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July 2006

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From:
Kevin Glidden <[log in to unmask]>
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Date:
Mon, 10 Jul 2006 09:38:40 -0400
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Question for the reliability guys...

We use copper core PCBs, typically with an .060" core.  There is some
discussion on whether these edges should be left as copper OR finish plated.
We use two finishes: ImAg and ImSn.  We specify the edges to be plated, but
the PCB vendor has requested to leave them as copper.  To plate the edges,
the PCBs must be depanelized, which creates excess handling and manual
processing for the vendor.

From a reliability standpoint, there are pros and cons.  The silver and tin
could be subject to electro-migration under temperature and humidity cycling
conditions (all of our products are for avionics and therefore receive ESS
testing and also an avionics environment in the end use).  As for corrosion,
none are safe: copper, tin, or silver, though the silver is most susceptible
I believe?  I was even forwarded a nice paper entitled "Galvanic
Compatibility of Immersion Gold and Immersion Silver Printed Wiring Board
Finishes with Aluminum, by none other than David Hillman and Matt Hamand at
Rockwell...  If I understand it correctly, looking at the Galvanic Series
chart, tin, copper, and tin-lead (solder) are all about the same, and
slightly closer to Aluminum (likely chassis material) than silver.  This
would suggest that leaving the copper edges vs silver plated may be
better???

All the products are conformally coated with type UR coating.  But it has
been pointed out many times in this forum that given enough time, all
conformal coatings will allow moisture through...

Thoughts / opinions?

As always, thanks in advance!



Kevin Glidden
Manufacturing Engineer
Luminescent Systems Inc.



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