Gary,
     In my past I have experienced similar "popcorning" of .020" pitch, 100 
     pin TQFP's (thin plastic). The original evaluations of the package 
     assembly process demonstrated excellent processing capabilities, then 
     when assembling components with die enclosed, we encountered 
     "tombstoning" of the QFP's. One corner was lifted. When looking 
     closely, the component appeared to be warped. Further investigations 
     (C-SAM) clearly demonstrated popcorning on the underside of the die. 
     
     The only resolution for the popcorning we found to work was to prebake 
     the parts prior to assembly. You are correct that there are some other 
     measures you can take to temporarily resolve the issue, but popcorning 
     is a long term field reliability issue. Methods like C-SAM and 
     Sonoscan are ways to non-destructively verify delamination. The 
     equipment is costly though.
     
     I have encountered other issues where the component characteristics 
     change over temperature. When exposed to cold environments (and 
     sometimes ambient), the component reset. When heated it returned to a 
     normal state. This appeared to be die related. This is probably 
     unlikely in your case, but it is one alternative. 
     
     Regards,
     Allen Hertz
     Racal-Datacom
     (954) 846-5829
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: ASSY: GEN: Popcorning during reflow
Author:  [log in to unmask] at ftl03
Date:    10/30/96 09:27 PM


Has anyone else observed the following?
     
We have a CMOS PQFP that was "popcorned" during reflow soldering.  Cross 
sectioning clearly shows that the die, along with its mounting epoxy, has 
separated from the leadframe by approximately .002" (2-mils).  There were 
no visible cracks in the plastic overmold material at 500X magnification.
     
The problem is...some parts don't fail until after a few thermal cycles 
of 0 to 50 degrees C during live circuit testing at-speed.  And...we can 
make them recover after failing by merely baking the entire board from
1.5 to 24 hours at 125 degrees C.  The baking fix is not permanent and the 
parts eventually fail again.  Some parts don't work initially but do work
after the above baking treatment (we discovered, quite by accident, that baking 
"fixes" the parts).
     
     
My question is...is the popcorning likely to be the only problem with
                 these parts?
     
Has anyone else been able to anneal a part so that it will recover from 
popcorning?  Doesn't sound likely to me!
     
Gary P.
---
                                  Gary D. Peterson
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