Nancy, Phil
I have to disagree with Phil somewhat, if good precautions are taken.  If
Nancy's line operates on a quick enough basis that there isn't a huge WIP
build-up between assembly and test then go for it.  If there is (ideally) an
inline ICT machine to make sure the caps/resistors are good, then your
rework exposure is minimal.  If however the more usual batch build concepts
are applied (100+ WIP between build and test) then I agree with Phil's
warning.

Sometimes space limitations make it worthwhile to do these things - if this
is a laptop product then that extra space required can make the unusual
worth doing.  Somewhat like the stacked memory chips IBM used to use in
mainframe memory boards...

Good luck!
 - Graham Collins

-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Tuesday, March 02, 1999 8:34 PM
To: [log in to unmask]
Subject: Re: [TN] 0402's under BGA problems?


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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