After the dry film is applied to the copper layer, it can be etched. There is an option to use ferric chloride (FeCl3), but it can not be reutilised and needs to be disposed. An alternative is to work with a mixture of hydrochloric acid (HCl) and hydrogen peroxide (H2O2), which can be regenerated with air and reused for the etching process.
To create the solution you will need 30% solutions of (10 mol) hydrochloric acid and 3% hydrogen peroxide. In my case I have poured 500 ml of hydrogen peroxyde into a jar and then added 250 ml of hydrochloric acid into it. So the ratio is 2 hydrogen peroxyde to 1 hydrochloric acid. There is an old chemistry saying: "Do like you oughta, add acid to water", because it's much more likely that the liquid being poured into will splash out of the beaker and get on your clothes. If it's just water, then it doesn't matter. Always wear eye protection and gloves. A mask is also advisable as the solution gives toxic chlorine gasses. Also keep the area well ventilated. These gasses are also produced after etching, so make sure to store it in a well ventilated area.
The chemical formula during this process is: H2O2 (aq)+ 2 HCl (aq) + Cu -> 2 H2O + Cu2+(aq) + 2 Cl- (aq). The end product will contain Cupric chloride (CuCl2), which in the long run will be doing the etching.
When you have your solution ready, prepare a container were you will be doing the etching and put the PCB inside it.
After a couple of minutes the solution will become light green. When you do more etching the color will darken.
Stir once in a while for the process to accelerate.
After about 15 minutes all the copper will dissolve and only the dry film layer shall remain.
This is a close-up before removing of the dry film layer.
Now it's time to get rid of the dry film layer. To do that we will need to prepare a solution of sodium hydroxide (NaOH). It is also known as drain cleaner. Be careful and wear gloves as it dissolves fats. Weight 10 grams of the granulate and add it to 200 ml of water, which will create a 5% NaOH solution.
Put the board into the container and rub it with a brush to dissolve the dry film.
And this is the end result.
The close-up shows that maybe I should have left the board a bit longer inside the etching solution. Or the error could also come from the previous steps. Anyway, there is still room for improvement and some calibration work to do.