Hi Matt - The problem with exposed Cu on pwb's is that many fluxes are not
active enough to breakdown the oxidized Cu. Since you have a HASL finish
I'll bet that you have an expose Cu/Sn intermetallic rather than just Cu -
oxidized Cu/Sn surfaces are practically nonsolderable which means the
solder joints that appear to be made on those surfaces are at best very
poor metallurgical bonds and that means poor reliability for most use
environments. Some good info can be found in "The Mechanics of Solder Alloy
Wetting & Spreading", ISBN 0-442-01752-9. Chapter 6. Your real issue is the
location of the "exposed Cu". The edge of pads and the sidewalls of traces
do not cause problems ((for many use environments) in comparison to the pad
location where leads will be present. Many assemblers have "exposed Cu" on
assembles that have no corrosion problems. Check the TechNet archives for a
reference someone gave a while back on a paper that detailed exposed copper
on assemblies in the telecommunications industry over a 30 year period.
Good Luck.

Dave Hillman
Rockwell Collins
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Matthew Sanders <[log in to unmask]> on 05/15/98 09:29:32 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to Matthew Sanders <[log in to unmask]>

To:   [log in to unmask]
cc:    (bcc: David D Hillman/CedarRapids/Collins/Rockwell)
Subject:  [TN] Non wetting surfaces (exposed Cu)




Hello,

I've been seeing boards from some fab vendors recently that have non
wetting of the HAL finish on copper surfaces. Now, according to IPC-6012
(3.5.4.6), if the non wetting is on non soldered surfaces, which is the
case with my boards, 1% of conductor surfaces can have exposed Cu for
class 3. If you count each of the holes we have twice (once for each
side), we have less than that percentage. I'm still not very comfortable
with this, though, so I was hoping someone could tell me the
ramifications of exposed Cu in finished assemblies; it seems like it'd
potentially be a reliability problem potentially. Can someone shed some
light on this?

Thanks,
Matt Sander

Matthew Sanders
PWB Procurement Engineer, Trimble Navigation Limited
[log in to unmask]
Phone: (408) 481-7817
Fax: (408) 481-8590

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