Hi Peter - I am a little late to the party but wanted to add on last chunk
of information. The TechNet crew gave you some very good possibilities:
organic contamination, solderability degradation or improper plating. IMHO,
the best "black pad" explanation and discussion is the following journal
article "The root cause of black pad failure of solder joints with
electroless nickel/immersion gold plating", authors Zeng,Stierman, Abbott,
Murtuza, JOM, June 2006, pp. 75-79.

Good Luck.

Dave Hillman
Rockwell Collins
[log in to unmask]



                                                                           
             Peter Wong                                                    
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                                                                   Subject 
             03/21/2007 03:17          [TN] Solderability problems on      
             PM                        through holes only.                 
                                                                           
                                                                           
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                Peter Wong                                                 
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Hi Technetters,



We are having a solderability problem confined to only through hole
locations where we need some help or advice.



We are soldering a no-clean SAC305 alloy to an ENIG finished board.  All
SMT solder joints (both sides) are fine.  During hand soldering in our
dedicated lead free area, we are having problems with solder wetting to
some of the annular rings and barrels of through hole locations, not all
holes.  The assembly is an 0.062" thick 4 layer 1"x1" board so it is not
thermally challenging.



We've built thousands of these in the past without this particular
problem.



The time lag between smt and through hole assembly is 1-2 days.



We've tried using a more aggressive flux such as OA flux with no
improvement.

The only way that we can get solder to wet is to mechanically scrape the
annular rings with a pick.



Has anyone seen this before?  Why would this only occur on some of the
through hole locations and not on smt pads?



We have isolated this to a specific lot code of boards and are waiting
for some advice from our board vendor as well.



Thanks

Peter




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