What a PCB is and how it’s made

Before you touch a single tool, it helps to know exactly what you are setting out to build. In this chapter, I’ll show you what a printed circuit board really is. You will learn what a PCB what it’s made of, what each part does, and the names we use for those parts. Then we’ll follow a design from an idea on your screen all the way to a finished board in your hand, install KiCad 10 on your computer, and take a quick look around the KiCad workspace so the rest of the book feels familiar. This is the most “reading” you’ll do for a while; from the next chapter on, you’ll be doing.

PCB anatomy

As a child, my interest in electronics grew from admiration of what clever engineers had come up with into plain curiosity about how those things actually worked. That curiosity led me to take an old screwdriver to anything electronic with a screw that fit. A record player, a VCR, and a radio were only a few of my “victims.” I am still surprised I never got electrocuted, though I did at least have the good sense to unplug things from the mains first. Inside those devices I found all sorts of wonders: resistors, transformers, integrated circuits, coils, and power supplies.

The engineers had attached all those parts to small boards (Figure 1.1). A board like that is a ‘printed circuit board,’ or ‘PCB’ for short. It is the flat green (or red, or blue, or black) slab that almost every electronic device is built around. Its job is twofold: to hold the components in place, and to wire them together electrically without a single loose strand of hook-up wire. Let’s look at what a PCB is made of, what each part is called, and why it’s there. The example I’ll use throughout this section is a simple board I designed for this book (Figure 1.1).

Figure 1.1: The top side of our example board, with the main features we’ll meet in this section labelled.

Soldermask

That coloured coating over most of the board is the ‘soldermask’. On our example board it happens to be green, but soldermask comes in a whole menu of colours these days. Green is the classic, but red, blue, black, white, yellow, and purple are all standard options from most manufacturers at no real extra cost. The colour is up to you; the job it does is the same regardless.

The soldermask does two important things. First, it seals the copper away from the air. Bare copper slowly oxidises, and oxidised copper conducts poorly, so the mask keeps your tracks healthy over time. Second, it makes the board far easier to solder. The mask coats everything except the pads, and because solder won’t stick to the mask, it stays where you want it, on the pads, instead of wandering between them. That matters because pads can sit very close together, and the tip of a soldering iron is often as wide as a pad or wider. Without soldermask it’s easy to accidentally join two neighbouring pads with a blob of solder, a defect called a ‘solder bridge’ (Figure 1.10). The mask doesn’t make bridges impossible, but it makes them much less likely.

Figure 1.10: A solder bridge, a defect that soldermask helps prevent.