How is a trace formed?

Question

I know it's the conductive connection between different points on a circuit board, but what is its form, how is it made?

My confusion stems partly from this image of a PCB cross-section, which I encountered when I started reading about how PCBs are manufactured:

This image suggests that there's a continuous layer, a sheet of copper on a circuit board that spans the whole surface. If that's true, how is a trace formed? What breaks that sheet of copper into separate wire-like lines that are isolated from one another?

Answer 1

Let's examine the manufacturing steps for a very simple single-sided PCB. ¹

PCB fabrication is a subtractive process. The PCB starts life as a solid sheet of copper laminated on a sheet of insulator ². Most of the time, traces are formed by etching. But how to etch away the excess copper between the traces, while keeping the copper that forms the traces?

1. Copper is covered with photoresist.

2. Parts of the board that will remained covered by copper (e.g. traces) are exposed to light.

3. The board is washed with a solvent. Unexposed photoresist is washed away. Exposed photoresist remains. So we've got a board [still] completely covered with copper. Traces are drawn on top of the copper with photoresist.

4. The actual etching takes place.

   • The etching solution attacks copper.
   • At the same time, the chemistry of the photoresist is such that the etching solution doesn't attack it. As a result the traces that are made of copper aren't etched away.
5. Exposed photoresist is washed away with another solvent (different than the one used for unexposes photoresist), which doesn't attack copper.

Thus, we get a board with copper traces.

¹ _{This question is about traces, and I'd like to focus on the traces. The sequence of steps would be somewhat different when there are have multiple copper layers and vias have to be plated.}

² _{Typically fiberglass. If the PCB has to be extra cheap, phenolic paper can be used .}

³ _{Sometimes during prototyping traces can be made by milling.}

Answer 2

First you need an image of what the traces should look like. Then you transfer the image to the PCB, and etch it with an acid. Then you can optionally apply a silkscreen and solder mask. There are many ways to do each of these steps. To transfer the image, you can use the toner transfer method or with a photoresist, as a film, liquid or pre-applied to your PCB.

To etch your PCB, three common acids are Ferric Chloride, Sodiumpersulfate, and Copper Chloride in Hydrochloric Acid. You submerge your PCB in your acid with where you want your traces to be covered, and the acid will etch away the exposed copper

Here's a good guide that runs through the whole process.