In my experience a stiffener is the way to go.

1. If you don't use a stiffener you'll risk a lot of failures in the outer
row of balls due to handing. With no stiffener any bending or shear loads
are taken by only the outer row of balls. You've also got the problem of
maintaining coplanarity between BGA and PWB pads during soldering.

2. Interesting point and I can see the logic. An insubstantial sliver of
polyimide will subject the joints to little in the way of loading when
subjected to temperature cycling. However If the particular package is
qualified for mounting on a rigid PWB then you should be able to stiffen the
polyimide with a piece of FR4 and achieve the same performance.

Choice of material depends upon the application. However I would start by
considering ~0.50mm thick FR4.....



> 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.


Eric Christison
Mechanical Engineer
STMicrolectronics
33 Pinkhill
Edinburgh
EH12 7BF UK

Tel 44 (0) 131 336 6165
Fax 44 (0) 131 336 6001

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