A copper board is made of an epoxy resin board covered with a thin layer of copper. The etchant reacts with the layer of copper and dissolves it, except where a mask protects the copper. The epoxy is left intact as it does not interact.

Here are the two chemical reactions taking place : (1) is ferric chloride’s (aka Iron (III) chloride) and (2) is vinegar/hydrogen peroxide/salt mixture’s.

(1) FeCl3 + Cu --> FeCl2 + CuCl

(2) H2O2 + 2(CH3COOH) + Cu --> 2(CH3COO-) + Cu2+ + 2(H20)

The reactant is the result of the combination of acetic acid (which represents 80 grams per liter of white vinegar according to the bottle [which says 8° acidity]) and hydrogen peroxide (which is at 3% mass concentration).

We can calculate the optimal ratio of each component in order to get a maximum amount of it.

Hydrogen peroxide is 34 grams per mole and acetic acid is 60 grams per mole.

So, in a liter of vinegar there are 80 grams of acetic acid which represents 80/60 or 4/3 moles of acetic acid. In a liter of hydrogen peroxide, which approximately weighs a kilogram, we have 3% x 1000 grams = 30 grams, nearly a mole, of pure hydrogen peroxide.

Because the reaction uses twice the number of hydrogen peroxide molecules of acetic acid molecules we can say that we need to put roughly a 2/3 ratio of hydrogen peroxide/vinegar in volume to get things optimal.

Now you may be asking yourself “Why do we want to add salt to that ?”.

Well, one possible explanation (I'm not actually sure it is the right one) is that the reaction brings neutral Cu atoms of the board to Cu2+ ions in solution that would form copper acetate (it is the combination of one Cu2+ ion and two CH3COO- "acetate" ions). And, at one point the bath would reach an equilibrium (because there is as much Cu2+ forming from Cu than Cu2+ getting back at a solid Cu form).

Generally this happens before your board gets its beautiful tracks, which is quite sad. Table salt, or NaCl, brings chloride ions Cl- to which Cu2+ ions will bond to form cupric chloride or CuCl2 instead of being left in solution (those ions would endlessly come back and forth from Cu to Cu2+). You can see this during the reaction (if you leave the reaction with no salt it will turn blue which is the color of copper acetate and stop, whereas if you put salt it will turn green, the color of cupric chloride, and carry on).

2Cl- + Cu2+ --> CuCl2

By doing so, the equilibrium point is pushed forward and more copper can be dissolved, so that, if you are generous on salt (if you introduce table salt in excess) you may get your PCB in the end ! One tablespoon of salt on the board is generally sufficient.

Hypothesis : Given the concentrations of hydrogen peroxide and acetic acid in our "recipe" ferric chloride should be way more efficient. However the Salt/Vinegar/Hydrogen peroxide seems to be both less expensive and safer.





Now comes the fun part, let's put up the lab coat and etch PCBs !