Steve,
As others have mentioned, these defects are mouse bites or plating pits.
They are cause by air bubbles attached to the copper surface in the copper
plating bath.  The air bubbles form because of air supersaturation in the
bath.  The most likely cause of supersaturation is air bubbles being sucked
into the filter pump.  Three key features in your pictures verify that
these are plating pits:
   Circular defects in plated copper only.
   Pits are at the edge of the line.
   The pits tend to be more prevalent on one side of the line.  The
   photoresist on the upper edge of the line (as it is positioned in the
   copper plating bath) tends to hold the bubbles in place.   The resist
   acts as a ledge.

The pits do not extent to the epoxy and therefore would not be detected by
electrical testing.  AOI would have found them, but not everyone AOIs
outerlayer boards.

I've included links to two papers that describe the defect and possible
solutions.

http://www.dupont.com/pcm/techinfo/plating/plating12.html

http://www.dupont.com/pcm/techinfo/plating/plating4.html


Regards,

Sidney Cox
DuPont Printed Circuit Materials





"Stephen R. Gregory" <[log in to unmask]> on 10/06/2000 11:33:54 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
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To:   [log in to unmask]
cc:    (bcc: G Sidney Cox/AE/DuPont)
Subject:  [TN] Etched Pits in Traces...




Hi Ya'll!!
It's picture time again! Got some boards in at incoming inspection that are
really weird looking Go to:
http://www.driveway.com/share?sid=e25a88c4.8e904&name=Pictures
Look at Pits.jpg and Pits2.jpg...very, very strange! Never have seen
something like
this before...and it's only on the backside of the board? What could cause
this?
Looking in the IPC-A-600F, it only talks about reductions in conductor
width
of more than 20% (for class-2), but doesn't talk about reductions in
thickness by pits such as these...it's hard to tell if they've gone all the
way through the trace because the pits are filled with solder mask and you
can't see all the way down to the bottom of the pits.
Common sense tells me that these boards are rejectable...am I right?
-Steve Gregory-
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