Here I describe my method for making 1 and 2 layer PCBs at home via lithography and etching
A etching chamber similar to the one I use from Gie-tech. Can be cut from 4 mm acryllic with a laser plotter. Simply add a aquarium heater, bubbler and thermometer.
svg+xml - 8.69 kB - 06/22/2020 at 17:30
For designing my schematics and the PCBs layout I use KiCAD from CERN. Use whatever software / online tool you like.
For plotting / exporting the design I set the following parameters (File -> plot):
- Plot as SVG
- Drill holes 'small' so you have a center point for drilling
- Mirror the design
The following steps are only nessesary if you are making two layer PCBs. For one layer simply print out the mirrored design and center it on the PCB raw material. Then continue with 'Exposure, developing and etching'.
Open the SVG files in Inkscape and align them so that they are on the same height. Flip one side vertically so that the vias / holes are superimposing when you mentally fold one side over the other (vertical hinge). Then draw a rectangle between them with the height of your design and width of your PCB thickness. I use raw PCB material from Bungard which is 1.5 mm thick. Position the front and back layout 3-5 mm apart from the rectangle and then distribute them equally around the center rectangle.
The spacing is important for the etching process. It garantees a good positioning in the holding rails of the etching machine and insures that your design is not covered by the holding mechanism. The next step should not be nessesary but I experienced some trouble when printing direct out of inkscape. One side of the PCB had way thinner trances then the other side although in inkscape the paths where the same width. To avoid this, export a PDF file and print this. You should choose the darkest / uneconomical settings possible to print. The darker the print, the better. If your toner is almost empty the print will be to light and the underlaying photoresist will be developed. I have only tested this with a laser printer, not sure if an ink jet printer will work.
I got the tip that the resolution after exposing is way better with transparency paper than a clear foil, so print on this if available.
Cut out your design and fold it along the edge of the center rectangle so that the printed side lays on the PCB (Thats the reason one mirrors the design in KiCAD export). Use a metal ruler to get a nice sharp bent in the paper. Take speciall care that your bends are not crooked. This will result in a shifted design of the fron/back layer of your board and vias / solder pads will not end up superimposed.
Remove the protective foil of the PCB and center it within the layout 'pocket'. Use a nice straight edge of the PCB to align it. Fixate the layout on the PCB with sticky tape and exposure both sides (see next step). Now put it quickly into the exposure unit.
Exposure and developing
Wear eye protection when handling the solutions!
I use a exposure unit from Gie-Tech GmbH (sold by Reichelt in Germany) with 4x8 W UV-tubes. With this unit the exposure takes about 130 sec. per side, but your times my vary depending on the power of your UV source. Make some tests with small pices of PCB to find the best exposure time. After exposure the layout is usually not visible on the PCB. Immerse it immediatly in the developer solution. I use a sodium hydroxyde solution (1 g NaOH / 100 ml water). With this concentration it takes about 2 minutes to remove the exposed photo film. Make sure everthing is dissolved, otherwise the etching process will not work. Rinse the PCB with cold water and put it immeadiatly to etching. You can use the developer solution multiple times.
For removing the copper layer at the exposed parts of the layout I use sodium persulfate (200 g/l water) in an etching machine (again Reichelt Germany, Gie-Tech) with optional air bubble column at 45-50 °C. Bubbling air through the solution makes a huge difference in the time it takes to etch the copper away. I have tried it in a flat bowl without air support and the results were disappointing. The whole process should take about 5 min until you see most of the copper gone. A couple of minutes more and all the exposed copper should have dissolved (about 7-8 min). Do not over etch ! Traces will get very thin and SMD pads smaller. Rather than trying to etch the last tiny bit of copper of one corner or solder pad off if the rest of the PCB is done, you might be better off if you leave it and scrape it off with a knife afterwards.