Hi Mason,

Almost certainly it is a sulphide Ag2S. This is the compound seen on
tarnished silverware eg cups,trophies etc.There are anti tarnish processes
but these are probably not thought wise in the electronics industry.Please
advise more about test conditions as this is not the only possibilty.

Best Regards,

Ken McGowan
----- Original Message -----
From: "David Hillman" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, June 06, 2001 3:34 PM
Subject: Re: [TN] Immersion silver boards tarnish


> Hi Mason! There are a number of different surface reactions which could be
> present (silver sulfide, silver chloride, silver oxide, etc.). I use my
> SERA test unit with a borate buffer solution to nondestructively analyze
> the immersion silver surface to determine who is present. You also could
> use  Auger spectroscopy too but that would require a destructive analysis.
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
>
>
>
>
> Mason Hu <[log in to unmask]>@IPC.ORG> on 05/30/2001 05:56:25 PM
>
> Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please
respond
>       to Mason Hu <[log in to unmask]>
>
> Sent by:  TechNet <[log in to unmask]>
>
>
> To:   [log in to unmask]
> cc:
>
> Subject:  Re: [TN] Immersion silver boards tarnish
>
>
> I recently ran a few test assemblies (with immersion Ag finish, of course)
> through thermal cycling (for 2nd level reliability).  At the end of the
> test, some of the non-soldered metal area (EMI contact) got the yellow
> tarnish.  When probed with a multimeter I measured significant increase in
> contact resistance.
>
> I read a lot of comments on the presence of sulfur and chlorine (or their
> ionic counterparts) as being the possible root cause.  My question is:
what
> is the chemical reaction(s) here?  Are those agents attacking the Ag or
the
> organic co-deposited with the Ag?  Does anybody know what the yellow stuff
> is?
>
> Mason Hu
>
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