Microcontroller layout on the KU+ test board.
This is a STM32L431 in the UFBGA100 package (500 μm pitch).
It's not technically compatible with OSHPark rules (125 μm trace/space) as you need 100 μm trace/space to fan out the second ring of balls, but it's for such a short distance that I think there's a good chance I'll get away with it. This is a quick and dirty prototype and I only need one of the three boards to be usable anyway. And if the MCU doesn't work at all, the board will still be usable, it's just a bonus.
I'm not sure how you're *supposed* to use this without microvias since the nominal land size is 285 μm on 500 μm centers. That leaves only 215 μm routing channel width, enough for a single 71 μm trace with 71 μm clearance.