PCB - The Making of Blaster Shark (Sumo)
May 10, 2010
The details of why I picked what parts I picked are covered in the Design blog. What I am about to describe here is the tools and process I go through to design and etch my own PCB.
The first thing is to get the layout of your PCB. I always start with pen and paper to get a block diagram then move onto a schematic which will then be transferred to the computer. Boot up your preferred electronics cad software (I use Eagle) and begin to lay out your circuit in schematic form. I find that if I skip this step I will always miss something or put a signal trace on the wrong pin. There are a lot of tutorials out there for Eagle if you don't know how to use it (Google is your friend). Next step is to lay out the actual components and tracks in the software. After that is finished I print out the top and bottom layer and inspect them cause it is a lot difference then looking at a computer screen.
I buy my copper clad presensitized with a special photo resist film. The toner transfer method works too, but i find this method easier. So with the piece of paper I had previously printed the circuit layout on I will tape a piece of inject overhead projection film onto it an put it back in the paper tray. I will then print that layer again with the best ink settings so all the traces and fill areas are completely blacked out. I do this for the bottom layer as well and then inspect them to see if they are alright. I then line up the two pieces of film and attach then either with tape or a staple so that they stay registered during the exposure process.
Photo Resist Exposure
I have built myself a UV exposure box with a bunch of LEDs that I purchased off ebay. It works great and gives be clear, crisp traces. So first thing is to cut the PCB to size and take the protective film off the presensitized board and place the film with the image of the circuit layout on it. I then sandwich the board between two clear pieces of plastic or glass so the film doesn't move when being exposed. I will then place it under the exposure box and start a time for about 3- 5 minutes. After which I will flip it over and expose the other side.
Photo Resist Developing and Etching
While the photo resist is exposing I will prepare the developing and etching solution. Once the board is finished exposing, dunk it into the developing solution and swirl it around a bit and you will see the image of the circuit appear. Rinse off the board and you are now ready for etching. I use Ammonuim Persulfate as an etchant. It comes in powder form and you can leave it in the garage to evaporate making clean up easy. Mix up the solution as instructed on the container and place the board into the echant. The etching process is accelerated with heat and gentle agitation. It should take about 20ish minutes depending on board sized and temperature. After eching is finished I throw the board back into the exposure box and expose the remaining photo resist again. This makes it easy to remove the remaining resist as I just put it back into the developing solution and the resist will desolve away. At this point I will tin the copper with a tinning solution. Tinning is purely optional but it helps prevent oxidation on the copper. All that is left is to drill the holes and populate the board.
I forgot to take pictures of the actual etching process, but you should get the general idea. Happy building till next time.