RE>Re[5]: Soldermask Tented Vias 6/20/96
I would be very careful with this report, if I were you. I was in DEC during the time that this work was done and was in a position to review it carefully. While I strongly AGREE with the results of this work (that Bare Copper does not present an added reliability risk and can actually be more reliable) and that Battelle does a great job with Flowing Mix Gas tests, I disagree with how this test was applied. They took a test environment/procedure that was originally designed to stress gold-plated connector mating surfaces and tried to run an SIR degradation test in it. In my opinion, this "test" didn't really show anything conclusive.
Greg Bartlett
Mercury Computer Systems
Chelmsford, MA
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Mike - do you have a title, report number, etc that I could have our
info center chase down a copy of the Battelle Labs report for me?
Dave Hillman
Rockwell Collins
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Subject: RE: Re[3]: Soldermask Tented Vias
Author: [log in to unmask] at ccmgw1
Date: 6/20/96 9:43 AM
I also have a copy of this report. It addresses metal migration of fine line
circuits under bias on FR4 and Polyimide. Major questions were raised
concering the use of polyimide PWBs. So, if you have FR4 substrates, bare
copper via edges or bare copper untented vias are probably OK. If the solder
mask covers the bare copper -- great. The level of residual ionic
contamination is critical (according to the Batelle report) to the
reliability of fine line circuits. We have a design with untented bare copper
vias in the preproduction phase now.
Mike Pisansky
Unisys
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From: TechNet-request
To: TechNet
Subject: Re[3]: Soldermask Tented Vias
Date: Wednesday, June 19, 1996 5:23PM
Yes!!!
I have a copy of a report from Batelle Labs (study commisioned by DEC)that
concludes tin/lead is more problematic than bare copper from a corrosion
standpoint. If any one out there has any data to support the negative
position on exposed copper, I'd love to see it posted here.
Dennis Mitchell
Zycon
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Subject: Re[2]: Soldermask Tented Vias
Author: [log in to unmask] at corp
Date: 6/19/96 3:53 PM
Rodger -
I vote for your suggestion! If you look at an assembly with solder
mask closely, one will find that the sidewalls of many surface
features are bare copper. I think that the electronics industry in
general has quite a bit of exposed copper functioning in the field
with few problems. The exposed copper issues I have been working seems
to be a "culture" issue (military product primarily) - the standard
question has been "won't it corrode if the copper is exposed?. There
will be design and use environment specific cases where bare copper
will be a problem but overall there are gains to be realized. Using
bare copper would mean that the printed wiring board would see one
less thermal excursion (no HASL or Refuse operation) which can only be
a good thing from a reliability standpoint. Lee Parker of AT&T gave me
a paper on the corrosion of exposed copper in several environments
over a 30 year period - if I can find that paper I'll post it here on
TechNet. Well TechNet - anyone else in support of bare copper?
Dave Hillman
Rockwell Collins
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Subject: Re: Soldermask Tented Vias
Author: [log in to unmask] at ccmgw1
Date: 6/19/96 10:05 AM
I don't know much about your soldermask issue, but I would like to see
some discussion of the exposed copper issue.
We have felt for a long time that any exposed copper was unacceptable
but with the industry switching over to OSP more and more, we have to
accept a limited amount of copper exposure (when SMT pads don't get
paste). If you are making SMT-only boards which don't flow across a
wave solder then the amount of exposed copper goes up tremendously. I
recently talked to people at a company which is very prominent in the
electronics industry and has been allowing a large amount of exposed
copper on their PCB assemblies (look inside your PC and see how much
exposed copper you can find). They have done a significant amount of
temperature and life testing and have seen no problems due to exposed
copper.
If this is true, isn't it time to stop worrying about exposed copper
and start making boards and assemblies cheaper?
Regards,
Roger Held
Hitachi Computer Products (America), Inc.
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Subject: Soldermask Tented Vias
Author: [log in to unmask] at Internet-HICAM-OK
Date: 6/19/96 8:48 AM
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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