In my first post I’ll describe how I etch PCBs at home with a less conventional but cleaner method, using vinegar as etchant. Yes, vinegar! Vinegar is somewhat slow compared to strong acids like ferric chloride or muriatic (hydrochloric) acid, but its much safer and less hazardous, and there’s an easy way to speed it up. When I first tried it I instantly liked it and never looked back.
If you etch PCBs at home, you may not care if the etching part takes 45 minutes instead of 10. So why not trade ferric chloride with vinegar! It’s delicious in my salads, but why not etch with it? Yes, it’s also an acid and will react with copper, but slower (it’s also much more diluted than ferric or muriatic). To help speed the reaction we can add hydrogen peroxide which acts as oxidant and salt which acts as catalyst. So much for my chemistry expertise!
The vinegar is regular 5% acidity found at the local grocery store. The hydrogen peroxide is standard 3% concentration also found at your local pharmacy. You can already tell these are dirt cheap. A bottle of vinegar, and some hydrogen peroxide will yield a good etching supply for less than $3!
The mix is about 60% vinegar and 40% hydrogen peroxide, with a good shake of regular salt (I use kosher pure salt which has larger crystals but will dissolve quickly and I get the impression is more effective than ionized salt but I could be wrong here. Feel free to experiment with the mixing and let me know if you find a much more effective combination.
I use photosensitive copper clad boards which I expose to direct sunlight for not more than 15 minutes (10 is probably enough), but I’ve seen some people expose with a regular CFL bulb (daylight range, about 5000K light temperature). No need for fancy UV light boxes. For exposing I use regular transparencies that I laser print. One issue is to make sure there is enough toner on the transparency, otherwise UV will peek through the light toner spots and your final copper surface won’t be entirely uniform, see photos for what I mean. To avoid that issue I print several of the same PCB layout on a transparency and then sandwich 2/3 together with tape on the edges, the resulting negative is much more opaque. When I’m ready to expose, I peel off the protection from the copper in a dark room (bathroom) and tape the transparency to the area i want exposed. I usually pre-cut the copper clad to roughly the final size and keep the transparency the same so I can tape it easily to the copper clad. I then use a glass surface to pressure push the transparency onto the copper clad and take it outside to sun bathing for 10 minutes. A photo frame works really well to keep the transparency very flat to the copper and avoid moving.
Once the board is exposed, I remove the transparency (back in the dark) and I place it in a 10% positive developer solution for a few seconds until the exposed parts wash away. You have to be very careful not to let it too long in the developer, or your thin traces will get damaged. This all depends on how opaque the transparency is, how long you exposed the board, etc.
The exposure part is the most important, if you get this right and your transparencies are crisp, you can etch very thin traces – probably 10mil or even less!
Then comes the etching part. After the board is developed, place it in your premixed vinegar-peroxide-salt solution, and add more salt if you don’t see anything happening very soon. Usually a table spoon of salt is enough to start the reaction. Once the salt is dissolved, the board should bubbling consistenty. It takes anywhere from 45 minutes to an hour depending on the size of the board and your solution mix. It will gradually start turning blue as the copper etches away and forms copper salts. You can add more salt, or if you want to speed it up even more, put the whole container in a warm/hot water bath, that seems to help a lot.
The best teacher is yourself, getting this right is a matter of experimentation. I messed up a few times when I was in a hurry. Just be patient with the exposure part, the etching will pretty much handle itself. You just have to check it to avoid over-etching if you have very fine traces. You get an accomplishment feeling when the PCB is done and looks good, without the ferric chloride mess, staining, expense etc.