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February 2006

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Subject:
From:
Richard Kraszewski <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Richard Kraszewski <[log in to unmask]>
Date:
Mon, 20 Feb 2006 18:46:41 -0500
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I've posted a rather interesting image to Steven Gregory's website. You
can view it at the attached link:

http://stevezeva.homestead.com/files/QFPcrud.jpg

It shows what we sporadically see on QPF solder joints with certain
batches of boards from a PCB supplier. Other details are as follows:

*       Sn/Pb process
*       Always seems to occur at a certain QFP pad location of one
specific board on a 6 board panel.
*       Solder paste & stencil have pretty well been ruled out.
*       This QFP is soldered on the second side reflow operation.
*       Occasionally we'll see black Crud float to the top of these pads
after first side reflow.
*       If we print over this black crud & solder, sometimes we won't
see any issue with the solder joint, sometimes we'll see as the image
depicts, occasionally we'll cross section and see a thick black layer
beneath the lead in the solder joint but the fillet around the lead
looks fine.
*       Boards are HASL coated.  (Horizontally I suspect).
*       Depending whom you believe, this row of QFP leads is either in
the leading or trailing edge of the panel as it proceeds thru HASL.
*       Inspection of the boards prior to any reflow does not show any
anomalies with the suspect pad. 


My belief is that dross is being included in these pads beneath the good
HASL coating, and coming to the surface in our reflow operations. 

Any thoughts other thoughts on what this is and how to prove it would be
appreciated.


Rich K / KEDS
  

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