Hi Kelly,
You are mixing totally different animals together,.
You do not say when these opens occur. SOT23s have Alloy 42 leads and
internal paddles. As a consequence they have radically different CTEs in the
three different directions: ~7 ppm/C lead-tip-to-lead-tip on opposing sides,
~11ppm/C on the same side, ~28 ppm/C vertically. You definitely do not want
your soldering pads run under the SOT body allowing the space between PCB and
body to be filled with solder. You do however want the SOT gullwing lead feet
enveloped with solder, because you are likely not get a good metallurgical
bond between the Alloy 42 and the solder [pry one of thewse SOT's from the
PCB, you will see the solder joints will separate from the lead
interfacially]. So pad size differences top to bottom are not the problem,
your problem is likely inadequate heat for good wetting [which includes the
formation of IMCs].
Now, 0402s up thru 1206s are a different story; here pad size can make a
great deal of difference in terms of tombstoning, wetting, and reliability.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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