How does the circuit board work? Video and text explanation

The reason electronic devices can perform different functions is that the internal PCBA board provides it with various capabilities. We usually refer to the printed circuit board in the equipment as PCB, which is actually not the correct name. The printed circuit board itself cannot work, it acts as a carrier for the electronic equipment, and the PCBA board is the main body that makes the electronic equipment work.

The smartphone you use has a long, uncluttered line. The main function of these lines is to integrate the hardware devices of the mobile phone into a single PCBA, including Wi-Fi, antenna, GPS, fingerprint sensor, etc. In the following content, FS Technology will take a mobile phone as an example to introduce how the circuit board works.

Before the text begins, let us watch an introduction to the working principle of PCB from a YouTube blogger.

Basic concepts of circuit boards

The development of PCBA is very rapid. Humans have never known what PCBA is, and it only took a century to understand the working principle of printed circuit boards and become the core of the electronics industry. Circuit boards were first invented in 1936 by Austrian Paul Eisner. Its development was aided by the use of the technology in radio equipment by the US military, as the technology became more well-known in the United States. In 1948, PCBA’s ability to work was recognized around the world and used for commercial purposes. In the mid-1950s, printed circuit boards became commonplace in people’s daily lives. As printed circuit boards dominate the electronics manufacturing industry, more and more attention has been paid to how circuit boards work.

For practitioners in the non-PCBA industry, it is easy to confuse the concepts of PCB and PCBABefore explaining the working principle, please let FS Technology clarify several confusing concepts in the PCBA industry for you.

PCB: Also known as circuit board, printed circuit board, PWB, bare board. You may have seen a board like this, it may be green or some other color. What appears to be a simple circuit board is actually a multi-layered labyrinth of hundreds of copper wires. The PCB provides support and effective blocking for all components mounted on its surface, and the lines in the middle of the board provide a platform for communication and collaboration between components.

PCBA: The abbreviation of PCB assembly, although it is only one letter more than PCB, there is an essential difference between the two. Usually, we call the conductive pattern formed by the combination of the circuit and the printed component on the insulating material according to the predetermined design as the PCB, and the board on which electronic components have been installed is called PCBA. The PCBA board is an electronic component that plays a decisive role in electronic products.


SMT assembly: an assembly technology that mounts SMD components on a bare PCB. The full name of SMT is surface mount technology.

THT assembly: A traditional technology for installing plug-in components on bare boards. The full name of THT is through-hole technology.

How Printed Circuit Boards Work?

PCBs are made by isolating the surface copper foil conductive layer through the board base insulation material, which allows current to flow through various components along a pre-designed route, ultimately achieving functions such as power making, amplification, attenuation, modulation, demodulation, and coding. This is a description from a PCBA government website. It may not be easy for non-industry practitioners to understand. FS Technology will disassemble the working principle of the entire mobile phone PCBA to show you.

Connection of PCBA traces to components

Understanding how a printed circuit board works should start with its composition. The entire PCBA board is composed of bare boards and components, such as microchips, resistors, capacitors and connectors. Manufacturers attach components to the PCB by solder mounting or other techniques. Take, for example, a single-sided PCB, a board that has electronic components and wires on only one side of the board. Usually we mount the components on a non-conductive board by SMT assembly technology or PCBA DIP assembly technology and connect them through small paths called traces. The traces allow the electrical components on the entire board to function by energizing them. Not all hardware devices are directly mounted on the PCB, components such as monitors and cameras are attached to the PCB through mating connectors and flat cables.

The working principle of double-sided and multi-layer circuit boards is to introduce process gas (Ar, N2, O2, etc.) in the high vacuum link, and the gas is ionized into plasma. Under the action of the electric field, these plasmas move toward high potential and low potential respectively. The atomic groups moving toward the low potential bombard the target (copper), so that the copper atoms are stripped from the copper, and finally covered on the substrate (FRP), that is, the copper clad laminate. This is a traditional physical method, which has the advantages of no pollution and mature technology. The disadvantage is that the efficiency is slow and the cycle is long. If you want to quickly realize the PCB manufacturing process, you can lay the etched pattern in advance, and then form the PCB in the above way, and the inner hole for connection can also be plated with metal copper to make it metallized.

The picture below is the PCB board taken by FS Technology through X-ray. The dark area in the figure represents the wire or trace, and its part is made of a non-conductive insulator material made of woven glass fiber and an FR4 epoxy resin material adhesive. In this X-ray image, you can see that even though there are many wires stacked on top of each other on the board, they are not in any contact with each other.

PCB board photographed by X-ray

The picture below is the key to the operation and function of the mobile phone circuit board – the microchip, referred to as a system-on-a-chip or SoC for short. Because it is mounted in a grid of connection points on the PCB (pads of the ball blue array), the hub is connected to a wire or trace that runs through the PCB. There are also many pads on the PCBA where other microchips are mounted, such as memory chips and wireless chips as well as resistors, capacitors, and other components.


