There are several guides on Instructables which explain how to make your own flex-circuits. I've been making these for years now, and thought I'd share my own workflow. I learned how to do this on Instructables myself. Be sure to check out the flex-circuit tutorial where I learned about all of this and the tutorial on etching which was also extremely useful to me.
What you need to get started:
Also, make sure this is the right method for you. In my experience these circuits are very easy to fabricate. It is also possible to make very intricate designs (I use 0604 parts on a regular basis). However in my experience they are not very durable. I like it for prototyping, however, with time my circuits will accumulate tiny cracks where flexible and rigid materials meet. I personally love this method and I find the results beautiful, but if I want to make something rugged, I go with rigid PCBs. (btw. If anyone has suggestions on how to increase the lifespan of these circuits, I would love to hear them. DuPont sells various interesting materials which could be used to seal them, but I have never tested any of them. Some type of elastic coating might also help, but I myself have not done any testing in this direction yet.)
Step 1: Create your design
I use Eagle which is as powerful as it is counter intuitive. I recommend learning it, but if you just want to get your toes wet, you are probably better off using some vector-graphics software. If all else fails you could even use Microsoft Paint. Some things to keep in mind
Step 2: Print your circuit
Before you print, clean the Pyralux with industrial alcohol. Fingerprints can lead to wax pealing off. You can cut the Pyralux to smaller pieces and then specify to print in that size. My prints usually are A6.
(The nice thing about this method is that you can do multiple test-prints on paper, and once you have it right, switch over to the Pyralux.)
Step 3: Etch your circuit
Create a bath using Hydrochlorid Acid (HCl) and Hydrogen Peroxide(H202) in a 2:1 ratio (Half a cup of Hydrochlorid Acid to a cup of Hydrogen Peroxide). USE YOUR SAFETY EQUIPMENT. Freak accidents *do* happen. Instead of thinking how cool you are, relish the opportunity to don all your safety gear and feel like a real mad scientist. (Yes, I *am* speaking from experience here.)
The HCL will give off steamy vapour. Once the HCl and the Hydrogen Peroxide are mixed, the fluid will get warm and somewhat bubbly. Put your printed circuits into that mixture. The first time you do it, you will probably want to watch it to get a feeling for the timing. You will need to agitate the circuits every once in a while, to avoid bubbles building underneath them, which would prevent the circuit from etching properly.
The fumes are probably toxic (I am not a chemist, so I don't know these things, but common sense suggests so). The fumes are *most definitively* corrosive. I have seen them eat away at all non-stainless steal tools in the workshop I use. I strongly suggest doing this out-doors if feasible. If not, consider building a ventilation system. If you follow this link you will find the one I built together with Antonio Gomes. It worked fairly well, however, it would be good to have some type of filtering system as well.
Have some baking soda at hand. You can use it to neutralize the Acid and turn it into a green salty muck.
Bottom line: You are dealing with Toxic stuff. Be smart.
Step 4: Clean it
Either dump it in a bath of nail-polish remover, or use the Acetone with some cotton to remove the paint (I find this step incredibly satisfying)
Step 5: Soldier it
Those tiny resistors? SMD capacitors? Surface mount components? Get out your pincers, listen to some relaxing music and soldier them. You can do it, no problem.
I generally first apply some soldier to all the pads. I then pick up LEDs, resistors, capacitors with thin tweezers and place them where I want them. Without letting go of the component, I heat the soldier, and let it flow into the component.
Surface mount components like the Atmega328p in the image above I usually attach to the circuit with some double-sided tape. Once it is in place I solder it, foot by foot, with a thin-tip soldiering iron (the tape just stays there once its done.)
I learned how to soldier smd components by following this tutorial.
NOTE: There is no pressing reason not to use Through-Hole Components as well. Just remember in your design to add them in as a mirror image, as you will be connecting them from the 'back' of the circuit (just punch holes where the feet go). I sometimes attach headers this way, though I prefer to attach them at right angles (as seen in the first two images) when possible.
Thats it. Feel free to ask questions. Also, here is an older write-up of mine, if you are curious. I am also considering writing tutorials on multi-layer DIY PCBs, neutralizing the acid or designing your own Arduino compatible circuit. If you are interested in any of those things let me know, it might inspire me to actually sit down and do it.
Hope this is useful to someone. Let me know if it is, and if you know of any method to make the circuits more rugged, let me know as well