The final design will use the 100-pins Hirose FX8-100 connector. The PDF contains these pictures, to give you a rough idea :
The stacking height is only 3mm between two boards. Actually it will vary because the PCB wil bend ;) but pairs of PCB can be easily mated and moved up/down the stack as needed, that's important for modularity and repairs.
Why this connector in particular ? Well, in another life I was a ETX PC/COM format enthusiast and by chance I got a half-reel of FX8-100S-SV from eBay (this can happen why you buy a box with "various parts" that the seller has no idea what they are or why they are useful and can't list what they sell... you're never safe from a good surprise !)
Since this connector is used for the ETX COM format, availability is rather good. It has a lot of pins and the stacking height is about the thickness of SMD parts such as SOIC ICs.
Now let's see the little issues :
- The pin pitch is .6mm, it's not extremely thin, but thin enough to prevent wires from crossing from one end to the other with a standard 150µ etching rule. 150µ wire width and spacing is ok to run 2 wires in .6mm but there is no room left for the vias. The approach today is simple : use 2 contacts for one signal, so only 50 signals are available. This increases the number of required connectors... Maybe 3 or 4 are necessary.
- I only have S-type connectors (a few hundreds). The complementary P-type is harder to get. I'll be investigating in the coming months, to find decent prices (sub-$) in medium volume. It seems that compatible connectors are made by another manufacturer, that could be a clue...
When I have enough male/female pairs, I'll start designing a support board and generic flexible "pages" :-)