Ingemar, et al,

Tin whiskers are a fascinating and confounding subject.  For the past 4
years or so here at NASA Goddard Space Flight Center we have been reviewing
the refereed literature, trade magazines, private reports, etc. as well as
conducting some of our own experiments/research and publishing our own
work.  We have a publicly available www site dedicated to the subject
(  http://nepp.nasa.gov/whisker  ) which has among many other things a
fairly exhaustive list of reference materials on the subject and pointers
to "PUBLIC DOMAIN" reports of actual field failures attributed to tin
whiskers (satellites, missiles, pacemakers, etc.) and various
representative photos of EEE parts and associated hardware with tin
whiskers growing on them (many are "recent" observations).

Now... on to "matte tin".  The following www site link (also at the NASA
www site) describes an actual field failure that has been attributed to a
tin whisker growing from a "matte" tin plated
IC.    http://nepp.nasa.gov/whisker/ancedote/20year/index.html  The user
who experienced this failure was courteous enough to allow me to document
it on the www site if they could remain "anonymous".

Next week I will be publishing/presenting the following paper
--  Brusse, Jay, "Tin Whisker Observations on Pure Tin-Plated Ceramic Chip
Capacitors", Proceedings of the American Electroplaters and Surface
Finishers (AESF) Society SUR/FIN Conference June 2002, Chicago, IL

In this paper I discuss multiple independent accounts of tin whisker
formation from matte tin plated ceramic chip capacitors under certain
environmental conditions.  We have in our possession chip caps with tin
whiskers up to 240 microns long (post temp cycle and subsequent ambient
storage).  All of these capacitors also have an underlayer of Nickel (~ 5
microns thick) which some have prescribed as an effective way to prevent
whiskering.  I also describe examples where the chip cap terminations form
whiskers (~30 microns) AFTER vapor phase installation with 63/37 Sn/Pb
solder and subsequent temp cycling.  These whiskers grew from areas of the
capacitor terminations that were remote from the solder joint and remained
pure tin.  The accounts in the paper cover four different manufacturers and
multiple lots and capacitor sizes.

In my involvement with a consortium sponsored by the National Electronic
Manufacturing Initiative (NEMI), I have heard multiple accounts from the
participants (including connector & IC suppliers and electroplating
suppliers) of their experiences and observations of tin whiskers from
various "matte tin" products/processes.  Ingemar indicated that Sandra
Winkler considers "matte tin + tough discipline in the plating
organization" is the remedy to tin whisker formation.  What exactly does
"tough discipline" mean? and how do I as a user of electronic parts
ascertain whether the product I buy was built using "tough discipline"?
Especially when the occurrence of tin whiskers can take several YEARS to
initiate, there are no industry-accepted test methods to judge whisker
propensity (indeed many have observed 'conflicting' test results) and the
growth may be affected by a number of factors including introduction of
stresses through handling, assembly and application environment.

As far as tin ALLOYS and whiskers... I too have seen accounts of whiskers
from alloys such as tin/lead, tin/copper and tin/bismuth.  What I have also
observed is that the references to THESE growths generally lack detail
regarding the length, density, time to form, etc.  Where such data is made
available (pictures, etc.) the whiskers I've seen for some tin/lead alloys
are generally  very short (few 10's microns) and are growing in very few
numbers.  Compare those observations with whiskers from various PURE TIN
specimens (few 100's microns to millimeters and high density) and you start
to see apples and oranges.  Also, in my own review of the "publicly
reported" field failure information attributed to whiskers, I have only
seen accounts that reference PURE TIN and ZINC whiskers as the root cause
of failure.  Does anyone on the TechNET forum have specific documentation
they can share about Tin/Lead whiskers?

This is a topic I could go on about ad-infinitum... let me stop here.

Sincerely,

Jay Brusse
QSS Group, Inc. at NASA Goddard
 >-----Original Message-----
 >From: TechNet [mailto:[log in to unmask]]On Behalf Of Ingemar Hernefjord
 >(EMW)
 >Sent: Wednesday, June 19, 2002 5:11 AM
 >To: [log in to unmask]
 >Subject: [TN] tin whiskers - matte tin
 >
 >
 >Hi,
 >had a chat with NASA about a little tin whisker problem we had.  Pure tin
end caps on ceramic capacitors. Now, Sandra Winkler wrote that tin whiskers
can grow on ALL tin alloys, not only pure tin, as some thought. Remedy is
matte tin and tough disciplin in the plating organisation. I'll check with
AVX and the others what solder they will use from now on. Happened to read
this last year:

 >"At the one-day joint MEPTEC (MicroElectronics Packaging & Test
Engineering Council)/San Jose University Lead Free Summit on January 10,
2001, matte tin was universally supported by the attendees by the end of
the session. Over 50 companies and nearly 100 individuals were represented.
Remarkably, matte tin has made a comeback and will hopefully be here to
stay." Sandra Winkler
 >
 >Q: do you ask for matte tin on cercaps for both solder- and epoxy mounting?
 >
 >/Regards/
 >Ingemar Hernefjord
 >Ericsson Microwave Systems
 >
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