Hi ehb,
Yes, I have seen the solder spread beyond the solder-mask opening; it depends
on the weight of the component. If you would reflow them upside-down, this
effect would diappear of course and you would 'stretch' the solder joint
somewhat. This would give you improved reliability because of the resulting
greater solder joint height as well as the better solder joint geometry. In
the really early days of BGAs (~1983!) we used rectangular soldering pads to
achieve better solder joint geometries. You may want to look at this early
work reported by a number of (now mostly ex-) Bell Labs authors.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]

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