In my papers on double sided reflow and Intrusive Soldering, I use the
formula:

Weight of component in grams
total pad to lead mating surface area (in inches)

and if the result is < or equal to 30 grams, the component will not fall off.

Last week, at the SMTA Int'l conference in San Jose, Bob Willis and I
compared further notes and experience on this.  We have both used components
that exceeded 30 grams/in sq but we are sticking with this number.  This
incorporates a lot of components including most fine pitch QFPs, TSOPs, as
well as 68 pin PLCC.  Trouble components are larger mass components and many
ceramics including some large tantalums.  So experiment with any component
first.
It also assumes that you have a smooth running reflow oven conveyor - no
"shake and bake".
Regards,
Phil Zarrow
ITM, Inc.
Durham, NH  USA
www.ITM-SMT.com

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