RE>Soldermask Tented Vias 6/19/96
Kevin,
I don't think that exposed copper is a reliability issue. OSPs have been used for years without any problems to my knowledge. The October Project published a pretty good report a few years ago which verified Bare Copper as a viable surface finish. We at Mercury have been using this finish for 2.5 years without any problems. Customer acceptance, particularly in the military arena, can be a trickier issue.
Consider what was probably the primary reason to prohibit exposed copper on circuit boards: copper salts (corrosion by-products) tend to be colorful, while tin salts tend to be clear. Open circuit potential and other measures of corrosion susceptibility on tin and copper in realistic environments tend to show very few differences, however. Field failures that I analyzed in a previous life were normally because of tin or silver migration.
As far as the vias go, we design our soldermask apertures to cover a good part (but not all) of the via pad. Our apertures are 5 mils greater than the nominal finished via diameter, which leaves some exposed copper for test/debug purposes, but which inhibits solder slivers between adjacent vias during the wave soldering process.
Regards,
Greg Bartlett
Mercury Computer Systems
Chelmsford, MA
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Good Morning,
We usually require soldermask over bare copper and tented vias (less than
.020"). This typically means that the soldermask is dry film. If a tented
via is not required and liquid soldermask is used, it appears that the
soldermask is suspect to flake off near the via knee, leaving a small amount
of exposed copper.
Is liquid solder mask over bare copper compliant with vias when the plating
is eletroless nickel - immersion gold?
Does the plating type matter?
Is the suspect of exposed copper a non-issue?
Thank you in advance of any comments.
Kevin Thorson
Lockhead Martin
Eagan, Mn
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From: "Thorson, Kevin J @EAG" <[log in to unmask]>
To: "'IPC Technical Forum'" <[log in to unmask]>
Subject: Soldermask Tented Vias
Date: Wed, 19 Jun 96 08:48:00 CDT
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