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May 2001

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From:
"Ingemar Hernefjord (EMW)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 9 May 2001 12:41:14 +0200
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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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