Jim & Syed,
I agree with Jim's findings. Other temp. cycle tests showed similar results.
Actually, in quite a few tests, cycles to failure for the bottom side PBGAs
(those that were reflowed twice), were higher because of the higher stand-off
provided during the second reflow. This holds true, in general, as long as the
joint remains "spherical" or column-like. When the component weight is large
enough for the joint to take an hour-glass shape, reliability may get
degraded. The key parameter to control for this is the "component weight on a
per joint basis" (unfortunately, I can't quote a universal limit for this).

Things also get worse when BGAs are mounted back-to-back because, locally, the
board cannot flex anymore. That is, the compliance of the assembly, which
helps relieve solder joint stresses, is gone. But that's another story and
fortunately, we don't see too many of these back-to-back PBGA assemblies.
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
(last updated on April 23, 1998)

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