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.

[SparkyLabs]

Boards are stencil pasted before part placement. the paste indeed will hold parts in place which is why I'd rather move the board around than the PnP head.

Large items that there are not many of I would still put on by hand. I'm trying to speed up the fiddly small parts that there are lots of and not try to do the easy big ones that would be near impossible to pick up and move like large electrolytic's as I can put them on faster by hand anyway