PCB:Etchants
From K9spud
Once you have a copper clad board with a resist layer of your PCB layout, you'll need to etch the copper off of the board in the areas that are not supposed to conduct. This step involves dipping the board in a bath of chemical capable of eating away at exposed copper. You have several choices of chemical baths to do the job: Ferric Chloride, Ammonium Persulfate, Sodium Persulfate, and Cupric Chloride.
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Ferric Chloride
Ferric Chloride's biggest benefits are that you can buy it almost any where, it is cheap, and it's pretty easy to use. Fry's Electronics has it, Circuit Specialists has it, everybody has it. You can buy it either pre-mixed in liquid form, or you can buy it in powder form that you later add to water. If you're just starting out, you'll probably want the pre-mixed liquid so there's one less thing for you to worry about messing up.
Dirty & Messy
The biggest downside to ferric chloride is that it's dirty. When brand new, the liquid is a brownish-yellow color that you could possibly barely see through if you shine a bright light directly through it. As soon as you start etching copper boards in it, it will turn a very dark brown, eventually almost black, that you can not see through at all. The chemical will permanently stain anything it touches, so you'll want to plan ahead a little bit. Keep plenty of paper towels and some water nearby to quickly mop up any splashes outside of your container. Buy some disposable latex gloves to wear. Wear your crappiest shop clothes.
Etching
When ferric chloride reacts with copper, it cools down. Cold ferric chloride will stop eating away at copper. Therefore, it is advantageous to continously agitate the solution during the etching process. This will help move the cold ferric chloride away from the board, and replace it with warm ferric chloride that can continue etching away copper.
At room temperature, etching your board can take a really long time (1-2 hours?), even with agitation. A better way to do it is to heat the ferric chloride a little bit during the etching process. If you get it too hot though, it will produce fumes and even start eating the resist layer of your PCB. M.G. Chemicals recommends heating it to a temperature no higher than 55°C (135°F).
What we did was buy a small tupperware container for the board + etchant, and a large tupperware container for warm water. We boiled water on the stove, let it cool a bit, then dumped the hot water into the large tupperware container. The small etchant container then sits in the middle of the large tupperware container, surrounded by warm water. If the etchant gets too hot, you can pull the small tupperware container out of the hot water until the etchant cools itself. If the water gets too cold, you can add some boiling water to raise the average temperature upwards.
Storage
Ferric chloride keeps very well. As long as you keep it sealed up from contamination, you can re-use the same bath over and over again until the chemical is completely used up and won't etch any more copper.
Ammonium Persulfate
Ammonium Persulfate's biggest advantage is it's transparent. It feels a little cleaner because it isn't the ugly brownish-yellow color that Ferric Chloride is and it doesn't permanently stain quite as bad. It's pretty easy to find, Fry's Electronics has it. You can buy it as a white powder only. When you are ready to use it, you will mix it with water slowly, as it will produce a lot of oxygen bubbles. Freshly mixed, it will be a clear or slightly milky-clear liquid.
Etching
During the etching process, ammonium persulfate will produce tiny bubbles of gas where it reacts with copper. These bubbles will prevent the chemical from continuing to etch areas of the PCB. Therefore, it is necessary to agitate the solution during the etching process. It also seems to react very poorly at plain room temperature, so you will most likely need to heat the solution. We used the ammonium persulfate with this $40 Etching Tank and our boards were etched in just a few minutes with excellent results.
Once ammonium persulfate begins to eat away copper, it will turn a beautiful transparent blue color. The blue color will darken as the solution becomes more heavily loaded with etched copper. Ammonium persulfate seems to become saturated much faster than Ferric Chloride -- I can only re-use the same batch a few times before it becomes ineffective.
The transparency of ammonium persulfate makes it much cleaner to use. Instead of having to pull the board out of the solution to check progress, you can simply shine a bright light through the solution and the board. As a result, you only need to pull the board once, after the etching process has finished.
Storage
Ammonium persulfate doesn't keep as well as ferric chloride, so you should try to use it up as soon as you've made it. If you do need to store it, keep in mind that the solution will continue to produce some oxygen gas. The gas will need to be able to vent safely instead of building up pressure and causing the container to burst.
Sodium Persulfate
Sodium Persulfate is an alternative to ammonium persulfate that will work with etch resist pens. We've never used it. It doesn't seem to be as readily available.
Cupric Chloride
Cupric Chloride's big advantage is that you can recycle the same bath over and over again indefinitely, just by adding a little HCl acid and bubbling air through the bath to regenerate the solution. Unfortunately, we don't have any experience using it, but here are some pictures from Kyoorius at techfreakz.org. It looks like he got his initial bath started from crystals bought at ArtCraftChemicals.com.
From the pictures, it looks like a transparent green solution that would be just as clean as the Ammonium Persulfate. Together with the recycling potential, this could be the best etchant to use.
Etching with Air Regenerated Acid Cupric Chloride by Adam Seychell provides a lot of good information about using this etchant.
