Looking for an economical XY (well and Z) solution that is not for the normal use

[Doddy]

T8.= Trapezoidal screw - that's likely to bugger your positional accuracy requirement unless you introduce some anti-backlash mechanism. Stepper "42" - can't see that in the link, but could be related to the frame size that translates to a NEMA17 motor - it's just an indication of size and expected power range. Most common steppers are 200 steps/1.8 degree/step resolution. On a T8 that's 25 steps per mm or 40 micron - but you could pretend that micro-stepping offers higher resolution. Of course that assumes everything about the screw thread linearity / accuracy, as well as the frame geometry. Do you really need 100 micron accuracy? I don't think so (I'm pretty sure that working with 0402 components I'm not that accurate by hand).

How are you planning to feed your P&P - what's the component delivery mechanism? How are you going to achieve component alignment (and necessary re-alignment...particularly with vibration moving through the frame). Have you not thought of solder stencilling or solder-paste deposition as a "glue" for the components before placement?

I spent a few months in a clean room pick&place PCB assembly room when I was "a lad" - okay, quite a few years ago, but working incredibly compact boards (and high layer-count PCs) and the early generations of SMD devices. P&P with a less than heavy investment can go spectacularly wrong quickly.

I'd be interested to hear how you get on; unfortunately none of my projects get anywhere near complicated enough to make me desire such a machine myself.

Edit: reference to 0402 - I tried to prove I could... I much prefer 0603 for sanity, or 0805 to rent out as student digs :).
Surface tension of molten solder will draw components into alignment over distances substantially greater than your 100 micro accuracy.