OVERVIEW:
This is a simple procedure for preparing 1 liter of cupric chloride etching solution. The tradeoff for the simplicity is a bit of patience as it may take a while to complete the preparation. The value of this patience is that the resulting solution won't be overburdened with hydrochloric acid and thus won't be as smelly and as corrosive to materials around it as more quickly prepared solutions tend to be. It also won't have any residual hydrogen peroxide which will result in a very slightly lower tendency to undercut your PCB traces. That last effect is really quite small, but why not? Also, since we won't be depending on the presence of H2O2 once the etching solution is prepared, we don't have to worry about over diluting our solution by trying to regenerate it with the weak 3% H2O2 commonly available in drugstores. We'll be regenerating with an air bubbler, which can serve double duty to agitate the solution when etching.
MATERIALS:
One large (at least 2 liter/0.5 gallon) glass container with a plastic lid. A 1/2 gallon "iced tea" jar with a plastic lid (plastic liner or no liner) and a small pop open spout would be ideal because you can open the spout to insert a fritted glass bubbler for use in rejuvenating the solution after use. If you need more than 1 liter (1 qt) of solution, consider using a larger container and increase the amounts of materials below accordingly. I suggest the container be only about half full when done. Only partially full plus a small opening in the lid keeps any spray resulting from regeneration by air bubbling inside the glass container.
One small aquarium pump. The cheap vibrating kind is fine.
One fritted glass bubbler, preferably long enough that it can reach the bottom of the large glass container with at least an inch or more of glass tube sticking out - preferably. If you can't find one that long, then your plastic tubing will extend into the container. I don't like that, but that's just me.
Plastic tubing to connect the pump to the fritted glass bubbler. I like enough tubing that I can place my pump higher then the top of my jar. Not likely the solution will syphon out if the pump is lower, but it's 100% impossible if it's higher.
200 grams (7 oz) copper wire, copper pipe or other pure copper. I used solid copper wire, but stranded copper will dissolve faster for those in a hurry. Pennies (even the old type) and misc plumbing hardware aren't pure copper and are unsuitable. 200 gms of copper is about 14 feet of #10 copper wire.
Hydrochloric (muriatic) acid concrete driveway cleaner, free of detergent or other additives. (Read the label.) This should be 32 - 37% HCl. Weaker will work, but will require more. You can get this at Home Depot, Lowes and assorted hardware stores. It will probably cost more at pool supply places.
Hydrogen peroxide. The typical 3% solution from the drugstore is fine. You only need a few ounces.
SOLUTION PREPARATION:
Pour 1 liter (4-1/4 cups) of water into your container and mark the level on the outside of the container. Pour out the water. This is the level you will want to maintain later after etching and rejuvenating.
Add the copper to the glass container.
Place 0.6 liters (2.5 cups) of hydrochloric acid in the glass container.
Add 0.1 liter (0.5 cup) of hydrogen peroxide. This should be all the hydrogen peroxide you'll ever need.
The copper wire will begin dissolving as evidenced by a green color forming in the solution and bubbles being generated on the surface of the copper wire.
Place the lid on the container, open the spout and insert the fritted glass bubbler connected to aquarium pump and begin bubbling to mix the solution.
Here's where patience comes in... continue bubbling until the copper wire is completely dissolved. If the solution becomes a brownish, possibly muddy, color, add 1oz of hydrochloric acid and continue bubbling until the wire is completely dissolved AND the solution is a deep emerald green. The color will be fairly dark, so hold the solution up to a bright light, or pour a bit into a smaller container, to gauge the color. If the acid you're using is on the weaker side, you may need to repeat the addition of a bit (less than an ounce this time) of HCl if several days of bubbling don't result in a deep emerald green color with no trace of a brownish or muddy color.
Dilute to 1 liter with water and that's it.
Use it as you'd use any solution to etch PCBs and pour the used solution back into the main glass container when done. Inspect for color each time you pour solution into your etching tank. (It should be easier to gauge the color in the etching tank because you won't be looking thru as much solution.) As long as it's emerald green or only slightly off color, it's good. No harm in bubbling to rejuvenate after every use, but not necessary until it's no longer a pure emerald green.
The amount of excess HCl in the solution is designed to be quite low to none. If excess HCl is present there's no avoiding some of it being driven off by bubbling, but at very low concentrations that amount is essentially negligible. For those whom I might consider overly concerned... you can have the greater peace of mind you desire by avoiding unnecessary rejuvenation... and when HCl must be added because the solution won't fully rejuvenate, you can add it in smaller increments than I recommend until the solution can be rejuvenated to a pure emerald green.
At some point you'll find you can't rejuvenate just by bubbling. When that happens, add 1oz (less if highly concerned about corrosive fumes) of hydrochloric acid and bubble until emerald green. Add less than an ounce if worried about corrosive fumes, but realize you may need to add several of those smaller amounts to fully rejuvenate.
Maintain the solution level in the glass container at 1 liter - or whatever your initial volume was by adding water if it falls lower. If you only add HCl very sparingly, you can easily find the volume of the solution doesn't increase because some water evaporates when bubbling.
Over time, depending how much etching you do, copper will build up in the solution. When this occurs you can remove a bit of solution (maybe 0.5 - 1 cup), pour it into a pint jar and set it somewhere to evaporate. Make up what you remove with water. If you accumulate any significant amount of copper chloride crystals in this jar, you can scrape it into a small freezer baggie and give it to someone who wants to use it to kick start his own etching solution. Just dissolve it in water and add a small bit of HCl and you have etching solution. Add copper wire to it and bubble air thru it, adding HCl until the wire dissolves and you have stronger etching solution. A weaker solution will etch a PCB, but may undercut a bit due to the much longer time it will take.
That's it.
Jim H
