PhiI, you said:

>Technetters
>
>For someone with a metallurgical backgroun, I turn on to whiskers which have
>a specific metallurigcal definition and have nothing to with dendritic
>growths or cathodic andodic filaments which are electrochemical in origin.

Now that's what I call kinky.  ;-)

I thought "whisker" was a non-specific term. The only reference I have at
hand _The Contamination of Printed Wiring Boards and Assemblies_ Carl
Tautscher, 1976) calls the formation of tin whiskers "a result of stress
corrosion", citing an article by S. M. Arnold "Repressing the Growth of Tin
Whiskers" in American Electroplaters Society, 1966.

I would have thought "dendrite" was an alternative term for "whisker" for
thin filaments of metal.

>Metallic whiskers which are formed on tin and cadmium coatings and a few
>other metals are a single crystal growth the may extend out from the coating
>for about a half an inch.  The tin-whisker is usually only a few microns in
>width and have been shown to be the cause of failure of electronic devices
>including printed boards.  The tin whisker has no branches but may make a
>sharp angular turn. The early papers on the suject came out of Bell Labs in
>the 1950's where, I believe, some field failures were investigated.  Tin
>platings or coatings when placed in compression in a humid atmosphere
>produced tin whiskers in a few hours to a few months as the tin seem to
>squirt out of the tin coating.  Unfortunately, some of the bright tin
>deposits such as bright-acid-tin had a built in compressive stress which also
>produced whiskers without external compression.

Yes, I agree and definitely Zn will do this, the stress can be measured and
is quite significant. Thin coupons plated on only one side can actually
warp.

My own experience with this was on Zn-Cr finished steel, plated in acid
zinc baths. The whiskers could be formed at room temperature, but formed
very quickly in high temp, low humidity environs. As to whether it can be
termed "corrosion", I'd have to assume that when a filament forms, it moves
metal from one place to another, and that removal could be termed
corrosion.

>Tin plating or fused coatings on brass were shown to be a large contributor
>to tin whiskers, on copper the growth was less likely, and on nickel very
>unlikely.  Tin coated brass terminals and tin plated screws is where I have
>seen whiskers most often.  However, I did see a tin plated printed board that
>seemed to have have had extensive Rogaine treatment, as it had sprouted
>everywhere and caused extensive shorting.  The use of small amounts of other
>metals such as lead or silver in the tin solder or plating/coating does
>inhibit tin-whisker growth and there are some reports of heat treatment below
>the melting point will also reduces whisker growth.

>Some metals such as silver, lead, copper etc. do form dendritic growths in a
>slightly humid atmospheres ( or in aqueos solutions) or in the presence of
>some gasses and grow dendritic filaments that also cause shorting.  Often
>these filaments have twigs or branches from which the name dendrite
>originated.  These filaments may grow in air, through dielectric or between
>soldermask and the printed board; however whisker growth is largely prevented
>by  barriers other than gaseous atmospheres.

OK, I see the point, that a dendrite is branched? I'd concur that the
filaments of zinc are not branched, they are more or less straight. Also,
I'm sure that there are great distinctions between the processes which
induce the growth of the little tiny metal filaments - I did not mean to
muddle terms together.

>It sounds to me that the failure reported by Rolf is a dendritic filament
>rather than a whisker in the true metallugical sense. .

Maybe so. My point was that the filaments of metal may have come from a
plated surface near the board when it was put into a card cage or with a
stiffener (attached to the assembly later) or steel fastener plated with Zn
or Ag. The process of formation is of course interesting, thanks for your
clarification.

cheers,


Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com/

Jerry Cupples - [log in to unmask]
Interphase Corporation
13800 Senlac Dr.
Dallas TX 75234
(214) 654-5150
(214) 654-5500 FAX


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