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March 2012

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Tue, 20 Mar 2012 12:48:36 -0500
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Boy, this raises a whole bunch of questions in my mind:

1. What technology is used that can tell NASA for certain what caused the "short circuits in the instrument that led to unexpected voltage shifts" on a satellite still in space? How do they know tin whiskers were the root cause? Could not small conductive solder slivers, fines, component malfunctions, etc. have caused the issue? Are there self-test diagnostics that can be performed electronically that "prove" a tin whisker caused an electronic malfunction?

2. In the deep vacuum of space, whiskers present a much more serious threat to electronics than they do here on Earth. Because there is no oxygen in the vacuum, a whisker half the diameter of a human hair can pass much more current in space than even a .010" diameter copper wire can here in Earth's atmosphere. So the statement "It is believed that these or additional tin whiskers that may grow on Cassini cannot carry enough current to cause problems, but will burn out on their own like a lightweight fuse" does not sound quite right.

Any NASA engineers out there who can shed some light on this?


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gervascio, Thomas L
Sent: Tuesday, March 20, 2012 12:08 PM
To: [log in to unmask]
Subject: [TN] Out of the world tin whiskers

Tin whiskers at Saturn

PASADENA, Calif. -- The Cassini plasma spectrometer instrument (CAPS) aboard NASA's Cassini spacecraft at Saturn has resumed operations. Mission managers received confirmation on Friday, March 16, that it was turned on. They plan to monitor the instrument for any unusual behavior.
Last June, short circuits in the instrument led to unexpected voltage shifts on the spacecraft. As a precaution, mission managers turned off the CAPS instrument while engineers investigated the issue. The investigation led to the conclusion that tin plating on electronics components had grown "whiskers." The whiskers were very small, less than the diameter of a human hair, but they were big enough to contact another conducting surface and carry electrical current. Researchers are still trying to understand why whiskers grow on tin and other metals, but they know now that whiskers can grow in space and on Earth. It is believed that these or additional tin whiskers that may grow on Cassini cannot carry enough current to cause problems, but will burn out on their own like a lightweight fuse.



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