They carry 50 mA for a short time and in doing so create a plasma (metal vapor enters the arc from the tin surface and the surface from which the whisker touches.  The result is quite dramatic as can be seen in the hole in the side of this relay my business partner Dr. Gordon Davy discovered when he worked at Northrop Grumman.

http://www.hlinstruments.com//RoHS_articles/davy2002_relay%20failure%20caused%20by%20tin%20whiskers.pdf

See also this busbar meltdown reported to NASA Goddard by a manufacturer in Sweden.

http://nepp.nasa.gov/whisker/anecdote/2009busbar/index.html

http://nepp.nasa.gov/whisker/anecdote/2009busbar/2009-brusse-bus-bar-tin-whiskers-sweden.pdf

According to NASA Goddard http://nepp.nasa.gov/whisker/background/index.htm#q3

What are the Risks/Failure Mechanisms Associated with Tin Whiskers?

Tin whiskers pose a serious reliability risk to electronic assemblies.  Several instances have been reported where tin whiskers have caused system failures in both earth and space-based applications.  To date, there are reports of at least three tin whisker induced short circuits that resulted in complete failure of on-orbit commercial satellites.  There have also been whisker-induced failures in medical devices, weapon systems, power plants, and consumer products.

The general risks fall into four categories:

    Stable short circuits in low voltage, high impedance circuits. 

    In such circuits there may be insufficient current available to fuse the whisker open and a stable short circuit results. Depending on a variety of factors including the diameter and length of the whisker, it can take more than 50 milliamps (mA) to fuse open a tin whisker.

    Transient short circuits.

    At atmospheric pressure, if the available current exceeds the fusing current of the whisker, the circuit may only experience a transient glitch as the whisker fuses open.

    Metal Vapor Arc 

    If a tin whisker initiates a short in an application environment possessing high levels of current and voltage, then a VERY DESTRUCTIVE phenomenon known as a Metal Vapor Arc can occur.  The ambient pressure, temperature and the presence of arc suppressing materials also affect metal vapor arc formation. In a metal vapor arc, the solid metal whisker is vaporized into a plasma of HIGHLY CONDUCTIVE metal ions (more conductive than the solid whisker itself). This plasma can form an ARC capable of carrying HUNDREDS OF AMPERES. Such arcs can be sustained for long duration (several seconds) until interrupted by circuit protection devices (e.g., fuses, circuit breakers) or until other arc extinguishing processes occur. This kind of arcing is happening in the metal vapor.  When an arc quenching agent (e.g., air) is present, more power must be installed into the event to replace power lost to the non-interesting processes happening in the quenching agent.  Therefore, as air pressure is reduced, less power is required to initiate and sustain a whisker-induced metal vapor arc.  For example, past experiments** have demonstrated that at atmospheric pressures of about 150 torr, a tin whisker could initiate a sustained metal vapor arc where the supply voltage was approximately 13 Volts (or greater) and supply current was 15 Amps (or greater).  Tin (or other materials) from the adjacent surfaces can help to sustain the arc until the available material is consumed or the supply current is interrupted. Metal vapor arcs in vacuum are reported to have occurred on at least three commercial satellites resulting in blown fuses that rendered the spacecraft non-operational.

    **  J.H. Richardson, and B.R. Lasley, "Tin Whisker Initiated Vacuum Metal Arcing in Spacecraft Electronics," 1992 Government Microcircuit Applications Conference, Vol. XVIII, pp. 119 - 122, November 10 - 12, 1992.

Bob Landman
LDF Coatings, LLC
www.ldfcoatings.com
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Goodyear, Patrick
Sent: Saturday, May 28, 2011 10:43 PM
To: [log in to unmask]
Subject: Re: [TN] New report finds tin whiskers to blame in Toyota UA

I have personlally removed whiskers from tin plated metal pots on Basler power supply regulators that were upwards of 3/8" in length and about 30 or 32 ga in diameter, more than enough to carry a sizable current. I had another board that had a hole in it from a short, suspected a whisker but the proof had poofed, although the pots looked like they had mold
growing on them.   The boards were 35 or so years old.  

Pat goodyear
Control Technician   

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kenneth J. Wood
Sent: Saturday, May 28, 2011 2:15 PM
To: [log in to unmask]
Subject: Re: [TN] New report finds tin whiskers to blame in Toyota UA

The report claims that Tim Whiskers can be up to 9mm long and carry 10mA of current...
That's quite a bit of amperage for a 1-2 micron whisker, I didn't know they were that hearty.
Ken
Saturn PCB Design, Inc.
www.saturnpcb.com

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Buetow
Sent: Monday, May 23, 2011 2:14 PM
To: [log in to unmask]
Subject: [TN] New report finds tin whiskers to blame in Toyota UA

A new report is blasting the NASA and NHTSA reports on unintended acceleration in Toyota vehicles, and specifically says tin whiskers is among the culprits. 

 

http://bit.ly/jDUofM

 

 

Best,

Mike

 

Mike Buetow

Circuits Assembly

w/m 617-327-4702

 

>>> The CIRCUITS ASSEMBLY EMS Directory -- over 2,100 listings, in Excel
www.circuitsassembly.com/dems <<< 

 

Follow me on Twitter: twitter.com/mikebuetow

 


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