>Following from the EMPF HelpLine in Indianapolis, prepared by Jeff Kukelhan.
>This supplements previous replys to this subject.
>
>62/36/2 paste has been used in the hybrid industry for years to solder to Ag
>bearing thickfilm materials (it cuts down on leaching).
True enough, and it has a lower MP. For soldering silver metallized
ceramics, it offers that benefit. But silver metallization is also a
migration hazard; IMHO, unless it has a nickel barrier, in which case you
don't need to worry about leaching...
> Apparently it does
>not cause any problems in hybrid applications. Just remember, in order for
>electromigration to occur, there has to be condensed water present. Keep
>condensed water off of the finished assembly, and there will be no
>migration.
That's not what I read. Humidity alone will accelerate the process. In my
experience with silver plated leadframes of DIP's on memory boards, this
was not the case. Computers in non-condensing conditions had memory boards
with dead shorts caused by apparent migration. Mostek and TI used this on
leadframes in the late 70's. It is unknown to me whether the dissolution of
Ag in solid phase alloys prevent migration, but I would guess it must slow
it down a lot.
>Remember that Sn and Cu also will migrate just as easily as
>silver under moist conditions.
See: _The Contamination of Printed Wiring Boards and Assemblies_, Carl J.
Tautscher, Omega Scientific Services, 1976 p. 6-32.
"When tested under the same conditions, silver was found to migrate more
than 1,000 times as fast as copper."
I've never seen copper migration, but I've seen silver migration several times.
> ----------------------------------------------------------------------------
>
>>I have a question about reflow soldering to gold with 2% silver paste.
>>
>>I heard some folk lore at our company about using 2% silver solder paste
>>on gold plated lands. The legend states that 62/36/2 paste on gold has
>>long term reliability implications, these joints being subject to
>>dendritic growth to a greater degree than 63/37 solder to gold.
>>
>>If anyone is aware of published data on this subject, I would be very
>>grateful if you would give me the references so I may review the
>>information.
>>
>>Also, if anyone is using silver solder on gold at this time, I would be
>>interested in hearing about your experience.
>>
>I think this has more to do with Ag in the solder than the Au lands.
>Ag is known to be a bad electromigrator because of the low activation
>energy for Ag dissolution/redeposition reactions and the soluability of
>Ag-organic acid salts. Krumbien's review on electromigration discusses
>this in general, though it doesn't discuss 62/36/2 specifically.
>
>However, we at Alpha Metals frequently run electromigration tests for
>pastes with 62/36/2 as well 63/37 powder to ensure reliability with
>both alloys.
regards,
Jerry Cupples
Interphase Corporation
Dallas, TX
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