Alcon: Mel P. is correct that the "bigger the blob" tests conducted by the
Air Force failed to define superior solder connection.  A large solder
connection DOES last longer than a lean connection (all stresses being
equal). The reason it lasts longer is not because it is more tolerant of
stress but because it takes a fracture/crack a longer period of time to
completely penetrate the (larger) solder mass (crack propagation rate being
constant for a given stress).
- AN OPINION: IMHO (for whatever that is worth) the down side of a bulbous
solder connection is more insidious. A "normal" solder connection on a chip
component (as depicted by Fig. 12-18 on page 12-16 of IPC-A-610) functions as
a compliant lap joint with stress being relieved by movement of the solder
beneath the on both ends of the  component.  In a bulbous solder connection
(as depicted by Figs. 12-25 & 12-26 on page 12-19 of IPC-A-610) the
connection is not "compliant". If there is a "normal" solder connection on
one end of a chip and a bulbous connection on the other end, the end with the
normal connection will absorb all of the stress generated by Tce
differentials because the bulbous joint (being noncompliant) "traps" the end
of the component so that the component body and the pwa move together as a
unit during temperature change.  If both ends have a bulbous connection I
believe that mechanical failure of the component can result with temperature
change. Comments?
Regards, Jim Moffitt

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