Brian,
I am not so sure about bare copper not being a reliability issue. It would depend on the design
of the circuit. High frequencies, impedence, or current limitations could be impacted by even
small amounts of degradation to the much smaller circuit trace sizes and spacing. A little
oxidation and/or corrosion can be become a problem.
David A. Douthit
Manager
LoCan LLC
Brian Ellis wrote:
> Jana
>
> The first question to ask is why the solder mask is blistering. This is
> probably indicative of some form of contamination before it is applied.
> This contamination, if it is ionic, or partially so, may be an indicator
> that dendrite formation may occur under unfavourable circumstances. If
> so, the reliability may be reduced.
>
> IMHO, bare copper, if the circuit is clean, will not reduce reliability.
> After all, this finish was common in the past, before solder masks
> became common (pre-1970), with hand-soldered assemblies and no problems
> arose.
>
> Brian
>
> "Jana L. Carraway" wrote:
> >
> > 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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