PCBs, copper pours, ground planes, and you

Ok I'm gonna ask a potentially stupid question as someone who knows little to nothing about electronics. The article (and every other source I looked at when trying to answer my question) points out the blank spots on the board as a copper pour. Where, exactly, is the copper? I just see green plastic. Is it on the back side and nobody is taking photos of it?

stupid answer: copper goes green when it oxidises

actual answer: PCBs are covered in a protective film to prevent damage to the extremely thin layer of copper on them from bumps

A PCB is a layer cake of fiberglass and copper, with the green stuff just being paint on top to make manufacturing easier. The copper is glued to the fiberglass and etched according to your design files. (This turns a plate of copper into a bunch of very thin wires, basically.) If it's a multi-layer design, a bunch of these etched boards are glued together. Holes are then drilled for vias (which are rivet like things that electrically connect arbitrary layers according to your design file) and through holes (for through-hole components, or for screws to attach a board to an enclosure, etc). Finally, a solder mask (the green stuff) is applied over the parts of the copper that don't need to be exposed (for connecting components), covering the copper on the outside layers.

The solder mask prevents solder from sticking to parts of the board that don't need solder on them.

To add components, solder paste (little balls of metal embedded in a flux) is applied to the areas without solder mask (using a metal plate that is laser cut to have holes where solder is needed), and then an optical/robotic system called a pick and place machine places each component at the right spot. (Or you can do this with tweezers.) Finally, the board is heated to the melting point of the solder, and because of the solder mask, pulls all the components into the exactly right place (by minimizing surface tension, something liquids like to do). Through hole boards are a little different, they get "wave soldered" where the board floats on molten solder and attaches to the metal areas. The solder mask is even more helpful here. If you didn't have solder mask, you'd just get a thick layer of solder on top of every trace, and potentially bridges between adjacent traces, which is bad. (But people do apply solder to exposed traces to increase their current carrying capacity.)

TLDR, the plastic on top is paint that makes manufacturing easier. If you make PCBs in your own shop with a laser or mini CNC, you won't have solder mask, and you can see how much more difficult soldering components is. It's not impossible. Just a little bit more work from your hand and brain is required, and that's expensive at scale. So, plasticy paint.

Thank you for the explanation! I had no idea about any of this, I figured that circuit boards were just plastic with metal traces etched in somehow. I didn't know that so much went into them.

Every time I order these things I am astounded at how cheap they are for what you get.

Sometimes volume really is magic!

The green is solder mask. It's just a coating that goes over most of the board, to prevent oxidation and accidental electrical shorts and to prevent solder from sticking where it isn't supposed to at the assembly stage. (You can get solder mask in different colors as well. The board itself is usually actually a sort of yellowish white and made of FR4, a kind of fire-resistant fiberglass.)

Generally you can sort of see through the soldermask to see the copper traces underneath. In the example images in the article, the lighter areas are copper and the darker areas are the space in-between.

This is a great explanation for the uninitiated. Kudos. You can get the green stuff in any color, of course. Hence red or blue circuit boards. There is lore about why green but whether it was chosen deliberately, copper traces appear darker through green than other colors because they're reddish. So one might prefer green for development.

The natural color of the most common fiberglass used in circuit boards is a straw color - you will see this from time to time, either on very low cost boards or where solder mask has been deliberately left off.

Vias were literally rivets in the very early days of PCB fabrication. Someone might still be selling a kit of that kind of thing for people who etch boards themselves. I'm not going to look, but they were still around 20 years ago, they were obsolete then, and things hang around in this industry for a long time after they're obsolete. Actual through-rivets have very poor mechanical performance and crack easily. Vias and through-holes for parts leads are typically plated now. You can only get plated through holes from the quick-turn suppliers hobbyists use, and that's a good thing.

> to prevent ... accidental electrical shorts

It frequently does that, so you are not wrong, but it is not a reliable insulator. If you need insulation or spacing for engineering or compliance reasons, you cannot rely on solder mask to provide it.

I mean the kind of shorts where you accidentally drop a screw on the PCB, drag a loose wire across it, or something like that. Not foolproof, but it probably reduces the risk of damage somewhat.

Right, and I think your language was careful to imply that, but others reading this should understand solder mask does that but it's not what it's for, if that makes sense. The bit of mechanical and environmental protection it can provide after assembly is similarly incidental.