Discoloration of copper plating under solder mask is not cause for
rejection. If it is indeed corrosion I would believe the solder mask would
be peeling off in those areas. If you are only seeing the discoloration
after washing I would suspect poor cure of the mask, allowing moisture to
penetrate the mask contacting copper surface.

One point of clarification from your email, you mention corrosion on traces
and plating, and then mention seeing it under mask. The corrosion you are
seeing, are these exposed (not covered with mask) traces? The traces under
the mask, these are bare copper correct?

Franklin

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kevin Glidden
Sent: Friday, March 10, 2006 8:53 AM
To: [log in to unmask]
Subject: [TN] IPC spec for plating corrosion?

Hi all,

We currently process flex circtuits with immersion tin finish.  The
thickness is specfied on the pcb drawing as 4 millionths minimum.

We were seeing PCBs come from our supplier with corrosion on the traces and
plating.  We screened for this, and allowed those without corrosion through
the two-sided SMT process.  After washing, we see corrosion on the traces
UNDER the solder mask.  I have photos if anyone would like.

A few questions..

1) Is there an IPC spec that deals with platings and corrosion?  I bought
IPC-A-600G, but I don't see what I am looking for in there.

2) Shouldn't the soldermask be preventing air & moisture from getting to the
traces?

3) I have read some old Technet postings on immersion tin, and it seems it
has known issues with corrosion, shelf life, etc.  True?  Can someone
summarize?

Thanks!


Kevin Glidden



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