----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Al Toothaker" <N6TA@arrl.net>; "'Bob Lombardi'"
<boblombardi@cfl.rr.com>; <towertalk@contesting.com>
Sent: Saturday, April 02, 2005 8:16 AM
Subject: Re: [TowerTalk] re; Whiskers growing where you cannot shave them
> > Whiskers are an issue in reliability of (usually space)
> components and
> > circuits. The culprits are Tin >97% pure, 100% pure
> cadmium and 100% pure
> > zinc. If these metals are NOT pure, there is little
> problem.
>
>
>
> Unless we send our solder joints deep into space or
> otherwise place them in a vacuum for many years whiskers are
> not an issue.
>
> They only form in a high vacuum and it takes a long time, so
> why worry about it?
This is not true. They form in any atmosphere. There are reported cases of
them forming in things like GPS modules where there was a pure tin plated RF
shielding lid over part of the circuitry. "Inspection of one lid using a
binocular microscope indicated 5 whiskers (lengths ranging between 1 and 3
mm) in a 6 mm by 4 mm area. This gives a density of (5 whiskers)/(0.24
cm**2) = 20 whiskers per square cm, and allows an estimate of the total
number of such whiskers per lid: (8 cm)*(4 cm)*(20 whiskers/cm**2) = 640
whiskers per shield top. "
Also, check out this delightful example:
http://nepp.nasa.gov/whisker/anecdote/2003rf_enclosure/index.html
Photos of numerous whiskers on equipment just 2 years old which had broken
off and shorted between SMT components (since they weren't conformal
coated).
And here's another interesting one where whiskers were growing on circuit
breaker contacts and causing failures in earth surface applications
http://nepp.nasa.gov/whisker/anecdote/2003ckt_breaker/index.html these are
things in airconditioned spaces with component temps around 30-40C..nothing
too extreme.
We're very conscious of this in space applications because of the lack of
repair access, and because there was a big increase in tin plating to
replace the more expensive gold, particularly in consumer parts. High-rel
(aerospace/space/mil) is a very small part of the IC business these days,
and lots of useful and interesting parts may not be available as fancy
hermetic gold plated packages.
For instance, the standard copper 28 pin lead frames used for small SOIC
parts are (were) often pure tin plated for solderability. Crimp lugs are
also often plated with pure tin.
Zinc whiskers are another problem, given the prevalence of galvanized metal
in structural applications. The galvanizing forms whiskers, which then get
bumped or brushed off and redistributed by the ventilation air.
However, solder doesn't form whiskers.. only the pure metals, as indicated
above.
>
> 73 Tom
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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