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]> 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 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 Tuyen Tran
> Sent: Thursday, July 17, 2014 12:36 PM
> To: [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]] On Behalf Of Wayne Thayer [
> [log in to unmask]]
> Sent: Thursday, July 17, 2014 10:57 PM
> To: [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]] On Behalf Of Tuyen Tran
> Sent: Thursday, July 17, 2014 11:42 AM
> To: [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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