Exposed copper is not a defect per the commercial standards of IPC
(J-STD-001) nor was it a defect for later versions of the Military specs
(2000A).  We seldom considered exposed copper as a reduction in electrical
conductivity performance unless there was significant deformation of the
conductor or High Voltage applications. This is outside of the criteria of
board fab which is intended to consider the performance of solder mask to
prevent solder.
This allowance is not without some reservation that centers around increased
susceptibility of metal migration for exposed Cu surfaces. This phenomena is
also dependent on other factors such as cleanliness and moisture and the
like.  Consider that no-clean is in vogue today and boards tend to be less
clean possibly due to limited inexpensive solvent availability, and there is
a potential for concern for products that have consequential performance
parameters.

Mel Parrish
Soldering Technology International
Madison, AL
256 705 5530
256 705 5538 Fax
[log in to unmask]



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Jana L. Carraway
Sent: Friday, September 28, 2001 9:13 AM
To: [log in to unmask]
Subject: [TN] Exposed Copper Reliability Data


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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