PCB Basics

PCBs stands for printed circuit boards. They are called “printed” because you print your circuits out onto the copper. With the design printed, you then either mill or etch your prints into the copper. The general process is actually quite complex, especially with quality control considerations and efficiency measures implemented in large fabrication houses. However, the process can be simplified into manageable steps such that home fabrication is possible. We will also be skipping a ton of steps, just because some of the more complicated features such as silkscreen and multilayer (more than two layers) is impossible at home. But I digress, here’s what you need to know about the PCB itself.

The surface of the PCB has several features. You will probably notice right away, when picking up any typical PCB, that the majority of the surface is covered with green stuff. This is called the soldermask, and it is a dielectric (insulator). It actually has several specific tasks. First, it is there to prevent corrosion, as the oxygen in our atmosphere is quite toxic to the copper on the top and bottom layers. Next, it has the job of preventing accidental shorts from occurring. The exposed copper is very vulnerable to paper clip drops and loose screws. Best to cover it up with green stuff that won’t conduct.

The next thing you will notice are the tiny lines that run across the surface of the board (albeit covered in green). They are the copper that reside on top of the PCB. This is how electrical connections are created from one electronic element to another. The term used to describe these lines is “signal trace” or just “trace”; they describe the trace that a copper takes from one point to another. Next, there are the pads. These are exposed bits of metal covered in tin (through electroplating). They are exposed so that the pins on your ICs and your resistors can be soldered onto the board. The tin does not oxidize, but is still conductive. This property protects the underlying copper, while still allowing an electrical connection to occur to the component being soldered. A plus side is that the metal tin is a major component in modern solder, such that the flow of the solder is facilitated by the tinned pads. Lastly, the colored letters and markings seen on top of the soldermask are called the silkscreens. They are aptly named since the markings are applied to the soldermask through a silkscreen process. It is essentially a stencil made with a thin membrane, onto which colored ink is applied. This layer allows the PCB designer to label the components, and indicate switches and functionality.

However, there are things underneath the surface that cannot be seen with the naked eye, but play a key role in the functionality of the board. Below is an example of a 4 layer PCB, typically very cheap to manufacture.