The posts in response to Doug Romm's question on silver dendrite formation
do not seem to address the issue.  Maybe some elaboration is needed.

Years ago, c. 1978 and before, IC leads were plated with silver, this was
for through hole packages.  There was no post mold solder finishing required
and in fact the leadframe and component were Pb-free. However, some end
users saw silver dendrites or whiskers grow between adjacent leads that
resulted in lead shorting.  If I recollect correctly the growth was
facilitated by humidity and an electrical bias.  The dendrites are fine
filaments of silver that grow from a silver surface.  End users specified no
silver outside the plastic.  This phenomenom led to the silver spot plating
process with post-mold solder finishing, hot dipping or plating. Eliminating
silver from outside the package eliminated the dendrite growth outside the
package. (There were suggestions that there can be silver dendrite growth
inside the plastic, but with the large number of silver spot plated
leadframes in the field, this does not seem to be a valid issue.)

The question is: if there is silver on a component lead and it is tied up
metallurgically in a solder joint is there still a potential for silver
dendrite growth?  Or, as in the case of tin whiskers does the silver have to
be present as pure or nearly pure silver with no adulteration?

Regards,
Don Abbott

> ----------
> From:         Kirk D Mueller[SMTP:[log in to unmask]]
> Reply To:     Leadfree Electronics Assembly E-Mail Forum.;Kirk D Mueller
> Sent:         Tuesday, September 28, 1999 12:05 PM
> To:   [log in to unmask]
> Subject:      Re: [LF] Pb-free Alternatives using Ag
>
>       My comment on 10% silver in solder being an embrittlement problem is
> for
>    Sn63 / Pb 37 (Eutectic) solder.   Brittle Ag-Sn intermetallics start
> forming
>    when silver exceeds 10 % by weight in that alloy.  Most other
> intermetallic
>    compounds formed with the Sn, like SnCu2, SnCu, Sn2Cu, etc., are what
>    actually causes solder to stick to the metal being soldered to.  These
>    examples are not particularly brittle.
>
>       I would imagine the upper safe limit for silver in Sn25 / Ag10 / Sb
> would
>    be different (or nobody would be using it).  Perhaps the Antimony
> allows
>    higher concentrations of dissolved silver before the brittle AgSn
>    intemetallics start forming. Do you know what the limit is?  I'm
> curious.
>
> Thanks,
> Kirk Mueller
>
> Martin Weiser <[log in to unmask]> on 09/23/99 08:19:53 AM
>
> Please respond to "Leadfree Electronics Assembly E-Mail Forum."
>       <[log in to unmask]>; Please respond to Martin Weiser
> <[log in to unmask]>
>
> To:   [log in to unmask]
> cc:    (bcc: Kirk D Mueller/RWS/Raytheon/US)
> Subject:  Re: [LF] Pb-free Alternatives using Ag
>
> All,
>
> I wanted to add that Sn25Ag10Sb has been used for many years in high
> volumes for
> soldering power die on to Cu heatsink/leadframes.  Yes, this solder is
> quite
> strong and brittle, especially compared to Sn37Pb and similar solders.
> However,
> the Ag-Sn and Sn-Sb intermetallics are not the normal cause of failure in
> these
> packages.  The most common failure mode is die cracking during cooling of
> larger
> die due to the large CTE mismatch and high solder strength.  In general,
> if the
> die survive the initial cooling then they can be thermally cycled
> thousands of
> times without failure.
>
> Regards - Marty
>
> On Wed, 22 Sep 1999 16:24:27 -0700  Kirk D Mueller
> <[log in to unmask]> wrote in response to Doug Rohm:
>
>      I thought the problem with tin and silver was the formation of
> brittle
> Sn-Ag intermetallics when the silver content exceeded 10 % by weight.
>
> Kirk Mueller
>
> Doug Romm <[log in to unmask]> on 09/22/99 08:48:13 AM
> All,
>
> Several of the Pb-free options being offered for use in both solders and
> component lead plating include silver (Ag), for example Sn/Ag.  In the
> past
> some users of IC components have had concerns/problems with use of Ag in
> components because of silver dendrite growth.  Is there any concern about
> use of Ag in these Pb-free alternatives?  If not please provide a brief
> technical explanation of why not.
>
> This question may have been raised previously, but I can't find the
> answer.
> Best regards, Doug Romm
>
>


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