I misspoke.  I meant to use "whiskers" all the way through.  I should NOT
have said dendrite.
Bev Christian

-----Original Message-----
From: jong s kadesch [mailto:[log in to unmask]]
Sent: May 9, 2001 3:31 PM
To: [log in to unmask]
Subject: Re: [TN] Sn whisker problem


Hi Ingemar,

I support NASA Goddard and I'd like to point you to our web site where we
have been collecting information on tin whiskers and reporting results from
our own on-going experiments.

http://nepp.nasa.gov/whisker/index.
<http://nepp.nasa.gov/whisker/index.html>  html
<http://nepp.nasa.gov/whisker/index.html>

To date, our experiments have focused on the effectiveness of the conformal
coating in preventing tin whisker formation.  Our boards are often coated
with Uralane 5750 and this may provide "some" protection against tin
whiskers, but we have found one whisker that protrudes through this
conformal coating (exact thickness not known yet).  Conformal coating
certainly slowed down whisker growth but may not be adequate for long-term
programs.

Whereas earth-based tin whisker shorting events may lead to minimal and/or
unnoticeable system anomalies (whisker may fuse at low current), tin
whiskers can cause major catastrophic failure in vacuum environment (space
application) because the shorting whisker may create a plasma capable of
sustaining dozens, if not hundreds of Amperes!!!!!  The following web site
reports that three satellites have failed due to this phenomenon.

http://sat-nd.com/failures/index.html?http://sat-nd.com/failures/hs601.html
<http://sat-nd.com/failures/index.html?http://sat-nd.com/failures/hs601.html
>

This is why NASA has taken such an interest in this subject.

Bev has a good point where the electronic industry going towards Pb free
attracts parts manufacturers to use pure tin plating more than ever.  It has
good solderability, is economical, and is an easy plating process.  Until
further research is done, for high reliability application (especially for
long duration), I would not rely solely on reflowing, matte finishes, or
different types of underlying materials.

We, certainly didn't have any problems growing whiskers in our farm.
Whiskers (more nodule-like growths) were observed under SEM or optical
microscope within a month of plating.  Bev also mentioned "tin dendrites".
Dendrites are a different phenomenon from tin whiskers.  Tin whiskers are
reportedly caused by the internal stress during plating process whereas tin
dendrites form under humidity and voltage.  Whisker do not require moisture
nor voltage to form.  There's also "tin pest" problem.  Below 13 degC,
metallic tin is not thermodynamically stable; rather, semiconductor tin is
the stable phase.  The electrical conductivity decreases many orders of
magnitude, the material is now brittle (not ductile), and occupies a larger
volume --- hence, the phase change usually turns the semiconductor into a
powder. The rate of transformation is irregular, and so the problem may not
show up during testing; rather, it shows up as field failures. The rate is
reported to maximize near -50 degC.

I'd welcome further discussion and experiences of other TechNet members.
Contribution from Henning Leidecker, Mike Sampson and Jay Brusse.


Jong S. Kadesch
Sr. Engineer
NASA-GSFC/Orbital Sciences Corp.
562/Component Technologies and Radiation Branch
Greenbelt, MD 20771
tel:(301)286-2785
fax:(301)286-1695



Jong S. Kadesch
Sr. Engineer
NASA-GSFC/Orbital Sciences Corp.
562/Component Technologies and Radiation Branch
Greenbelt, MD 20771
tel:(301)286-2785
fax:(301)286-1695