good article and links on 'tin whiskers' avionics hazard

good article and links on 'tin whiskers' avionics hazard



Close Shave? NASA Dismisses Sn Whisker Worries

http://circuitsassembly.com/cms/content/view/3490/95/

By Mike Buetow

Friday, 09 June 2006

WASHINGTON, DC -- NASA has decided to push forward with next month's
scheduled launch of its Discovery space shuttle, even in light of an
internal report that noted the presence of tin whiskers in some computer
avionics boxes.

NASA's decision is a rebuttal to a recent report by The Aerospace
Corporation, which found that "The flight rationale currently proposed that
would allow flight 'as is' of critical Flight Control System (FCS) avionics
boxes suspected of whisker infestation cannot be validated with the
available data and, therefore, is not acceptable for flight.

...snip

NASA is aware of tin whisker-related problems: It's Goddard Space
Flight Center maintains a Web site that lists historical failures due to tin
whiskers (http://nepp.nasa.gov/whisker/index.html).

snip

GO FEVER Hey we MUST keep our jobs, this things got to go!

NASA learned nothing from columbia, same old same old

Tin whiskers are a very serious problem in electronics. They are also
not well understood either. The general impression is that tin is
stress relieving itself out of solder. Solder is often a Lead/Tin
alloy. From what I have learned, whiskering has become a problem as the
lead content decreases in lead/tin solder. The drive to reduce lead
comes from concerns over the environment and lead toxicity. From my
conversations with materials engineers working on aerospace projects,
whiskering is something to be avoided, especially in microgravity where
these whiskers can grow, detach, and float to short out other
components. I have seen electon microscope photos of tin whiskers,
they are very small.

Gene DiGennaro
Baltimore, Md.

wrote in news:1150077308.045552.40990
@g10g2000cwb.googlegroups.com:

Tin whiskers are a very serious problem in electronics. They are also
not well understood either. The general impression is that tin is
stress relieving itself out of solder. Solder is often a Lead/Tin
alloy. From what I have learned, whiskering has become a problem as the
lead content decreases in lead/tin solder. The drive to reduce lead
comes from concerns over the environment and lead toxicity. From my
conversations with materials engineers working on aerospace projects,
whiskering is something to be avoided, especially in microgravity where
these whiskers can grow, detach, and float to short out other
components. I have seen electon microscope photos of tin whiskers,
they are very small.

The "tin whiskers" problem in the shuttle is a little different. On the
shuttle, there is no concern with the solder because they use old-fashioned
solder with high lead content. Instead, the concern is the tin-plated card
guides in three of the shuttle's avionics boxes, the ASAs, ATVCs, and RJDs.
These card guides have been observed to generate whiskers. On the other
hand, the circuit boards in those boxes are conformally coated, so in order
to cause a short, a whisker would first have to break loose from the card
guides (no small feat - they're quite strong for their size, since they're
basically a crystalline extrusion), *and* float to where it can bridge two
contacts that either had inadequate conformal coating, or were not coated
at all because they were "shadowed" by the component above them (e.g. the
leads below a capacitor or transistor).

Further mitigating the risk is that the ASAs and ATVCs are Crit 1R3,
meaning that they're two-fault tolerant (catastrophe requires three boxes
to fail). The RJDs are Crit 1 during ISS docked ops, but they're powered
off right after docking until just before undocking, so the time of
exposure is very short.


--
JRF

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