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January 2001

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Subject:
From:
"Wenger, George M (George)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 26 Jan 2001 06:04:58 -0500
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Ingemar,
Our board vendors know not to send us IAg boards if there are "exposed
copper pads" (i.e., features not plated).  If they happen to miss them in
their inspection (accidentially or not)our operators at stencil printing
reject boards that have exposed copper pads.

We don't use lots of ENIG boards so we have little experience with ENIG
solderability.  I think it is safe to say that if a copper pad changes color
when ENIG plated the copper pad should be solderable. However, once plated
with ENIG you can no longer solder to the copper, you must solder to the
nickel.  Our limited experience indicates that nickel easily passivates and
if it does it solderability degrades.  One experiment we did was to
Temperature/Humidity condition ENIG and IAg boards and then do double sided
surface mount solder assembly.  The ENIG boards had solder defects and the
IAg didn't.  The 4 microinches of IAg will not stop diffusion of the
underlying material any more than 5 microinches of IAu on ENIG boards.
However, the under lying material on IAg boards is copper but for ENIG
boards it is nickel.  The flux we typically use were designed to work with
copper oxide but they don't touch nickel oxide.

Regards,

George
George M. Wenger DMTS
Bell Laboratories Princeton, Supply Network Solutions
Engineering Research Center FMA / AQA / RCA Lab
(609) 639-2769 (Office); 3210 (Lab); 2343 (Fax)
[log in to unmask]


-----Original Message-----
From: Ingemar Hernefjord (EMW)
[mailto:[log in to unmask]]
Sent: Friday, January 26, 2001 4:22 AM
To: [log in to unmask]
Subject: Re: [TN] Silver vs OSP surface finish


Interesting George,
is that a std among board makers or do you have to imply that method in your
own Incoming Inspection? In the later example it's too late, the boards are
already delivered, so I assume it's something one has to perform immediately
before final plating. My question: is there a corresponding inspection
method for judging nickel solderability before gold immersion is put on? I
talk for ENIG of course. There are many soaps that react with nickel but
using a std method would be best. Wetting balance on test coupons will
probably not tell the whole truth.
Ingemar Hernefjord
Ericsson Microwave System


The primary advantage of IAg is that it changes the color of the copper pad.
OSP doesn't change the color of the copper pad.  We use IAg not as a
solderable surface finish; we use IAg as a visual inspection aid to know
that the underlying copper is solderable (i.e., if the copper changes color
because the silver plated on it we know that the board fabrication process
didn't leave any residue such as solder mask or incomplete tin strip on the
pad that won't show up until you commit expensive components at assembly.

Regards,
George
George M. Wenger, DMTS Bell Laboratories Princeton
Supply Network Solutions
PO Box 900, Princeton NJ 08542-0900
Route 569 Carter Rd., Hopewell, NJ 08525
(609)-639-2769 (Office), 3210 (Lab), 2343 (Fax)
[log in to unmask]


-----Original Message-----
From: Michael Forrester [mailto:[log in to unmask]]
Sent: Wednesday, January 24, 2001 10:51 AM
To: [log in to unmask]
Subject: [TN] Silver vs OSP surface finish


What are the advantages of using Silver (Alpha level) Vs OSP (Entek) as a
surface finish?  Since Alpha level
has an OSP over the silver why bother with the silver?  Wouldn't less
interfaces
between the component
and trace be better?  Thank you.

Best Regards,

Michael Forrester
LeCroy Corp.

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