Hi guys - a couple of material tidbits - silver oxide is as conductive as
elemental silver (one of the cool elemental quirks of silver) but the
tarnish you are seeing is typically either chloride and/or sulfide based.
Silver sulfide can impact contact resistance for some sensitive circuit
applications. But in most applications, this condition is cosmetic as the
discussion as detailed.  Here is a good paper on the topic:

IEEE TRANSACTIONS ON PARTS, Electrical MATERIALS, AND PACKAGING, VOL.
PMP-6, NO. 4, DECEMBER 1970 129
Characteristics of Contacts Contaminated With Silver Sulfide Film, GEORGE
J. RUSS.

Dave


On Fri, Jul 18, 2014 at 6:58 AM, Wenger, George M. [Contractor] <
[log in to unmask]> wrote:

> Hi Wayne,
>
> I agree that the tarnish is not a problem for ohmic contact.  My comment
> had to do with some customers perception that it would be a problem.
>
> 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]
> [log in to unmask]
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
> Sent: Friday, July 18, 2014 7:48 AM
> To: [log in to unmask]
> Subject: Re: [TN] Staining immersion silver coating on ground plane after
> assembling
>
> Hi George-
>
> I don't believe the tarnish is a problem with getting good ohmic contact.
> If it was, then the world would have lots of problems with all of the
> switches and relays in use today!
>
> Wayne
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
> [Contractor]
> Sent: Thursday, July 17, 2014 11:25 PM
> To: [log in to unmask]
> Subject: Re: [TN] Staining immersion silver coating on ground plane after
> assembling
>
> Hi Tuyen Tran,
>
> The stain I see in your photo looks like a typical mild silver sulfide
> tarnish.  Are the ground planes on the PCBA exposed (i.e., not covered by
> solder mask) because they will be contacted by some shielding or other
> parts that need to be at a ground potential?  Although I don’t think there
> would be any detrimental impact of the contact due to the tarnish, if you
> customer isn’t contacting to these ground planes I would think they might
> be convinced to accept the PCBAs as-is.
>
> 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]
> [log in to unmask]
>
> From: Tuyen Tran [mailto:[log in to unmask]]
> Sent: Thursday, July 17, 2014 11:16 PM
> To: Steve Gregory; Wenger, George M. [Contractor]
> Cc: TechNet E-Mail Forum
> Subject: RE: Staining immersion silver coating on ground plane after
> assembling
>
> Hi Steve
>
> Pls help to post a photograph of the stain on your site.
>
> Thanks in advance for your support.
> Tuyen Tran
>
> From: Steve Gregory [mailto:[log in to unmask]]
> Sent: Friday, July 18, 2014 2:21 AM
> To: Wenger, George M. [Contractor]
> Cc: TechNet E-Mail Forum; Tuyen Tran
> Subject: Re: Staining immersion silver coating on ground plane after
> assembling
>
> Hi George!
>
> Photo is here:
>
> http://stevezeva.homestead.com/Immersion_Silver_Tarnish.pdf
>
> Steve
>
> On Thu, Jul 17, 2014 at 12:35 PM, Wenger, George M. [Contractor] <
> [log in to unmask]<mailto:[log in to unmask]>> wrote:
> Hi Tran,
>
> If I've read your TN post correctly it sounds like you have some PCBAs
> that have "stains" and are concerned about sending these to a customer.  It
> also sounds like you didn't see the stains immediately after reflow but saw
> the stains on ground plane areas ten days after reflow.
>
> If you look on line at the paper we presented at the SurFin 2000
> Conference and read the four bullets on our title page you will see that we
> indicate silver does tarnish when exposed to pollutants.
>
> * Silver Migrates !
> * Silver Tarnishes !
> * Silver Sulfides !
> * So why would anyone use Immersion Silver?
>
> Despite what one might think about the paper title immersion silver has
> been our surface finish of choice since 1996 and we've never had a product
> failure since then that was due to the immersion silver surface finish.  I
> can't tell from your text just how "stained" the ground planes are but it
> might be helpful if you could forward a photograph of the stains to Steve
> Gregory and ask him to post it on his site.  I've attached a photograph of
> a stained RF product PCBA to this email which you and Steve will see
> because you are on the email copy to but IPC TN will strip off the photo so
> others won't see it unless Steve posts it on his site.  The photograph
> certainly shows a tarnished immersion silver surface finish.  This PCBA was
> deployed in Asia in a non-controlled environment and it was subsequently
> exposed to a five day Battelle Level III MFG and then put back into a
> telecommunication system and operated properly.  Yes the tarnish is a
> cosmetic issue but it should not have any detrimental effect on
