Projects/PCB/Etchant

Background
The Etchant we use at Reading hackspace is Copper Chloride in Aqueous Hydrochloric Acid Solution. This functions in the same way as the more common ferric chloride, except, rather than getting weaker with use it gets stronger. this means it needs to be treated and handled differently to FeCl.

Theory
The etchant consists of Copper Chloride ions mixed with HCL. The full reaction this this.


 * Cu +  CuCl2   →  2 CuCl
 * 6 CuCl + 3/2 O2  + 3 H2O   →  2 Cu3Cl2(OH)4  +  CuCl2
 * Cu3Cl2(OH)4 + 4 HCl → 3 CuCl2 + 4 H2O

For the less Chemistry literate:


 * Copper + Etchant → Used Etchant
 * Used Etchant + Air + Acid → Etchant

Therefore, so long as there is acid in the solution, and you add oxygen (air), you recover the orignal etchant. Even better, the copper taken off the board is converted to Etchant as well!

Making
Copper Chloride Etchant is quite easy to make, though getting hold of the chemicals can be a pain.

There are 3 main ingredients to make it. Copper, HCl and O2,


 * Copper is obtained from the PCB itself. Though you can also use an old clean bit of copper pipe to make up a batch.


 * The HCl is normally used in swimming pools to control pH. A pool chemical supplier will stock it. Make sure it is as strong as possible, with no surfactants in it. It will work, but it causes interesting reactions, as well as problems bubbling air though it. (Surfactants = bubbles!)


 * The Oxygen can be provided by passing air though the solution. This is slow and would cause a poor etch. Initially we therefore use Hydrogen Peroxide (H2O2). Once the solution has enough copper in it, it is able to fully etch a board before needing regenerating and so the H2O2 is no longer needed. Hydrogen Peroxide can be obtained from a hairdressing supplier. It is also a good time to pick up some need Acetone (nail varnish remover) and rubber gloves (though I prefer washing up gloves, for there strength).

To make the etchant. Put 1 measure of Hydrogen Peroxide into a plastic container, in a VERY well ventilated place. Add an equal measure of Concentrated HCl to this. NEVER add water to an concentrated acid, it can heat up and boil the acid is seconds, always add the acid to the water (just like you oughter). Now, just place the copper into the solution. It will be VERY aggressive and eat the copper in about a minute.

At this point you are acid etching, which produces some nasty vapours. DO NOT INHALE THEM! Once the etchant as been used a bit, the CuCl reaction dominates, making it less aggressive and less fumey. Still use it outside though.

Usage
The solution should be used just like normal Ferric Chloride. Dip the board in till the copper does away, don't spill it don't drink it, don't stick your hand in it.

It is best however if you bubble air though it while etching. This acts to both agitate the solution and regenerate the etchant. If the solution is quite dilute it can run out of CuCl before it finishes etching. when this happens, the reaction will obviously slow to a crawl, limited by the oxygen available. Adding a little Hydrogen Peroxide can speed it up to finish the etch.

Aftercare
Once the etch is done, the solution needs to be regenerated. This is quiet slow, but can be done easily. Place the air stone into the solution, either in the etching tank or into it the storage bottle and attach to the aquarium pump. this will mix air into the solution. The solution will turn a vivid dark green when it is recharged. this can take 30 minutes - 1 hour depending on usage and quantity. pH measurements need to be taken periodically. There are pH strips in the etching kit. Place on in a small drop of etchant and read off it's pH. If it's close to neutral (pH 7) add a small amount of HCl to the mix (2-3% total volume).

Care should be taken in sealing a solution that has had Hydrogen Peroxide added to it. The H2O2 will degrade to Oxygen and Water and so pressurise the container, or pop a cork. Make sure you leave air space for this extra gas.

Disposal
Disposal wise. Copper ions are highly toxic to aquatic life and so etchant should NEVER be tipped down the drain. There are 2 ways of disposing of it. The first is to take it down the dump in a clearly marked bottle. They have a section for hazardous liquids.

The second is to neutralise and evaporate it. To do this add small amounts of bicarbonate of soda to the mix. this will react and neutralise the acid. Be careful it doesn't bubble or boil however. If the acid is too concentrated it can react strongly.

Once the acid is neutralised, soak the solution into paper towels. Allow them to dry out, and you can safely dispose of them in the trash.