I found a passing mention of this technique of using a sponge with a small amount of ferric chloride on the Pulsar web site, and I was very skeptical that it could possibly work. So naturally, I tried it.
Whenever I have made circuit boards in the past, I did it just like most of us do. I put the ferric chloride into a small tub, dropped the masked circuit board into the solution, and rocked it back and forth for a long time. Even with fresh, strong ferric chloride solution, it would usually take at least 10 minutes for the copper to be removed. As the solution grew weaker, the etch would take longer and longer.
A few months ago, I discovered the 1-part pool acid (muriatic acid) to 2 parts hydrogen peroxide method of etching a circuit board. You will find lots of wonderful Instructables on this method. That method works great, and it made me mad that I spent so much money and effort with ferric chloride over the years when I already had all the chemicals I needed right at home to use this method. The down sides of the acid and hydrogen peroxide solution are that the muriatic acid can cause skin burns and is a little dangerous and damages things that it contacts. Also, I found the etching solution to be quite aggressive which was great for fast etching, but I ended up with severe undercutting and partial obliteration of the traces, and the solution tended to be more corrosive to the resist materials I used, and partially dissolved the mask away during the etch.
This weekend I tried this sponge and ferric chloride method to etch 3 Arduino shield boards I am prototyping for our RFID-enabled member access system at TechShop(TechShop is the 15,000 square foot membership-based DIY workshop with locations in Menlo Park CA, Portland OR and Durham NC). I was so impressed with the success of this technique that I decided to write it up as an Instructable.
The method I will now show you gives you the advantages of all the other methods, and none of the downsides. Specifically:
o You get a fast etch (much faster than either method I know of),
o You use a tablespoon of solution, so disposal problems are eliminated
o A small bottle of ferric chloride will last for hundreds of boards
o No tank or tub is needed, no heating or agitation
o Undercutting is practically non-existent, and the resist stays in place
o There is no need to try to reduce the amount of copper being etched
o The etch is so fast that it is actually exciting to watch and show your friends!
Let's get to it, shall we?
Step 1: What You Will Need
o Ferric chloride (available at Radio Shack, 16 oz bottle for $10, part number 276-1535)
o Sponge (2" x 2" square, cut from any sponge, or paper towel will work too)
o Rubber Gloves (you don't want to stain your hands)
o Copper Circuit Board (one or two sided)
o Cup of water (to drop the etched board into to stop the etching)
Step 2: Clean the Copper and Apply the Resist for the Circuit Pattern
The key to any method of applying the resist is to make sure your copper circuit board is absolutely clean. I use a Scotch Brite pad and some dish detergent to scrub the copper clean, as shown in the first photo. Then I blot it with paper towel and let it completely dry. Never touch the cleaned copper, because oil from your fingers will cause the resist to not adhere to the copper, and the resist will come off during the etching process.
If you want to play with this Instructable right now and you don't want to make an actual circuit, just use a Sharpie pen to draw a little squiggle onto your cleaned copper circuit board.
In this case, I laser printed the Instructables robot onto Pulsar paper and applied it to the clean copper board with a laminator. Hey, it came out pretty good!
Step 3: Etch the Board (Instant Gratification!)
Open the bottle of ferric chloride and put the sponge over the opening, and tip the bottle to let about a tablespoon or so of solution saturate into the sponge.
Now with the circuit board in the palm of one hand, simply wipe the solution-saturated sponge over the surface of the board over and over. Don't scrub, just keep wiping it all over. In just a few seconds of wiping, you will see the copper start to disappear!
You will find that unlike the submersion etching method, the copper in the center of the board etches away first, so you might want to try to focus on the edges as you wipe.
In less than a minute of continuous gentle wiping, your board will be fully etched before your eyes!
Drop the etched circuit board into the bowl of water to stop the etching action.
If you are etching multiple boards, you can rinse out the sponge, squeeze out most of the water, then re-apply ferric chloride solution if desired, but I have found that I can etch two 2" x 3" boards with one application.
Step 4: Clean Up (Not Much to Do)
You can reuse the sponge over and over, so rinse it and let it dry, and keep it with your bottle of ferric chloride.
Step 5: Finished Product, and Your Results
I hope you will agree that this method is faster, cheaper, and more exciting to watch than other etching methods you may have used in the past. I'll bet you'll never use the ferric chloride tub or tank immersion method again.
I'm not sure if this low-volume wiping method will work with the muriatic acid and hydrogen peroxide etchant, but it is worth a try.
Some information I read indicated that adding a small amount of citric acid to the ferric chloride will make it an even more effective etchant for use with the sponge or immersion methods. You can find citric acid powder at beer and wine making shops, and even on eBay.
Go ahead and try this sponge method, and let me know in the Comments section if this will become your new method for etching circuits like it did for me.