Jim,

     About a year ago I investigated the same issue within our stock. I
     firtst identified the problem in our Assembly.  I kept noticing lands
     not soldering, even when using an OA flus and paste.  Reworking was
     either impossible or the problem was hidden.  You will get enough
     solder on there to mask the defect, but it eventually comes back to
     plague you in CRGs.

     You should first determine, what is the date code on the fabs?  Not
     that boards become un-solderable after 366 days, but depending on your
     storage conditions and the age of the fabs will provide you an idea to
     the degree of intermetallics.  If the fabs are close or greater than
     one year, then you probably have a condition of intermetallics (IMC),
     as you describe "where the SnPb is thin."  This condition will be more
     prevalent at those locations.

     Through repeatitive testing, I determine that IMC is very noticeable
     when reworking our fabs.  Using Nitric Acid I SnPb stripped 10 samples
     that were approx. 12 months to 24 months old.  In a converorized line,
     the speed and solution pressure had not yet been determine.  Following
     stripping the samples I could see the effects of IMC by the color of
     the underlying copper.  A majority of the fab surface represented a
     clean copper surface, typically what you would see after microetch.
     To prove the theory further, I HASLed the stripped samples and found
     the same defective lands un-solderable.  This occured in several
     locations within each fab, approx 15% affected (the fabs are 9U cards,
     typ. 8.5x14 90% SMT).

     At the same time perform a solderability sample per J-STD-003.  Hope
     this helps.

     John Gulley
     Inet Inc
     "A Smarter Vision"
     972-578-3928



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Subject: [TN] FAB: BOARD SOLDERABILITY
Author:  Jim Marsico 516-595-5879 <[log in to unmask]> at Internet