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From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Tue, 19 Dec 2006 16:14:57 -0600
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Hi Wayne! There is a big difference in the migration behavior of
electroplated silver and immersion silver. Some of the differences are due
to the specific chemical baths, the nature of the deposition mechanics, the
deposit thickness, and the deposit structure. The IPC committee work that
George W. described included electroplated silver samples in their testing.
And to quote Doug Pauls, the propensity of silver to migrate "depends"! Don
Cullen has presented some data on the creep corrosion of immersion silver
in a very harsh, high humidity environment.  The INEMI statement is not
incorrect but there are a number of other use environment dependences (such
as a sulfur contaminated atmosphere, etc.) which need to be considered.
Many high performance  product companies have determined that immersion
silver is an applicable finish for their use environments. There will be
some information on immersion silver durability in the next revision of
Clyde Coomb's Handbook.

Dave Hillman
Rockwell Collins
[log in to unmask]



             Wayne Thayer
             <WTHAYER@EVITECHN
             OLOGY.COM>                                                 To
             Sent by: TechNet          [log in to unmask]
             <[log in to unmask]>                                          cc

                                                                   Subject
             12/19/2006 12:29          Re: [TN] Fwd: RE: [TN] iNEMI Hi rel
             PM                        Pb-free finish report


             Please respond to
              TechNet E-Mail
                   Forum
             <[log in to unmask]>
             ; Please respond
                    to
               Wayne Thayer
             <WTHAYER@EVITECHN
                OLOGY.COM>






Steve-

Thanks for the link.

That paper actually increases my questions about the iNEMI
recommendations:

First, it IS about whiskers grown from silver, not electrochemical
migration.  The author claims to have seen such whiskers "6-8 cm long
and 1mm in diameter"!

Second, on page 4, it specifically mentions whiskers also being grown
on AgNi alloy, while the iNEMI recommendations list Ag over Ni as one of
their preferred finishes for separable contacts.  Maybe the plated
coatings won't make enough AgNi to matter, but it would probably have
been a useful tidbit to add to the recommendations paper.

The author, Bella Chudnovsky (Square D circuit breakers), of this paper
actually concludes that Tin might be better in some situations for
addressing this problem, since it isn't as reactive to the sulfurous
environment found within a pulp plant and the whiskers are relatively
tiny.  She does temper this by recommending the use of Tin alloys which
have higher whisker resistance.

She reports that thinner platings of silver actually makes the
situation worse, but the thinnest she could find data reported on was
still 2 microns--much thicker than ImAg.  The reasoning is that the base
metal, copper, contributes to the reaction (copper was found in the
whiskers in single digit percentages).

She claims to have solved her problem with a combination of air
filtration and circulation to minimize H2S and drop the temperature,
contact lube (to form a film to further block sulfur), and regular
maintenance solved her issue (circuit breakers failing in a pulp
plant).

Sounds like it would be an interesting challenge to attempt to grow
these structures on PCBs with ImAg.  Growth accelerated greatly above
140C, and you want about 1ppm H2S to get the precursors started.

Wayne

>>> [log in to unmask] 12/19/2006 12:19 pm >>>
Wayne writes:

"...The UL paper you and I both found suggests that ImAg is no more
susceptible to electrochemical migration (I hear you Bev!) than most
other finishes. The subject paper contains language which gives me the
impression some hydrogen sulfide tests were done with ImAg and the
results were of concern, but the only obviously relevant paper in the
reference area was 38 Chudnovsky, Bella, "Degradation of power
Contacts
in Industrial Atmosphere: Silver Corrosion and Whiskers". Proceedings
of
the 48th IEEE Holm Conference on Electrical Contacts, 2002 which I
don't
have ready access to, but I suspect it doesn't concern ImAg."


Hi Wayne!

I found it for you:

http://tinyurl.com/oo23f

-Steve Gregory-

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