How to etch a printed circuit board
by Dominic-Luc Webb
First, please do not sit around for hours wondering how etching of copper circuit boards is done. It is very, very simple. The basic idea with etching is that you want to 1) transfer your circuit layout to a copper coated fiberglass board and 2) dissolve away all the copper around the transfered layout. Electronics shops will usually have everything you need in stock. I list it all in the materials section. There are a couple ways to accomplish etching. With the diverse background of amateur astronomers in mind, I offer the following cheap and simple protocol that should work well regardless of your technical skills.
single or double sided copper circuit board, depending on your circuit
sodium or ammonium persulphate
transfer plastic (laser printer overhead transparencies work well)
black permanent marker
jar with lid just big enough for the board to lay in.
Printing and transfering the layout
First, you might want to cut a piece of the original PCB to the size you need, since its kind of expensive. This is easily done with a band or jig saw. The board is usually washed to remove fat, or dirt, which can hinder the etching chemical from reaching the copper and reacting with it, although I have never done this and never had a problem. The layout, which was drawn with a graphics program (with nice thick lines, I hope), is printed with a laser printer onto transfer plastic, or alternatively with a copier. It is important to keep track of which way the layout is facing or you could end up with a mirror image of what you wanted. The image on the transfer plastic is then placed onto the copper circuit board. A hot iron is then used to transfer the image to the PCB board. This is possible because the ink from laser printers and copiers comes off the plastic transfer 'paper' and sticks to the copper. The trick here is to not use too high temperature. The 'right' temperature is found by trial and error, but it will always be at or near the 'cotton' setting. If you see melting plastic and smoke, you should deduce that the temperature is too high. If you are not sure the image transfered, try peeling off the the plastic a little bit from a corner, etc. When most or all the image is sticking to the copper, you are at the right temperature.
Once you have the image transfered, it is important to look over the board and make sure you transfered the entire circuit well. Otherwise, you'll end up etching more than you wanted. It is therefore useful to use a permanent marker to go over any problem areas. You're ready for etching once you have an image on the copper that you're content with.
A note on chemicals... There are a lot of acids and other chemicals that can be used to etch a copper board. Conversely, it may surprise you that some well-known acids like hydrochloric acid, HCl, do not dissolve copper very well, at least, not without addition of an oxidizer like hydrogen peroxide, H2O2. I thought I would share some of my experiences about the use of some different chemicals. HCl could be used, as I mention, with presence of an oxidizer. This tends to be rather slow process, and unfortunately, many permanent markers chip off of the board if aggitated in this case. So you will likely end up having to re-draw the whole layout several times (at least) before your PCB will be ready. HCl also produces a lot of toxic outgassing. Acetic acid, CH3CO2H, also dissolves copper, but unfortunately is also a very good organic solvent, dissolving ink from virtually any source. Hot sulphuric acid, H2SO4, might seem like a good solution, and indeed, it will dissolve copper, but most inks are not resistent to this acid. H2SO4 reacts with copper to give off some terribly toxic sulphur and copper oxide (SxOy and CuxOy) gases that you really do not want to inhale, and won't do much for your social life either. Nitric acid, HN03, could also be used, but I am told it produces some dangerous outgassing, and it's a really nasty acid that you really would rather avoid. Ferric (FeIII) chloride, FeCl3, is comonly employed in the USA, but in much of Europe its use is illegal for environmental reasons. Thus far, the safest, most effective way I have found to etch a board is with one of the persulphate (S2O82-) salts, like sodium or ammonium persulphate. These do not generate outgassing, do not smell bad, and are not so objectionable to environmentalists. Further, persulphate solutions will not dissolve any permanent ink (that I know of). Unlike most of the other chemicals I mentioned, the persulphates are legal and readily available virtually everywhere in the world. I therefore choose sodium persulphate, which is often more available and about half the price of ammonium persulphate.
Make up the persulphate in water. About 1 teaspoon/25 ml of water is usually adequate. Heat this solution to near boiling in a glass container slightly larger than the board to be etched. Drop in the board, close the lid and submerge the bottle slightly in hot water to keep it warm. The reaction is faster at warmer temperatures. Something like 50-80 degrees C will work. This normally takes me around 30 minutes with occasional mild aggitation. Afterwards, the board can be rinsed, dried and drilled. For drilling, I have had great luck with 1 mm drill bits, which are pretty common. There are smaller, more expensive ones actually made for electronics, but I have never needed them for any of my projects.