Hi David,
What happens with all chip components and large pads is, that most of the
solder goes for the fillets and very little winds up between the chip and the PCB
pad. Thus, the stand-off height, one of the prime reliability parameters, is
very small and you get crack initiation underneath the chip very early. What
the fillets do in this case is provide functional life because of the larger
volume through which the cracks need to propagate for functional failure. Out in
the fillets the cyclic strains are smaller than underneath the chip, because
of a much larger 'effective' solder joint thickness.
However, you would be much better of if the solder were underneath the chip
instead of the in the fillet, thus preventing any premature crack initiation in
the first place.

Werner

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