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From:
"Wenger, George M. [Contractor]" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wenger, George M. [Contractor]
Date:
Thu, 26 Sep 2013 15:47:31 -0500
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Gerry,

Like Doug's comment about Dave, I have the same general view that silver tarnish is a cosmetic issue only and should not impact test or reliability.  But I'll also use one of Doug's favorite comments "That Depends".  If the immersion silver surface finish on your boards is totally black then I would say you have a real problem and shouldn't use the boards.  Although I haven't seen the tarnish you  have I have to assume that the immersion silver doesn't have a "nice", "whitish" "silver" looking color but probably has a "Yellowish-brownish-black" tarnished appearance that you might find on silver plated utensils in your home that have sat around unprotected in a drawer waiting for use at some special occasion. We've used immersion silver on telecommunications products since 1996 and have never had any production yield, quality, or reliability issues associated with immersion silver or tarnish.  Many of our telecommunication products are outdoor products and they are deployed all over the world.  The only issues we've had with tarnish is every now and then an assembly manufacturer sees tarnish on an immersion silver boards and doesn't want to use them.  The tarnish issue typically occurs when the manufacturer left immersion silver boards open in an uncontrolled storage environment or someone in a stock room handled immersion silver boards with bare hands after just have an egg salad sandwich or hard boiled eggs for lunch.  These cases don't happen often but when they do we tell the manufacturer to run a trail and see if the boards solder okay.  In every case they were okay and used for production.  We've also had product returned, for an upgrade or a failure not associated with the immersion silver surface finish, from the field where it was deployed in uncontrolled environments in Asia and the immersion silver surface finish on the boards was "very" tarnished and our repair organization wanted to scrap the product and replace it with new rather than do a repair.  However, in all cases they had no problems with the upgrade or repair and sent the product back to field deployment.

Having lived with immersion silver surface finish for a long time I do believe that your issue is probably not do to immersion silver tarnish.  From your email "Our SMT machine was down for more than 3 days that went through the weekends and the boards were not completed", it sounds like you are not experiencing a soldering issue with tarnished board but have a testing issue with boards that may have went through first side assembly and then several days later went through second side assembly and you than had problems at test.  You may have a contamination issue on your test points due to the first assembly process (e.g., flux contamination) and since you see some immersion silver tarnish the natural tendency would be to blame the tarnish. The other possibility as Ted indicated could be a test probe issue.

Regards,
George
George M. Wenger
Failure Signature & Characterization Lab LLC
609 Cokesbury Road, High Bridge, NJ 08829
(908) 638-8771 Home (732) 309-8964 Mobile
E-mail: [log in to unmask]
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
Sent: Thursday, September 26, 2013 4:00 PM
To: [log in to unmask]
Subject: Re: [TN] Immersion Silver Board Finish Tarnishing

Gerry,
You are correct.  Both silver and silver oxide are conductive.  Since my esteemed colleague Dave is in Texas steering dragonboats right now, I will pass along his general view that silver tarnish is a cosmetic item only and does not impact reliability.  Of course, that may depend on how the tarnish is getting there in the first place, but in general, tarnish is not a functional issue.

Doug Pauls



From:   Gerry Gagnon <[log in to unmask]>
To:     <[log in to unmask]>
Date:   09/26/2013 02:56 PM
Subject:        Re: [TN] Immersion Silver Board Finish Tarnishing
Sent by:        TechNet <[log in to unmask]>



Hi Ted,
 
Not 100% sure, but I believe silver and its' oxide(s) are highly conductive.
 
I suspect the "oxide" Ernesto is seeing is not silver or its' oxide.
 
Just my $0.02.
 
Gerry
 
> Date: Thu, 26 Sep 2013 17:47:14 +0000
> From: [log in to unmask]
> Subject: Re: [TN] Immersion Silver Board Finish Tarnishing
> To: [log in to unmask]
> 
> Ernesto,
> 
> Look at the boards you are having issues with test. Is it possible the
ones you are having problems with went through the reflow process first?
> 
> I ask because with an immersion silver finish there is typically an 
> OSP
finish to help prevent the silver from tarnishing. So unless the boards went through half the SMT process you should have had no problems related to them being out on the shop floor.
> 
> If the boards are having issues at test and you are in fact hitting 
> test
points with silver finish on them I would suggest you change your test probes to a sharper point or, print paste on the test pads. After the SMT process the silver finish is going to tarnish creating interment connections during test. This would also be dependent on how long the boards have been in process i.e. rework. The longer the boards are in process prior to testing will increase the problem with interment contact with test points during test.
> 
> Ted T. 
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Rueda, Ernesto
> Sent: Thursday, September 26, 2013 12:30 PM
> To: [log in to unmask]
> Subject: [TN] Immersion Silver Board Finish Tarnishing
> 
> 
> Hi all,
> 
> I am seeking recommendations from the experts of this forum about
tarnishing of the PCB board finish with Immersion Silver.
> 
> Please enlighten me about the minimum thickness of silver metal on top
of the copper layer. 
> What is the maximum process time limitation to build these boards?
> 
> Our SMT machine was down for more than 3 days that went through the
weekends and the boards were not completed. 
> It is possible that varying temperature and humidity affected some of
the assemblies. 
> We have problems with testability at some test points because of the
tarnishing issues.
> 
> We are planning to send out some panels for plating thickness test and
contamination tests as well.
> Any provider that you may recommend?
> 
> 
> Thank you in advance.
> 
> Ernesto
> 
> 
> Ernesto V. Rueda   |   PELCO by Schneider Electric   |   SMT Process 
Engineer
> Phone: +1 559-292-1981 ext. 3848  |   Fax: +1 559-388-3781  |   Mobile: 
+1 559-333-3913
> Email: [log in to unmask]    |   Site: www.pelco.com | 
  Address: 3500 Pelco Way, Clovis, CA  93612-5699 USA
> P please consider the environment before printing this email
> 
> 
> 
> 
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