic tubes and place them onto the panel according to the design documentation. A wide variety of components are placed this way, from big chips to small LEDs.
The PCB with installed components then moves along the conveyor belt right into a reflow soldering oven. The oven has several zones, each with a different temperature. The board enters the pre-heat zone and moves through zones with gradually increasing temperatures. This is to prevent a thermal shock. Once the board enters the zone with a high enough temperature, the solder paste melts, bonding the component leads to the pads on the PCB. The surface tension of the molten solder will keep the components in place and align them while the solder cools. After soldering, when the boards cool down a bit, they are washed to remove any possible residues.
Visual inspection of the boards for missing components or incorrect soldering follows. This can be done with an automated optical inspection system. This machine can control many factors, such as the missing components, deviations in components positions, polarity, etc. Boards with detected defects are sent to a rework station where a human manually repairs any errors.
After the SMD components are assembled, THT components are installed. There are two ways to do this: wave soldering and selective soldering. In wave soldering, the PCB passes over a torrent of molten solder pumped so it looks like a standing wave. As the board touches this wave, the components become soldered to the board. This is a great way to solder a bulk of components in a short amount of time. The other way is selective soldering. Here, individual through-hole components are soldered one by one. A fountain of molten solder moves to the position below the board and solders the components from underneath. Not only is this process faster and repeatable compared to manual soldering, but it also leads to better results and fewer mistakes.
The electronic module will go through the final quality control step when all components are on the board. Visual quality control will be performed with inspection equipment before the product gets packaged and sent to the customer.
How to design your PCB
Have you ever thought about creating your PCB, but you weren’t sure you could do it? We’re living proof that it is entirely possible! We could go in-depth about all the ins and outs, but we wanted to keep it short and informative. That is why we listed 6 quick tips before you get started.
- You need to understand the electrical parameters. There’s no way around it. These are the fundamentals before making a PCB. Current maximums, voltages, signal types, impedance, capacitance, etc. are all terms you should know before making and assembling a PCB.
- Choose a CAD system you’ll work with. There are some free and paid options available. We recommend KiCad because it’s free, open-source, and powerful enough for all your ideas. We used it when we started making our PCBs and we still use it professionally to this day! Search for a CAD that has a good community around it and is (relatively) easy to use. No community would mean you could hardly get any help if you get stuck, and complicated software will just frustrate you enough to quit quickly.
- Copy others’ work. Thanks to the open-source hardware, this is easier than ever. But don’t steal it! There are a lot of layouts available so you can take a look at how others have done it before you. Use them either as an inspiration or learning material, but never claim them as your work! That would be immoral and could get you in a lot of trouble.
- Schematics are crucial. Good schematics result in good boards. When you’re making a PCB, it is a good practice to begin with the schematic first. The schematics are basically like a blueprint to a new device. They show which components are used in the design, how they are connected, and the relationships between groups of components in different schematics.
- Communicate about placing the components. If you’re designing a PCB and get it from an outside provider, you’ll likely be discussing its layout and design. Good communication is key here. There might be standards and rules why you might not be able to use certain components together. This is because they might create an electrical noise in the circuit when near one another. The PCB provider will have datasheets for every component so you’ll get all the required information.
- Practice and don’t get discouraged. Designing your PCB might seem complicated at first, and it is! That doesn’t mean you shouldn’t do it. Quite the opposite! Your first board design will likely have a bunch of problems. Your next one will have slightly less. You will improve over time, as long as you keep learning. Our boards can still sometimes have some problems when we’re designing them, and we’ve been doing it for years. So don’t stress about mistakes. It’s how we learn, after all!
It might be well worth your while when you learn how to make PCBs. If you’re making more than one project, you can easily adapt your PCB designs to your needs. It can be much better than using a prebuilt board and adapting your project around it. And who knows! You might even earn some money with the boards if they get popular enough.
I want to learn more!
If you’re interested in learning more about PCBs and everything that surrounds them, we recommend you learn how to read a schematic . You can always brush up on your soldering skills as well. Check out our tutorials page to learn more about the world of electronics! 🙂
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