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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