silver and silver oxide are both conductive - no problem  for  
tarnish.  however, i would a bit careful about the tarnish -  
specifically containing S, Cl or others ionics.  just in case some  
tree grow on them.  switches and relays are relatively high voltage  
of 12V - a bit more tolerant to the type of contact - besides, those  
are thicker silver, not immersion silver layer, if the ground  
intended for 3V or below, you might have some issues - if the contact  
on either side is rough.  my 2 cents.
          jk
On Jul 18, 2014, at 7:47 AM, Wayne Thayer wrote:

> 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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