I'm struggling with an exposed copper/reliability issue with Class 3 (high
reliability) boards.  We have a group of boards with some degree of Solder
Mask blistering, no Electroless Nickel/Immersion Gold plating in those
areas, which will result in exposed copper after assembly for the life of
the part.  The effected areas are typically the junction of trace to pad or
the trace between SMT component pads. There are two concerns with this
condition: potential reduction of solder volume by increased copper area and
long term reliability with exposed copper.  I have been able to demonstrate
that the degree of solder volume reduction is sufficiently small so as not
to affect solderability.  However, I do not have reliability data that says
X amount of exposed copper will be acceptable or unacceptable.

Can you guys help?  Where can I find a/some studies regarding long term
reliability of exposed copper?  Are there any papers available?  Is such
data available with respect to different line widths and spacing or
different end use environments?  Our geometries are "fine line" and the
application is "high reliability", but in a benign environment.  Your
thoughts or references in this matter would be greatly appreciated.
Best Regards,
Jana Carraway
503-579-8595

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