This problem has been seen in pcb fabrication for years and years, and is
known under several names, the most common of which is probably
"mousebites." Unfortunately, I believe assembly also has a defect known by
the same name. Anyway, the defect is not due to resist breakdown. Rather,
during the copper plating process, bubbles attached to the edge of the
dryfilm, preventing plating from occurring. When the boards come out of the
copper tank and then progress to solder or tin plating, the traces resume
plating and are protected during subsequent etching. I am sure that if the
traces were cross sectioned, you would find that the copper was 1/2 ounce
thick at the bottom of the pit, which is typically the starting thickness
of the copper foil. What causes it? Too many small/micro-bubbles in the
plating tank, poor cleaning prior to plating, substandard dryfilm
developing... many people have suggested different failure modes. My vote
is less than ideal developing followed by marginal preplate cleaning. Like
beer in a glass, nucleation sites on the trace edges cause bubbles to
form/gather at those locations. One of your pictures shows the regular
spacing of the pits. This is sometimes a common observation and it can be
seen that the pits repeat with depressions in the starting copper foil due
to the laminate weave. Sometimes you will see that the pits all line up at
the same place on trace after trace after trace, as the weave transverses a
section of traces.

In any case, the result is ugly. The copper is half thickness at the
defect. Who knows whether or not it will affect the functionality of the
product, since numerous designs have half ounce copper on inner signals. I
would be more concerned if the board was high speed and impedance
controlled. But that is not for me to decide, it is between the fabricator
and the end-user.

As a fabricator, our position is: spec or no spec, critical or not, we
don't like ugly and would reject internally if we see this defect. But then
again, I have never presumed to think that my business philosophy might
represent an industry standard!





At 11:33 AM 10/6/00 -0400, you wrote:
>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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