The plot thickens… and I’m pulling my hairs out along the way.
The good news is that I have assembled 6 boards now, and they all behave in (nearly) the same way. The on-board ATmega is working and correctly running the blink test sketch installed when setting up the boot loader via ISP.
So the basics work. The voltage regulator is probably also ok.
The bad news is that none of these 6 boards is showing up as USB device. Whoops.
I’ve gone through a couple of build techniques, from reflow to hand-soldering, as well as various combinations. But after some obvious solder bridge removals and a few loose connections after hand-soldering, at least 5 of these boards exhibit identical behavior. I’m either getting it right or consistently doing something wrong.
Also found a problem in the PCB traces, i.e. a trace between 3.3V power and the 3.3V pins on all 4 ports is missing. But this is relatively minor – a tiny extra wire should fix that.
I’m at the point where I really need to step back to re-think what else could possibly be wrong. It must be some silly / stupid oversight… I’m still hopeful that the aha-slap-on-the-head-erlebnis will come soon.
Meanwhile, I’m also exploring some alternatives. Today, I visited a nearby “Fab Lab”, which has laser cutters, CNC routers, 3D-printers, and more such mind-blowing geek stuff. Absolutely fascinating – I’ll never look at an object in the same way again. Any object.
One of the things I wanted to try out is to create SMD stencils like the big boys do. A first trial on cardboard, paper, and 0.5mm polystrol came out as follows:
The layer on top was my first attempt, using thin cardboard. As you can see, it’s not quite perfect since the holes are too large – there is no vector adjustment for the 0.1mm laser beam overhead. Then again, the plain paper example in the middle shows decent separation, and having paste overlap is not necessarily a problem.
I’ll try some more adjustments next time and will get a bottle of paste with squeegee to play around with this stuff. Who knows, maybe even a 1-time use plain paper sheet can work?