What Is a Printed Circuit Board (PCB)?
This FAQ provides you with basic information about the most important and broadest techniques for converting theoretical circuits into functional physical devices.
We commonly understand, analyze, and design electrical or electronic circuits using diagrams called schematics, which consist of component symbols connected by lines.
The symbols represent everything from basic passive components like resistors or capacitors to complex integrated circuits like microcontrollers, and the lines represent conductive paths that allow current to flow freely from one part of the circuit to another.
What all the schematics have in common is a complete inability to drive motors, blink LEDs, filter noise, or do anything else useful and interesting that we would expect an electrical system to do.
After all, a schematic is just a drawing. In order to actually accomplish something with a circuit, we need to convert its schematic into physical components and physical interconnections. Simple schematics can often be implemented on a breadboard, but the vast majority of circuit designs enter the physical realm in the form of printed circuit boards, or PCBs for short.
Structure of PCB
A very basic printed circuit board is a flat, rigid, insulating material with thin conductive structures on one side. These conductive structures create geometric patterns consisting of, for example, rectangles, circles and squares. Slender rectangles are used as interconnects (i.e. equivalent to wires), and various shapes are used as connection points for components.
PCB Stackup
A stack-up is an arrangement of conductive and insulating layers in a multilayer PCB. The following side view shows the stack-up of a four-layer board.
The conductive material of choice is copper. Prepreg is an insulating material pre-impregnated with resin, and the core material is similar in composition to prepreg.
I recommend that you use a four-layer structure whenever possible. A four-layer board allows you to use one internal layer for the reference potential (i.e. ground) and the other for the supply voltage. The top, and if necessary, the bottom, will be a component layer. This arrangement facilitates PCB design and can also help you improve circuit performance.
Learn about PCB Characteristics and Terminology
There is quite a bit of specialized vocabulary that comes up when discussing printed circuit boards. This section describes the physical structures on a PCB and gives you the words we use to identify them.
The conductive interconnects are called traces, and the connection points of components are called pads (pins that are placed on the surface of the board) and vias (pins that are inserted into drilled holes on the board). Basic PCB design involves arranging pads and vias for proper component mounting, and then using traces to connect those pads and vias.
Not all drilled holes are used for through-hole components. We often need to transfer a signal or supply voltage from one PCB layer to another, this is done using small conductive holes called vias.
Many PCBs also include mounting holes that have a mechanical function rather than an electrical function, so no plating is required. The term “coating” herein refers to a conductive material that has been deposited into the interior of a drilled hole.
Copper cladding is a relatively large portion of a PCB layer filled with conductive material. Copper pours can be used to provide very low resistance or low inductance connections between components and to improve thermal performance.
A PCB layer consisting entirely of one large copper pour is called a plane layer. We often use internal layers as ground planes and create ground connections by placing vias next to component pins.
A via or via starts with a copper circle and then becomes a hole when the drill bit goes through the circle (ideally through the center of the circle). The term annular ring refers to the width of the copper remaining after drilling.
Printed circuit boards include various “supplementary” information that have no bearing on the electrical function of the device. For example, reference marks uniquely identify components, dots indicate correct component orientation, and item names or serial numbers help us keep track of the many boards accumulated in the lab. We call this information screen printing.
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