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May 2001

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From:
"Creswick, Steven" <[log in to unmask]>
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Date:
Thu, 10 May 2001 12:05:56 -0400
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Ingemar,

Sorry for being late in offering my two pence but:

1 - I think the 84-1 series is a pretty good material, but I would never use
it for attachment to solder, or tin.  Don't you see mechanical degradation
(loss of adhesion)?  Have used it in various applications where cure temps
ranged from 150-200°C with cure times ranging from 0.5-2 hours.  Did you
indicate your cure temp/time?  

2 - Never had the opportunity (mis-fortune) to use a Sn terminated component
within a hybrid (don't think I would want to, either!).

3 - Any chance to switch to a Pd/Ag terminated component??

4 - Maybe you should join up with Ball Semiconductor.  They have spherical
devices, and you have a three dimensional interconnect for passive
components (Put them in a beaker and shake well to create a circuit).  --
Oops!  That was levity.

Steven Creswick
Gentex Corporation

-----Original Message-----
From: Ingemar Hernefjord (EMW)
[mailto:[log in to unmask]]
Sent: Wednesday, May 09, 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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