Hi Mickey,
Both of your quotes can be right, DEPENDING...
Everything else being equal you are likely to get the best results with the
pads on the PCB matching the pads on the BGA. Reducing the pad size, can
indeed increase solder joint height (=longer life) if you do not decrease the
solder volume, but you can also get more of a stress concentration geometry
(=shorter life). However, these differences will be small (they are more
pronounced for accelerated testing results than for product life in service).
What you actually will get will depend on solder volume, BGA weight, number
of solder balls, solder paste vs. flux only, SMD vs. NSMD, near-eutectic vs.
high-melt solder balls, etc.
There are much more significant parameters affecting solder joint
reliability, PBGA and uBGA construction being one of them, service
environment another.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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