Hi John,
The reason tombstoning occurs at all is that the outer edges of the pads
protrude too much beyond the chip components--if you have no large solder
fillets you will not get tombstoning because the required surface tension
forces that cause tombstoning exist only with large fillets and unsymmetric
position. In reflow soldering these components there is really no need for
protruding pads (to be touched by the wave) and the resulting fillets at all.
These components are so small, that certainly there is no need for fillets
for reliability reasons. 
Some people also found success with narrower pads which diminish fillet size,
but the best way is to have pads that essentially are only underneath the
components-- if you really want fillets put them to the side of the
components.

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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