Next, let’s focus on a small set of wires. As mentioned above, there are a lot of wires inside the phone, and the total length of these wires can go around a football field. In the image below we can see that there are 15 traces connecting the SOC and the 16MP rear camera, if more traces are used, it means more data can be sent. If you look closely, you will see that as these traces are numerous, they are independent of each other. To make it easier to understand the traces, the diagram below shows just one set of wires, but the traces in the phone are actually routed along the edge of the printed circuit board and connected to a flat cable to the touchscreen display. All these tiny signal lines have to go through the mobile PCBA board to work. PS: The width is less than 1 mm.

15 neatly arranged PCB traces

In short, the PCB enables each component to connect to the SoC or other microchip through hundreds of wires and is organized quite precisely.

The composition of the PCB board

This smartphone PCB is actually made up of ten conductive layers, the top and bottom layers of which are used to mount the PCBA components and act as multiple antennas. In order for it to function properly, power and ground planes are included on every circuit board. In addition, the ground plane also has the functions of electromagnetic shielding and heat dissipation. The remaining four intermediate layers are used to carry all communication lines or signal lines. Each conductive layer contains copper and insulating fiberglass and epoxy. The smartphone PCBA shown here has only 10 conductive layers, but in other applications they can have 2 to 50 more layers. FS Technology can produce multilayer PCBA with up to 56 layers.

Ten-layer conductive layer PCBA board

In addition, on the top and bottom of the PCBA is a layer of colored solder mask, which acts as an electrical insulation and thus makes the pads of the components available. Above it is silkscreen, including ink for marking and letters indicating the location of the components.

Colored PCBA Solder Mask

For the circuit board to function properly, the signal must pass through each conductive copper layer and the insulating material between them. This smartphone has ten separate layers of conductive copper and insulating material between each layer. How does the signal propagate from one layer to another?

The PCB layers perform this function by drilling or metal plating the signals through vias or vertical interconnects. These vias connect the wires of each layer to each other, thereby connecting multiple components and realizing the work of multi-layer boards. There are three types of vias that can be achieved with the PCB manufacturing technology of Chinese manufacturers.

  • Through Hole: A hole from the top layer to the bottom layer
  • Blind Vias: Blind vias connect the top or bottom layer to the middle layer
  • Buried Vias: while buried vias connect internal layers
Three types of vias in PCBA

Since this process requires traversing many layers, when traversing unconnected conductive layers, the copper around the vias on that layer is removed. If we take an X-ray to look at the pads on the top of the PCB for mounting, you can see three sets of wires running in different directions, while holes go through each layer.

These complex wires exist on the PCB in the center of the phone, and they are so incredible. Each model of smartphone is different in shape and specification, so engineers need to use PCB design tools to design different types of PCBA. For example, some mobile phones have separate top and bottom PCB boards, the larger one is called the main board and the smaller one is called the daughter board.

Mobile phone PCBA disassembly drawing

Another important detail is that the PCB is lighter, weighing only a tablespoon of water, and will fit nicely in your phone. Engineers are constantly innovating to make microchips, resistors, capacitors, inductors, and connectors as small as possible, with some components smaller than a flea.

PCBA components smaller than a flea

In the past, PCBA board designs used components that took up too much board space and board space. Since the mounting mechanism for these components is that the pins pass through the connection holes, these older designs are called through-hole components. But here, these components are SMD components mounted by SMT assembly technology.

How to check if a PCB is working?

From PCB to PCBA is a complicated process, both individuals and assembly companies should fully inspect the assembly quality of PCB to ensure that the PCB can operate normally. FS Technology has explained the working principle of the PCB circuit above, then we will solve the problems that may be encountered in the self-made circuit board, that is, how to check whether the circuit board is working.

Schematic Accuracy

Drawing a schematic is the first step in manufacturing a PCB. These schematic diagrams are used to show the working principle of electricity and the function and connection relationship of various electrical components. Checking the schematic will be an important part to make sure your homemade board will function properly.

  • First check whether the power supply of the IC chip and the label of the network node are correct;
  • Check if network nodes overlap;
  • The model of the component package, the pin sequence of the SMD component;
  • Check whether the wiring is correct, including wrong wiring, missing wiring and multiple wiring.

Accuracy of component installation

As mentioned above, the work of PCB is to provide a platform for electrical connection for electronic applications, and PCBA is the core to realize the operation of electronic applications. After the PCBA assembly is completed, it is necessary to inspect the welding and assembly quality of the polar components on the board, such as whether the pins of the light-emitting diodes, rectifier diodes, and triodes correspond. It is worth mentioning that even components with the same function but different procurement channels have different pin ordering. When making the PCBA by yourself, you need to use a multimeter to measure it after the manual soldering is completed, otherwise it will not work when it is used for the PCBA of the keyboard and mouse.

Electric observation method

The power-on observation method is a method of PCB manual inspection, that is, after the circuit board is energized, it is not eager to test but to observe the changes after energization. Any mistakes in the circuit board manufacturing or soldering process may cause smoke, odor, etc. after power-on, which is not a common circuit operation phenomenon.


The above is all about “How the Circuit Board Works”. In this article, FS Technology has made a comprehensive description from“ how does the circuit work ” to “how to check if a PCB is working” and provides video explanations. If you are an electronic enthusiast and have a deeper understanding of PCBA, you can try to make a unique electronic business card by combining “How do circuit boards work” and “PCB Business Card”.