Paul -

Once upon a time in a previous life I had the occasion to be involved in a
similar effort.  We learned a bunch of stuff that does and doesn't work and
ultimately developed two approaches that were worthwhile:

1] Riser wires soldered, sequentially into the boards, with the boards being
spaced about .06" inches apart.  The separation distance allowed sufficient
heat dissipation to prevent reflow or melt of the preceeding solder
connections.  TCE of the spacers used to mount the boards needs to be a
reasonable match to that of the riser wires to prevent setting up a solder
fatigue risk.  We reached a maximum stack up of about 6 boards/cables on this
application;

2] Installation of "cage jacks" in plated thru holes in the boards, followed
by installation of riser wires (non-soldered) at time of board to board
assembly.  Riser wires and contact areas of cage jacks were Au plated.  Cage
jacks were supplied on headers which also served to prevent entry of
solvents/flux/etc., during processing.  We reached a maximum stack up of 10
boards on this approach.

Both of these design approaches were ultimately space qualified for USAF and
NASA applications.

Regards - Kelly

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