So far as Tin coatings whiskers and so on are concerned coincidentally
there is an excellent article in April 2001 edition of HDI " the
magazine of High-density interconnect". The article is entitled A look
at the Past reveals a lead-free Drop-In replacement. It is by Sandra
Winkler and Bance Hom. Its rather long but gives a comprehensive
rehearsal of why matte tin was the preferred coating why its now not
(in the authors' opinions anyway) and how it could be again. The bad
news is its a bit long for a one sitting read.
The mag gives its web address as www.hdi-online.com for those that
want to follow up.
BTW
Have to dissent on 2). Sorry if I have mislead you, but I never was an
employee of Ablestik. I did however work closely with them whilst
looking after the UK for about 10 years when with their UK distributor
where, then anyway, Ablestik was by far the dominant supplier.
Mike
----- Original Message -----
From: "Ingemar Hernefjord (EMW)" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, May 09, 2001 3:16 PM
Subject: Re: [TN] Sn whiskers, TT (Technical Topic)
> Bev,
> thought you were too high up nowadays to have time for the floor
queries.
>
> 1)) some small percentage Lead in the Tin is said to save from
whiskering. Do we always have to select sliding into the ditches, 100%
or nothing at all?
>
> 2))Mike Fenner's secret. Cooked at home, with additional saliva from
tse-tse. Bev, it's 841-LMI, a superclean adhesive from (now) E&C. Mike
was boss and expert there in 'younger' days.
>
> 3))on the way
>
> 4)) except if you let them (the caps) pass melting before delivery
to the user.
>
> 5)) vacuumbaked, refilled with Helium and laser seam welded. E.g.
'hybrids' in earlier days.
>
> 6))Ag/Pd
>
> 7)) have never heard about a modern tele company in Canada. Heard
they still use morse and copper lines up there.
>
>
> What you say is frightening. If your sura is true, why is this theme
'never' up in the forum here?
>
> Ingemar
>
>
> -----Original Message-----
> From: Bev Christian [mailto:[log in to unmask]]
> Sent: den 9 maj 2001 14:26
> To: 'TechNet E-Mail Forum.'; 'Ingemar Hernefjord (EMW)'
> Subject: RE: [TN] Sn whiskers, TT (Technical Topic)
>
>
> Ingemar,
> 1) I'm afraid we are going to see more and more of this, not less.
In the
> blind race to lead-free, some of the suppliers are becoming "enviro
> friendly" (Hmm, if a plane crashes because of tin dendrites does
that count
> as enviro friendly because there are fewer humans or enviro nasty
because it
> burned some trees?) by replacing perfectly good 63/37, 60/40, 90/10,
98/2
> coatings with pure tin - unrelieved of course since they can't take
the heat
> to reflow the pure tin.
> 2) What in the world is McFenner's glue?!
> 3) Can you post the picture on Steve's website? I would love to see
this!
> 4) I don't believe you need heat as a stressor. I think the
heat/humidity
> just accelerates the process that would happen any way. The
stressor is
> internal, in the tin coating. And since there is no place for the
tin to
> move in the x-y plane (no soft lead to push around) they (tin
crystals) grow
> in the z axis to relieve the stress.
> 5) What do you mean by sealed package?
> 6) Most academic studies to produce tin whiskers on demand have
failed.
> However, I do seem to recall reading a paper that said/showed having
nickel
> under the tin retarded whisker formation. What are the underlayers
of your
> tin coating on this part?
> 7) Certainly one un-named large Canadian-based telecommunications
equipment
> company would not use leaded devices with 100% tin coatings.
>
> regards,
> Bev Christian
> Research in Motion
>
> -----Original Message-----
> From: Ingemar Hernefjord (EMW)
> [mailto:[log in to unmask]]
> Sent: May 9, 2001 6:41 AM
> To: [log in to unmask]
> Subject: [TN] Sn whiskers, TT (Technical Topic)
>
>
> Hi all,
> thought Tin whiskers belonged to stories from those day when..etc.
To my
> surprise we do experience that phenomenon in one of our many
hundreds of
> products, so not a problem that hits the whole organisation, but
that keeps
> a small local group awake. Now, description: small Kovar package
with
> alumina substrate and Au thickfilm and among components there are a
number
> of ceramic caps which are all glued with ol' McFenner's glue,
nothing wrong
> with that. The but: the end terminals of those caps are pure tinned
instead
> of traditional Ag/Pd or Au which are much used materials for glueing
> components. Mc Fenner has my story in details and he has already
given his
> view on my problem. What even he did not predict was that single
Tin +
> stress + x+y+ ? can generate tin whiskers. After a number of
tempcycles the
> caps (terminals) look like a mature burdock, never seen something
like that.
> Thousands of needles with different cross sections: T, X, circular,
ellipses
> etc. All like piped fro!
> m a cream press for making tartes, but less in size of course. The
strange
> thing is (yes, I have read everything about tin whiskers in Wassink
and
> Manko) that the dominating driving force, heat, has created no such
> whiskers at all. Sealed packages passed 500hs +90Centigrade and
tempcycling
> 300 cycles -40/+90C. No whiskers in the heat aged, whiskers in all
packages
> that passed cycling. Also seen, is that the cap's terminal tin
finish is
> totally fatigued and structure is broken up into coars grains or
rather
> 'flakes'.
>
> Litterature says that using some percentage Lead in the tin will
stop
> whisker growth. Or, if you have 100% tin, you should pass all caps
through a
> temperature that remelts the tin, also a method for minimizing the
creation
> of whiskers.
>
> Is there anyone else that has seen Sn whiskers on inside of a sealed
> package, and if so, what components and what environment did start
the
> whiskers growing? Would you dare use burdock fastener caps in a
HiRel
> product?
>
> Obstinate as I am I conclude with something that is forbidden, a
line from
> the world of fun:
>
> "I bought my wife a new car. She called late in the evening and
said,
> "Sorry, know I'm late, but there is some water in the carburetor." I
asked
> her , "Where's the car?" She replied, "In the lake."
>
> > Henny Youngman
>
> See you all
>
> Ingemar Hernefjord
> Ericsson Microwave Systems
>
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