Wednesday, October 31, 2018

Creating PCBs

I have been working lately on perfecting a formula for preparing PCBs. If you're like me and like your designs tried out faster than a few days read on.

Assumptions

I am assuming you either are using the applications and materials that I used or you are happy to realize the same functions in the application you are using. I will be using Linux, KiCAD, a PDF viewer for printing and chemicals available in Poland, where I live.

Preparing the printout

The following assumes you can use KiCAD. KiCAD's printing functionality is basically broken. The quality of printouts is so low you can't do anything with it. The following mitigates that by using Plot.

When creating designs that contain through-hole components make sure to check the size of pads. By default they are tiny-tiny small and can easily break when heat is applied. To change the parameters of a pad right-click on, select Properties and set the Hole shape to Oval, Size X and Size Y to at least 2. With that there is probably enough space to solder.

  1. Create your design
  2. Select File / Plot
  3. Set Plot format to PDF
  4. Select the copper layers
  5. Select Mirrored plot and Negative plot
  6. Set Drill marks to Actual size
  7. Click Plot
  8. Open the created PDF and print it on a Canson calcque
  9. Use Density Toner to even the traces on the layout. Density Toner is basically rectified gasoline which melts the toner making it evenly black in all spots. This prevents the printout to leak UV rays through

Depending on your printer (I use OKI C332 laser printer) it is important to set the printout density to maximum. That black toner is all that will block the UV rays from the light-sensitive material!

Preparint the PCB

  1. Cut out a piece of laminat to the size of your project. You don't need any excess - that's a waste I don't like
  2. Cut out the printed layout to the size of your PCB. Again, I do that precisely which then makes aligning things easy
  3. Cut out a piece of light-sensitive foil with additional 5-10mm. That excess is needed for the board to be exactly covered form side to side
  4. Clean the PCB. I mean like really, really well. I use a special sanding block for cleaning PCBs that I bought ages ago but a very fine sanding paper will do just fine. Don't overdo it - you don't want the copper layer to get too thin!
  5. Wipe the board with a clean tissue so that any dust is removed from the surface
  6. Peal the cover off of the foil on the inner-side. I do that with a piece of tape - works every time with absolutely no hassle
  7. Place the foil on the PCB - the side that had the outer foil removed
  8. Make sure there are no air bubbles or dust bubbles between the foil and PCB
  9. Put the PCB between a folded sheet of paper (PCB towards the fold!) to form a sandwitch
  10. Run the sandwitch through laminator at least 3 times. Make sure the laminator is properly heated!
  11. Remove paper from sandwitch leaving only PCB. Beware not to peal of the foil from PCB
  12. Put the printed layout toner-down on the PCB
  13. Spray the PCB with layout on it with Transparent and remove all air bubbles
  14. Cover the PCB with a plexi and make sure there are no air bubbles. If there are then you either have to litle Transparent or the field is not even.
  15. Place a 32W energy saving bulb 22cm from the PCB.
  16. Turn on the light source for 12-13 minutes
  17. Remove the PCB and put it in a dark place for 10 minutes (necessary for the chemical process to finish)
  18. Remove the outer protective foil - watch out not to break the layout!
  19. Use foil developer for 2 minutes. While developing use a soft brush to remove the disolved foil. Do NOT exceed 2 minutes!
  20. Wash the PCB under warm running water and using hands delicately remove any remaining pieces of foil.
  21. Put the PCB under light for 5 minutes. That way if there are any parts of the foil lefy they will turn blue and you'll know you have to start again
  22. Put the PCB in a dark place for 5 minutes for the chemical process of hardening the foil to settle
  23. Use B327 to remove non-covered parts of the copper; Don't keep it too long.
  24. Once all the parts are disolved put the PCB under running water and wash excessively
  25. Pour acetone on the board using surface tension to flow the liquid from one side to another. Do that until the foil peels of from the traces
  26. Wash under running water

That's it! With luck the PCB can have extremely small traces (requires good printer which the C332 is not) and very delicate handling especially when washing out the unexposed foil. I stick to 0,25mm traces and above but even that can sometimes go wrong. 0,5mm traces are guaranteed to be 100% successful.

Important remarks

Don't rush things! With the exception of exposing the foil for too long and developing the foil for too long take your time! Rushing things is the worst you can do. This process requires you to be gentle with the board, precise and methodical. Rushing things will only get you to broken traces.

That is it! Happy etching!

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