Thursday, 2 August 2012

Playing with PCB's

For most of the last few months of messing around with the development of the BlueFlyVario I have been playing with prototyping PCB's. Before I go on I have some important advice:

DON'T DO THIS! At one point I figured it would be a good idea to be able to produce PCB's myself for a run of about 10 to 50 prototypes. After a few months of fiddling around, trying all kinds of different methods, I have one PCB that I am happy with. Just one. I think it has cost me quite a bit to produce it. I am about 50% confident I could produce another in about two hours, but I might screw it up. In this post I am going to describe what worked for me and what didn't. The bottom line is that I recommend getting PCB's produced in a fab house. I have just ordered a batch from SeeedStudio to check them out.

Artwork

I used Eagle. It is pretty common amongst hobbyists. After a few hours I got used to the interface. Sparkfun has some good tutorials.

Substrate and Etching

I started with standard double sided 1.6mm FR4, the 1 oz copper kind. After screwing up quite a few times I now understand how important it is to prepare the surface very carefully. The method that works best for me involves using wet 400 grit sandpaper in a random fashion. This works best with a small sanding block to ensure the surface is completely flat.

I tried all kinds of methods for etch resist. Drawing using a sharpie is useful only in patching up areas where fine detail is not important. I spent some time messing with dry film negative acting photo resist (the DuPont stuff you can get in small quantities on ebay). This is used in professional fab houses and I thought would be a great idea. It comes with the promise of being able to get 6mil traces in optimised production environments. In practice I was able to get 16 mil traces, but with most boards there were always errors. The obstacles to overcome include:
  • Getting the film to stick with no bubbles. This is tricker than trying to get a screen protector on with no bubbles. Just one small bubble with screw things up as the etch gets under the resist in the wrong places. Being very careful and using a pouch laminator helps a lot.
  • Producing artwork thick enough. Toner on clear film worked best.
  • Getting exposure right. I used a BugZapper lamp (40W I think) at 30cm for three minutes. That worked well.
I have ended up relying on toner transfer. I can now get 10mil traces and spaces with no problem. There are tutorials all over the net on how to do this. Some pointers that worked for me:
  • I use a cheap HP mono laser printer. The toner seems to be heavy enough, even on ground plane pours, and it melts at the right temperature.
  • I tried lots of different paper. In the end paper from an Australian Who magazine works best. The idea is to get the glossiest possible paper that is very thin and breaks down well in water. Glossiness prevents the toner getting too intertwined with the paper fibres, thinness ensures the heat and pressure gets through the paper to remelt the toner, and being easily dissolvable in water helps the paper come away.
  • I used a cheap A4 pouch laminator. It takes 4 to 12 passes to get the board hot enough to melt the toner. It is all about the right temperature, not the number of passes. If you do one pass, then wait until the board cools down, you need to start again. It is also important to ensure the paper does not move around. The best method I found was to ensure the paper was a little smaller than the board (so it does not hang over the side and get stretched in funny ways, attaching only one side with a piece of tape (the side fed into the laminator first).
  • To release the magazine paper it just needs a few minutes soaking in warm water, then very carefully pealing off the paper. Some will stick to the toner which is stuck to the board. I used light rubbing with my thumb under water to carefully remove the paper from between fine SSOP pads. If any toner comes off the fine bits you will need to start again. If the board is too dirty or too smooth the toner will not stick. This is tricky to get right.
This bit is dangerous if you do not know what you are doing. I use an etch consisting of one part HCl at 30% strength (Mutaric acid or Hydrochloric Acid from the hardware store) and two parts 3% H2O2 (Hydrogen Peroxide from the chemist). Constant agitation should see the exposed copper come off in a few minutes.
 
To remove the toner I use toluene/xylene based paint thinners. Any strong non-polar aprotic solvent will work better than acetone, which for some strange reason seems to be the standard.

Holes and vias

I thought it might be a good idea to have a crack at through hole plating the vias. This in theory involves drilling holes prior to etching, and some electrochemistry. I failed at the hole wall activation stage, mostly because I could not source a fine enough graphite powder without forking out a wad of cash and waiting weeks for delivery. I figured a really good hole wall activation mix would be toluene, graphite powder, and polystyrene. Polystyrene dissolves in toluene well, the resulting mix is not very viscous and flows well into the holes, it also allows a fine suspension of graphite powder to be maintained. Once dry, a very thin film of polystyrene holds the graphite in place. It further smooths if heated and forms a thin film of very conductive polymer, provided the graphite powder is thin enough. I gave up once I worked out that wire is good enough.

Quality fine PCB drill bits can be got on ebay for about $1 each. My solution for working diy vias involves drilling with a 20mil bit (after etching). I make the via pads 70mil in diameter which helps to ensure that even the two copper sides are not perfectly aligned, then it might still be ok. I then put a small copper wire through the hole (I use single core wire from stripped down CAT 5 cable - it is just about the perfect size). I make the wire flush on top, but with about 0.2 to 0.4 mm of protrusion on the bottom side which is stamped flat with a modified hole punch prior to a small flat dab of solder on the bottom side only. After SMD components are in place I then put a dab of solder on top.

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