PCBs themselves are like a sandwich made up of layers of different materials. The main ‘body’ of a PCB is made of some kind of substrate; typically a fibreglass called FR4. On top of that is a thin layer of copper-foil that lays out the traces and pads, then a layer of coloured solder mask which prevents solder from sticking where it shouldn’t.
This solder mask is often green, but you can get all sorts of colours, such as my favourite – purple. The copper that is not covered by solder mask then gets protected by a surface finish such as HASL (solder) or ENIG (gold plating). Finally, there is the silkscreen layer where text and symbols are printed.
Like any good sandwich, you can keep adding in more layers of copper, from two layers up to tens of layers. It’s also worth knowing that rigid fibreglass isn’t your only option: you can choose a material called Kapon (polyimide film) to make flexible PCBs. You can even make your PCB sandwich using both fibreglass and Kapon to make a board that is flexible in some places and rigid in others. One super-cool option, that both OSH Park and Evil Mad Scientist Laboratories have been using recently, is black fibreglass paired with a clear solder mask, so all your copper traces are visible.
In next month’s column, I’ll be talking about the awesome ways the hardware hacker community has been using PCB design to make cool electronics projects and beautiful art, as well as sharing some of the tools they’re using to make their boards. In the meantime, I recommend this excellent video that visualises the composition of a PCB: hsmag.cc/SI2m31 (How do PCBs Work? by Branch Education).