The Schematic:
An electronic circuit starts as an idea implemented in a schematic diagram. The circuit schematic is a visual representation of the electronics components and how they are connected. It looks like a road map with buildings and streets, but more chaotic. Components in a schematic look like blocks with lines representing wires or connections. The size of the components in the schematic does not represent their actual size.
The PCB:
The PCB is made of fiberglass. Similar to the fiberglass you can buy at a hardware store. Some PCBs are ceramic or metal(like aluminium) with an insulation layer. The PCB material needs to have the correct mechanical, thermal, RF, electric …etc. Properties for the specific application. Most PCBs used today are flat and use multiple layers with components on both sides.
Single layer PCB:
A physical circuit is made from the schematic with objects laid out on a flat surface. Simple circuits can be designed to fit on one layer. This single layer is made from a thin copper foil glued to insulating material where the components are soldered to the exposed copper. The insulator is most commonly some sort of glass fiber cloth with epoxy.
The circuit traces are made in a process similar to painting with a patterned stencil.
The copper foil is coated with special material called “Photoresist”
The circuit stencil is used with a light source to create a shadow on the photoresist.
The photoresist is selectively hardened where the light hits it, but stays liquid in the shadows
Washing away the liquid keeps the hardened photoresist glued to the copper
The whole thing is dipped in acid that dissolves the copper but does not affect the hardened photoresist or PCB material
A second acid dip is used to remove the photoresist without affecting the copper or PCB material
A second stencil is used to cover the exposed copper with a soldermask except for the solder pads
The end result is a single layer on one side of the PCB ready for soldering components and the rest of the copper is covered with a soldermask material.
Dual sided PCB:
Using both sides of the PCB for routing and component layout costs more but decreases the size. The same manufacturing process is used but on both sides of the PCB.
Most circuit designs need a connection between the two sides of the PCB The role of plated through holes (PTH) is to connect the two layers of the PCB.
A PTH is manufactured with a drilling and plating process.
The two sides of the PCB are designed together with matching pads for the holes.
A drill is used to make a hole through the top layer of copper, the PCB and the bottom layer of copper.
The drilling process is a whole other discussion on how to make a hole with flat sides with tiny drill bits through glass fibers in an epoxy matrix.
(It is possible to use a wire going through the PCB and soldering it to both pads. This kind of connection is rarely used.)
(In substrate manufacturing, the vias are LASER drilled, this is a different process)
It is more common to use a copper plating process to create a “Barrel” of copper that connects the two sides of the PCB
The end result is a dual sided PCB with connections between the two layers that look like little copper pipes. Each side of the PCB has some exposed pads ready for component soldering and the rest of the copper is covered with a soldermask material. A PCB can have thousands of these PTH with different diameters.
Adding more layers:
This is where you can get creative. It should be easy to take two or more “dual sided PCBs” and laminate them together with an insulating layer between them. Adding copper through-holes during a sequential lamination creates connections between various layers. As the number of layers increases, the complexity of the PCB manufacturing process increases (along with the cost).
Components can only be soldered to the external layers, but some components can be embedded inside the PCB. Resistors, capacitors, antennas, inductors…etc can be embedded in the PCB using different deposited materials, designing copper traces to act as passive components and other creative solutions. Some PCBs have a copper slug embedded to work like a heatsink. These creative and complex solutions are only used when the performance demands it.
The PCB serves as the mechanical support for the components as well as the electrical wiring and insulation. The PCB has additional considerations like Thermal, RF, humidity, CTE...etc. More complex manufacturing increases cost and usually reduces reliability. The industry is starting to see mainstream use of flexible, rigid flex, bent FR4 and curved boards.
Copyright Gil Sharon June 19, 2025 . All rights reserved.