> performance.  One of the reasons we use immersion silver is because unlike
> copper, silver oxide and silver tarnish is conductive, which is the reason
> we believe we haven't seen any degradation of RF performance.
>
> We were originally concerned that if we had tarnish the solderability
> might be degraded and we'd have problems soldering if we ever had to do
> subsequent component replacements or upgrades.  This has not been the case.
>  Our experience indicates that we can solder to almost any tarnished
> immersion silver surfaces.  If the tarnish gets extremely severe (i.e., it
> turns totally BLACK) we have seen solderability degradation.
>
> I can't give you a direct answer about shipping the product to your
> customer because I haven't seen how "stained" your PCBAs are and I don't
> know your customer.  There are ways of removing tarnish on immersion
> silver, however, our experience indicates unless one is able to effectively
> clean or remove any of the chemically used to remove the tarnish that even
> though the cleaning process removed the tarnish it left harmful chemical
> residues behind that impacted the long term reliability of the product so
> we don't try to remove tarnish on immersion silver PCBAs.
>
> You'll have to decide if you want to send your PCBAs to your customer.
>  However, my suggestion is moving forward you should evaluate what caused
> the stains and see if you can prevent them.  We've found some stains on
> reflowed PCBAs that were placed on mats on a work bench or laid on pieces
> of cardboard or other material that contained sulfur.  We've also found
> stains on PCBAs that were handled by operators who were not wearing when
> the PCBAs were being handled.  We even had a case of tarnish on immersion
> silver PCBAs that were on a wire rack next to a bench were an operator was
> peeling and eating hard boiled eggs.
>
> The immersion silver PCBs we receive from various PCB fabricators are
> always separated with Silver Saver paper, we avoid handling immersion
> silver boards with bare hands, we specify what kinds of materials should
> not be used to package PCBAs and we minimize exposure of immersion silver
> PCBAs to the environment until they are delivered to customers.
>
>
>
> Regards,
> George
> George M. Wenger
> Failure Signature & Characterization Lab LLC
> 609 Cokesbury Road, High Bridge, NJ 08829
> (908) 638-8771<tel:%28908%29%20638-8771> Home  (732) 309-8964<tel:%28732%29%20309-8964>
> Mobile E-mail [log in to unmask]<mailto:[log in to unmask]>
> [log in to unmask]<mailto:[log in to unmask]>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]<mailto:[log in to unmask]>] On Behalf
> Of Tuyen Tran
> Sent: Thursday, July 17, 2014 12:36 PM
> To: [log in to unmask]<mailto:[log in to unmask]>
> Subject: Re: [TN] Staining immersion silver coating on ground plane after
> assembling
>
> Wayne
>
> Yes, the staining on ground plane not affect 3F, because it were not
> soldered so it were stained during production time and looked so bad about
> cosmetic. Can you advise how to treat of stain? We not confident to
> delivery to customer these boards.
>
> Thanks
> Tuyen Tran
>
> ________________________________________
> From: TechNet [[log in to unmask]<mailto:[log in to unmask]>] On Behalf Of
> Wayne Thayer [[log in to unmask]<mailto:[log in to unmask]>]
> Sent: Thursday, July 17, 2014 10:57 PM
> To: [log in to unmask]<mailto:[log in to unmask]>
> Subject: Re: [TN] Staining immersion silver coating on ground plane after
> assembling
>
> What's to solve?
>
> Apparently the boards soldered OK, so it sounds like you are only talking
> about cosmetics. No long term reliability problem.
>
> Talking with the PCB supplier about getting an anti-tarnish applied over
> the ImAg will help with the cosmetics.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]<mailto:[log in to unmask]>] On Behalf
> Of Tuyen Tran
> Sent: Thursday, July 17, 2014 11:42 AM
> To: [log in to unmask]<mailto:[log in to unmask]>
> Subject: [TN] Staining immersion silver coating on ground plane after
> assembling
>
> Hi Everyone
> We received some PCBs where the finish surface was good, but after we ran
> reflow about ten days, the ground plane with immersion silver coating that
> were not covered by solder paste look to be stained on surface.
> We known the chemical silver [Immersion Silver finish] surface is very
> sensitive to surrounding condition, it is easy to stain from oxygen/water
> vapour or production time exposure in the air.
> We are looking for document on staining metal of the finished board. Has
> anyone else experienced something similar phenomenon or anyone have an idea
> to solve this?
> Thanks
> Tuyen Tran.
>
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