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August 12, 2007

Metal weirdness [Purple Avenger]

No, not Twisted Sister, Metal metal, periodic table type metal. You always thought metal was mundane stuff right? Predictable and prone to staying where you put it right? Wrong. Metals have a secret life that few of us are aware of.

[Fresh updates below the fold!]

Some metals grows whiskers. Yea, whiskers.

...

Conclusions

Catastrophic failures of electronic systems caused by tin whisker induced short circuits have been reported in commercial satellite applications. Despite decades of research into the tin whisker phenomenon, a comprehensive understanding of the mechanisms that affect their growth is not available. Currently, there are no industry accepted test methods for determining a particular component’s (or plating process’s) propensity to form tin whiskers. Some experimenters report the incubation time before initiation of whisker growth can be months, if not years. These observations are particularly concerning for long duration missions...


Historically, various forms of pure tin plating (especially, “bright” tin plating) have been found to be prone to tin whisker formation. Currently, the most effective risk mitigation technique against tin whiskers is to avoid the use of components that are most prone to whiskering of the form that is large enough to pose a reliability hazard. In the event such usage cannot be avoided or confirmed, users are advised to carefully review the use of pure tin plated components against application conditions, circuit geometries and mission objectives.
  1. Uralane 5750 does not prevent whisker formation
  2. Uralane 5750 shortens the incubation period of whisker formation
  3. Although, whiskers initiate more rapidly under Uralane 5750, the coating also retards the growth rate of whiskers
  4. Whiskers are capable of growing out from beneath a 1/4 mil thick Uralane 5750 coating
  5. Tin whiskers can bend under forces of electrostatic attraction, thus increasing the probability of shorting

More scary stuff here --- NASA

This whisker shit has shot down sats, screwed with weapons systems, and knocked out data centers around the globe.

[UPDATE]

eman asks:

Are they following external field lines or is it intrinsic to crystal growth?

One of their general traits is they seem to appear on areas where there's internal stress in the metal -- ex. stampings, die formed stuff, and the inside of rolled/welded pipe, castings that necessarily undergo differential cooling rates.

I saw quite a nest of zinc whiskers growing inside a piece of galvanized steel conduit a while ago. Obviously the rolling process imparts high compression stresses on the inside of a pipe. The growth was oriented towards the center of the pipe.

Each metal whisker appears to be a single growing metal crystal. Its been shown by observing unique features of tips, that they grow from the base because the tip feature(s) remained unchanged.

However, they do seem to deflect with electro-static stimulation.
Before:

After:
grounded whisker, 50V to the probe

Chad remarks:

A couple of years ago when the EU implemented their RoHS directive this was a big deal. One of the reasons leaded solder was so popular was that it helped prevent the formation of these whiskers...

Yep, this is going to be a big problem in electronics produced in the EU. The banning of traditional 50/50 solder means you gotta use something else. For lower cost commercial/consumer stuff, high tin solders are the obvious answer and they'll be whisker/failure prone. Politics trumps science in the EU.

I believe California decided to adopt the EU standards as well. CA produced cheap electronics will be similarly afflicted.

Can anyone say failing auto electronics boys and girls?

Someone will eventually say just use silver solder. The problem is, silver whiskers have been observed and can grow to several inches long in the right conditions. In a slightly corrosive atmosphere, the silver whiskers seem to be quite prolific.

Silver whiskers growing in electrical switching contacts:

Chudnovsky's (Schneider/Square-D corp) paper on silver whiskers. This is a VERY BIG deal for electrical gear makers like Square-D, Cutler/Hammer/GE/Siemens, etc. Silver has been used on electrical contacts for ages due to its low resistance.

digg this
posted by xgenghisx at 02:22 PM

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