Ideally one would like a solder joint shape that is slightly hour-glass
shaped, because it provides the most uniform strain distribution (see papers
by Satoh, Hitachi; Engelmaier, Bell Labs).  That is however only obtainable
by 'stretching' the joints. Solder joint shape, except for height which is a
first-order parameter, is however only a second-order variable compared to
other parameters. More barrel-shaped solder joints experience somewhat
shorter lives because the thicker sections strain less therefore forcing more
strain onto the thinner sections, which also experience strain
concentrations, particularly for solder-mask defined (SMD) pads. The
difference between SMD and non-SMD (NSMD) solder joints is that NMSD solder
joints have fatigue lives between 1.3 to 3 times longer than SMD joints,
depending on the severity of loading (more difference for highly accelerated
tests) as long as everything else is the same (see papers by Mawer,
Motorola). 
Voids are primarily process indicators. There is experimental evidence that
voids retard crack propagation locally around the void on a temporary basis.
There is no standard, IPC or otherwise, on voids--nor should there be. See
also J-STD-013, Implementation of Ball Grid Array and Other High Density
Technology.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 201-543-2747
E-Mail: [log in to unmask]

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