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Do-It-Yourself PCB Etching (method 1)
Hey there, kids! Are ya ready to get down n' dirty with some corrosive chemicals? Fantastic! Today we're going to learn what is probably the easiest way to design and etch a copper circuit board. Before we get started, you'll need the following:

1.) A copper-clad board
2.) Some etching solution (ferric chloride)
3.) A small plastic dish/basin of some sort
4.) A Sharpie marker
5.) Something to grab the board with (tweezers/tongs/etc).
6.) A scouring pad or steel wool
7.) Gloves
8.) Some protective eyewear is probably a good idea too

The first thing you'll want to do is use your scouring pad and/or steel wool to scrub the board clean. You want to make sure it's free of any greasy fingerprints, dirt, thin protective coatings, etc. The board should have a nice even shine to it.

Next, use the sharpie to draw your highly complex circuit design on the board (or in this case, a dog yelling "STUPIDFINGERS!"), making sure that all of your traces are nice and dark/solid. The permanent marker will act as the resist. Or in other words, anything NOT covered by the sharpie will be eaten away when we dunk it in the etching bath.

After you're satisfied with your circuit design, it's time to get dirty with the acid. Pour some etching solution in the plastic tray and carefully place your board in it, face down, preferably floating on top of the solution rather than immersed in it. The etching process usually takes about 20 minutes or so, but don't you go walking off just yet. You'll need to stick around to agitate the solution every few minutes, or it might not etch completely. To agitate it, just scream at it gently tap the board around a little bit with a toothpick or whatever. That'll usually stir the solution enough.
PS: I don't know how the hell that crow got on the other side of the board. I mean, it's not like I tried two designs in case one came out looking like crap or anything.)

After about 20 minutes or so, use your tweezers/tongs to lift the board out of the solution. Now RUN to the bathroom sink, dripping etching solution all over the carpet on the way. Rinse the board under cold water for about a minute to get all the solution off the board. You might notice some spots that didn't etch all the way. This is caused by either not etching long enough, not agitating the solution well enough, or dirty spots on the board. If that's the case, dry the board off and return it to the etching bath for a few more minutes. (Not TOO long though, because it can over-etch and start eating away at your traces).

After all the undesired copper has been etched away, use your scouring pad/steel wool to scrub the board clean under the sink. Keep scrubbing until all the sharpie marker is removed from the board and your traces show. (Rubbing alcohol will speed things up if you've got some handy). Note that I managed to screw up my own tutorial by etching the board too long. (Some of the text was etched away). Ahem... VIOLA! You just done up and made a PCB, sucka! (Or dog head saying "upidfingers", depending on your skill level). Of course, you'll still have to drill holes and populate the board with electronics of your choosing, blah blah blah.

I used this Sharpie method as an example for the sake of simplicity, but there are some other really keen methods of transferring your design to the board which you might want to try too. There's a pretty good method which uses laser transparencies or glossy paper and an iron that you might wanna try. That method can be found here.

Now you wait a gosh darn minute! What about all that used etching solution, eh? What are you gonna do with that? It's usually good for several etchings, so I suggest saving it. Just pour it into a Gatorade bottle or whatever you've got laying around. Once the solution gets too crappy to work well anymore, THEN you can dump it. BUT WAIT! I noticed that the back of my bottle says it's safe to flush down the toilet or dump in the sink, but let's think about this for a moment... This stuff has one sole purpose in life, and that's to MURDER COPPER. After you flush it down the toilet or sink, where do you think it goes? Through some pipes which, you guessed it, are probably made of copper. Chances are it won't hurt anything right away, but it can't be good for it. So instead, do the right thing and get rid of it properly at your nearest hazardous waste disposal center.