Peter,

To answer you questions directly::

Q:  what material do I use as the barrier.
A; Try CDM

Q:  what minimum material thickness do I need to prevent material
warpage over repeated heat cycles.
A: about 8mm ot thicker.

Q:  what minimum opening size is acceptable to allow solder to flow
up to the circuit board.
A: depends on components sizes and surounding components

Q:  would using the chip wave help?
A:  Always


KHFC-HK