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Chemical etching is one step of some popular PCB fabrication techniques.
In this technique,
- one starts with a copper-clad board.
- one puts a mask over all the copper he want to keep. There are a variety of ways to do this -- see Toner Transfer and Photoetching.
- one removes the parts he don't want to keep, by chemically etching away the copper.
- cleanup: wash off the board in the sink; carefully store or dispose of the acid.
There are a lot of different chemical techniques for doing this, each with its own advantages and drawbacks.
vinegar and salt
This is the most common hobbyist etchant. Ferric chloride, FeCl3, is a brownish substance. It's usually sold in a bottle (dissolved in water, perhaps with a little acid or peroxide) or as a powder (which you have to dissolve in water).
When in solution, ferric chloride is a ferric ion (Fe3+) and a chloride ion (Cl-). The ferric ion reacts with the metallic copper on the circuit board in a redox reaction, producing a ferrous ion (Fe2+) and cuprous or cupric (Cu1+ or Cu2+) copper. The chlorine is just along for the ride. The copper ion, unlike the metallic copper, is soluble, so it leaves the circuit board and goes into solution. The reaction products form a black sludge which settles to the bottom of the etching tank. After etching enough copper, all your Fe3+ is used up and your solution is full of Cu1+, and you need to get more etchant.
Expensive & hard to control and optimize the process parameters (such as specific gravity & pH value).