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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