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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