If I was a betting man, I would bet that your opens are due
          to plated through hole failures.  Most rigid flex stackups
          heavily depend on special adhesive materials that enable the
          polyimide material to bond to the FR-4 laminate.

          One unfortunate side effect of this is the fact that these
          adhesives have an extremely high Z-Axis expansion of the
          PTH during thermal excursions such as assembly, particularly
          IR for SMT assembly.  Conventional Rigid flex materials and
          technology does not mix well with SMT assembly processes.

          This problem is much worse if you are using small vias (i.e.
          much less than 30 mils diameter).  If you have an SMT design
          I'd bet you are using small vias.

          You can solve this problem by using larger vias or using
          less adhesive in your stackup.

          If this is not possible you might go to some of the more
          advanced rigid flex technologies that are out there such as
          Regal Flex which has a clever way of avoiding this problem.

          Contact someone at Zycon or Automata and they can help you
          with Regal Flex.

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Problem:
We experience an unacceptable failure rate with open in rigid flex boards
AFTER assemby and thermal cycling.

Question: 
I feel there is a good chance the problem is in the assembly process.  What
kind of things should I look for?  The boards are mixed surface mount and
thru-hole, with much hand soldering and rework.

Thanks
George Franck