Let's say you're mounting a BGA to a flex.....

1.   Will the use of a local stiffener behind the BGA increase yields
through reflow or improve the solderabilty of the BGA to the flex by
preventing the substrate from warping?

2.   What would be the effect of this stiffener on the BGA joint
reliability over thermal cycling, thermal soaking, etc? CTE differential is
obviously a concern here. One Technetter that I already talked to said that
in his experience the use of stiffeners for BGA's on flex decreased the
mean time to failure significantly. Does anyone else have experience with
this? What type of stiffener do you recommend?

After looking through the archives and IPC-7095, I wasn't able to find
anything that addressed this issue directly.

Thanks for the help,
Chris

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