In a message dated 98-10-21 11:31:18 EDT, you write:

<<  The questions I have are related to back-to-back BGA assembly on
      double sided surface mount designs.  A common application is reverse-
      pinned memory devices, i.e. which lets both parts share similar
      signals (data line, address line, etc.).  Specifically,
       1.   Are the BGA's commonly mounted with their pads exactly mirror
      imaged?
       2.   Can they share common vias?
      3.   Are there any known reliability issues with implementing directly
      mirrored pad configurations?
      Bruce Tostevin >>
Bruce,
For BGAs mounted back to  back on organic substrates, the fatigue life of
solder joints gets reduced by a factor 2 to 3 X, compared to the reliability
of "single-sided" assemblies.  Problem is the same with Chip Scale Packages.
Whether this is an issue depends on use conditions.

The reason is that board flexure under the "single-sided" component is what
provides most of the compliance of the assembly. With a "back-to-back" design,
the board area under the components cannot flex anymore. This is suppported by
thermal cycling data and several life prediction models.

Jean-Paul
 _____________________________________________________
Jean-Paul Clech
EPSI Inc., P. O. Box 1522, Montclair, NJ 07042, USA
tel: +1 (973)746-3796, fax: +1 (973)655-0815
home page: http://members.aol.com/Epsiinc1/index.html

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