Whisker growth is depending on the base material. Any alloy with Cu and
Zn is risky, tin over Ni will not cause much whiskers, Pt and Pd no
whiskers at all etc. Reason is that whisker growth is mainly caused by
diffusion of e.g. Cu into the tin plate. Cu/Sn IMCs grow and cause
stress in the boundaries and. The stress is in the order 40-100 Mpa at
the root of the whisker embryo. The unique thing is that this can only
be created in a specific range of tin thickness: approx. 2-5 um. 10um
thick tin or more is said to produce no whiskers at all, as well as
thinner than 2um. Our company together with Bosch and ESA have done a
study and made tests, probably the most advanced today to get to the
root of the phenomenon, but still there are work to be done. There are
many contradictory facts, e.g. two bath suppliers can have 'exactly'
same chemicals in their soup, one giving whiskers, the other not. Other
questions are about growth speed, where some claim that whiskers can't
grow with more than some um/month, while I myself have seen whisker grow
to 10 mm length in a couple of weeks! I'll see if I can put a picture up
on Steve's wall. Finally, fusion seems to solve all whisker problems,
but how do we know that? Who can predict what happens after 5 or 10
years.
Interesting topic, anyway
Ingemar Hernefjord
Ericsson Microwave Systems
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Franklin Asbell
Skickat: den 14 oktober 2005 16:41
Till: [log in to unmask]
Ämne: Re: [TN] Electrolytic Tin Plating
I'm thinking the chemistry involved with electrolytic tin is going to
result
in whiskers/growth and I would suggest it not be used for a final
finish. If
you are like many shops you use the tin/lead as an etch resist in
process,
which is fine, but for final finish immersion tin would be better.
Check out Florida Cirtech's website, they have a nice study you can
download
and print regarding the differences between the two type baths
(electrolytic
and their Omikron).
(and again, this is not an endorsement for any specific chemistry, the
site
mentioned offers greater volumes of literature is all, and I like my
literature)
I would think storage would not be an issue with plated tin or tin-lead
provided they were sealed properly, but with the plated tin, some
humidity,
some contamination, and mild current...these are all good conditions to
cause growth.
And I'm thinking tin will indeed flow, it just takes a higher
temperature...
Franklin
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Fineline Circuits,
Inc
Sent: Friday, October 14, 2005 9:20 AM
To: [log in to unmask]
Subject: [TN] Electrolytic Tin Plating
Hi Technetters:
Now that Leadfree is here or close to, we would like to change the
electrolytic tinlead plating tank to electrolytic tin tank.
I would like to know if the other fab houses have done this and if so
which
vendors tin chemistry has a wider window of operation. Are they seeing
any
tin whiskers/sliver problems etc..especially if the boards are stored
for
few months.
Also it is my understanding that you can mask over this plated tin since
it
does not flow. In this case you can totally avoid stripping the tin and
tin
levelling.
We currently do offer other finishes including electroless tin ,
immersion
nickel/gold, silver....
If there are other consideration in making this change I would
appreciate
your input.
Franklin, like you I would like to see more fab houses participating but
I
would like to remain with technet.
Thanks
Sona Sitapara
Fineline Circuits Inc.
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