Hi Nancy -
Your question reminds me of a "concept" that was done - for a while - in the
early days of SMT.  People would place a discreet component (usually a
decoupling capacitor) under a PLCC>  There was no problem, per se, soldering
it, even the then common Convection/IR ovens (heat is conducted through the
substrate).  A lot of folks were patting themselves on the back for this
wonderful PCB real estate saving set-up - particularly memory module
assemblers.  What they soon found out was that, if there was a problem with
that inexpensive little discreet  - component failure, bad joint, etc -  they
had to remove an expensive PLCC to correct the problem.  In light of the
exponential "fun" of removing and replacing a BGA to get at that 0402, my
recommendation is DON'T DO IT!

Phil Zarrow
SMT Process Consultant
ITM, Inc.
Durham, NH  USA
www.ITM-SMT.com

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