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July 2006

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Subject:
From:
Colin McVean <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Colin McVean <[log in to unmask]>
Date:
Mon, 10 Jul 2006 14:44:58 +0100
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I don't quite understand what your PCB vendor is saying. The PCB does
not have to be de-panelised to have the edges plated. He should simply
create the profile at the same time as he creates the through holes,
leaving the panel on lugs, which are removed at the final profile stage,
then plate up either by panel plate, or by leaving apertures in the
primary image then pattern plating.

Either way, the core or plated edges will still be subject to whatever
solderable finish he will apply, be it Imm Ni/Au or ImmSn. I guess is he
simply profiling the panels at the end, leaving the bare copper core
exposed. I would not recommend this, as the copper will oxidise.

We manufacture plated edge PCB's all the time, and work with the
designer/customer to determine what works best with them. Usually we
make the boards with built-in "button-hole" lugs which do not need any
finishing of with a linisher even after assembly.

Colin McVean M.Inst.C.T.
Production Manager
Artetch Circuits Limited
Main: 01903 725365
DD:    01903 712926
Email: [log in to unmask]
www.artetch.co.uk
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kevin Glidden
Sent: 10 July 2006 14:39
To: [log in to unmask]
Subject: [TN] Edge plating on copper core PCBs

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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