siy - thanks for feedback. I think our target users and goals are very different.
I understand your view on modern printer resolution. My target users bought their printers years ago ;-)
Unfortunately, my colleague's employer has decided to force all of their network of laser printers to 300dpi. Some are 600dpi or 1200dpi according to manuals, but it really doesn't seem to matter. They act as 300dpi to our test patterns. It seems to be a function of their network printing software, and we haven't found a way to override it.
I also feel that for the boards I am currently doing 10mil/10mil is fine. After the footprint 'tweaks' 300dpi seems to be good enough. I try to avoid finer tracks in case a board needs to be reworked, when a thicker track is a bit more robust. I know some colleagues who aim for very wider tracks, i.e. 1mm or more, wherever possible! People new to soldering tend to hold the iron on too long and 'lift' pads and tracks.
We like photo-resist a lot. We are aiming at schools and not the home lab. Many schools seem to have UV exposure systems. We feel we get very nice results, and have been through the learning process to get those results. We think they are consistent too, so we can make multiple, identical, double-sided boards. In the few experiments, student-made boards seemed pretty-good too.
Clearly, single sided photo-resist could give superb results, and still be pretty straightforward for schools.
So far, we have not put any holes in the boards to get alignment between sides. We make a 'pocket' of the two masks, and expose both sides simultaneously. Another colleague has used holes to align the masks when he uses a single sided UV box. He says he has regularly achieved 0.25mm alignment with some practice, but we haven't done that experiment yet.
A new UV box will need a bit of testing to calibrate it, but once the exposure time is known, they seem to be consistent for a while.
We discovered the right printers settings through trial and error:-(
Now we find a hatched fill, proper photo-mask film, and cleanliness overcomes most problems in the printing, exposure and etching chain. We did try ordinary film for weeks, but got patchy, inconsistent results. Then we were introduced to 'proper' PCB film, and instantly got much denser black and sharper edges. We don't post process the film. Since we switched to 'proper' film, use the right printer settings, and hatched fill, we get good masks (at 10mil/10mil) and PCBs.
Etching - yes, etching constrains resolution more than high resolution printers, so does cleanliness. Our experience with photo-resist technology is a spec of dirt during exposure is much more of a problem than almost anything else.
We started with ferric chloride, but that gave very poor results compared to 'clear' etchant, which we now use exclusively.
press-n-peel - I don't feel confident that I could get the two sides aligned. With photo-masks, side to side alignment is better than 0.2mm with a double sided UV box (based on microscope examination with a fine steel rule showing 0.5mm divisions). We need to do experiments with single sided UV exposure. One idea is to use 'press-in' 'plate through' 'tubes' to get alignment.
Drills & vias - we use Dremel-style PCB drill, carbide drills and a drill press. Even then, my colleague finds students break carbide drills "continuously", especially for drills under 0.8mm. My old eyes find it hard to drill small annular rings anyway. Using larger vias and pads also allows for slight misalignment between the two sides. So these are the constraints I obey for my boards :-)
Even with more experienced users, who could use thinner drills reliably, and single sided boards, I'd still favour my eye sight and big pads and vias :-)
Another friend, who is a technician at a college, has a nifty little CNC mill that is very popular in UK schools. I intend to do some experiments with that to drill our etched boards. He showed me how to mill and drill PCB's using the CNC mill. My young colleague Stephen has got very good at this for single sided boards. Stephen has made his own sensor boards, and motor controllers for his championship-winning robots (he is 16yo). PCB milling is a nice process for schools, but it takes some skill to set up. Resolution is only moderate, but good enough for some stuff.
SMD parts - IMHO 64pin LQFP in a toaster oven isn't as bad as 0603. It is almost the last part to be placed (the way I do it), and is the only one, so I find it less stressful or tiring than placing twenty 0603's. I use 0805 or 1206 SMD parts (or bigger) as much as practical. My colleague is experimenting with making cheap solder-paste stencils. We currently apply solder paste with a very fine syringe. This is quite slow, and skillfull. If he can crack solder-paste stencils, we think kids will be able to make boards more easily. Paul Gardiner (a local Electronics teacher and consultant) has been using solder paste, SMD and a toaster oven, for most of his 12yo to 16yo pupils for a couple of years, and he finds kids get very good at it, much better results than soldering with an iron, quicker, and finer.
Noise - how many bits of an ADC input are stable when it is tied to ground (or Vcc)? When continuously sampling the ADC, and concurrently doing other things, is the 'noise' on the ADC effected? For example, if the program briefly 'flashes' an LED, on a nearby digital output pin during an ADC sample, does that effect the ADC reading? If you put an oscilloscope on digital input pins next to the 'flashing' LED, does the digital input bounce around?
