In a message dated 97-08-16 05:05:31 EDT, Yuan Li writes:
<< Any restrictions on putting components on the opposite side of the BGA
      site? My opinion is NO. >>
Yuan,
Solder joint reliability of leadless assemblies gets degraded significantly
when components are mounted back to back. I have seen LCCC test data that
showed this effect very clearly (LCCCs were mounted back to back). In a
single sided assembly, the board can flex during thermal cycling, thus
providing compliance to the assembly and some stress/strain relief. However,
when similar components are mounted back to back, the board cannot flex. The
assembly stiffness increases by a very large amount and solder joints see
much higher stresses and  strains.
Whether this matters depends on your application, BGA and die size, CTE
mismatch between board and BGA, use conditions and expected design life.
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
For SMT / BGA / CSP reliability info, go to:
 Home page:
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(last updated on June 28, 1997).
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