[tinwhiskers] NASA experiments: conformal coat vs. tin whiskers
- From: "Brusse, Jay A. (GSFC-560.0)[QSS]" <jay.a.brusse@xxxxxxxx>
- To: <tinwhiskers@xxxxxxxxxxxxx>
- Date: Fri, 14 Mar 2008 12:55:03 -0500
Below I describe our ongoing research at NASA Goddard Space Flight Center into
the effectiveness of (Polyurethane) conformal coat to reduce the threat from
metal whisker induced electrical shorting. In December 2007 I presented an
update of our ongoing examination of Arathane 5750 (formerly Uralane 5750)
conformal coat vs. tin whiskers as part of a larger presentation on metal
whiskers.
"Metal Whiskers: Failure Modes & Mitigation Strategies", J. Brusse, H.
Leidecker, L. Panashchenko, MRQW, Dec. 2007
http://nepp.nasa.gov/whisker/reference/tech_papers/2007-brusse-metal-whiskers.pdf
<https://mail01.ndc.nasa.gov/exchweb/bin/redir.asp?URL=http://nepp.nasa.gov/whisker/reference/tech_papers/2007-brusse-metal-whiskers.pdf>
Slides 16 thru 29 encompass a good discussion of our observations after 9 years
of monitoring profuse whisker forming test articles that were selectively
conformal coated to evaluate the effectiveness of the coating to inhibit growth
and/or escape outward through the coating. The experiment described in this
recent presentation is a continuation of Leidecker and Kadesch's report from
2000.
Capsulizing our research:
- Dec. 1998 - we procured tin-plated brass coupons as whisker growing test
vehicles.
- Within 1 week of plating: applied Uralane 5750 conformal coat to ½ of top
side of each coupon (nominal 2 mils thickness)
- Tin nodules and whiskers began to grow on the coupons within the first few
months.
- We stored coupons in lab ambient conditions as well as at 50C
- In 2000, Leidecker/Kadesch published observations
- Conformal coat did NOT stop whiskers from forming
- HOWEVER, 2 mils of this particular polyurethane was effectively stopping
outward escape of whiskers growing beneath the coating.
- Whiskers were able to push through THIN areas of our coating
- After 9 years of watching our samples
- We have profuse density of whiskers growing on the NON-conformally coated
control areas.
- Where the coating is nominal 2 mils thick we find ZERO whiskers growing
outward through the coating. Instead the whiskers lift the 2 mils of coating
slightly in the shape of a dome (e.g., circus tent lifted by a single
pole/whisker), but fail to completely penetrate. Instead, we believe the
whiskers crumple/buckle before penetration
- Where the coating is THIN (i.e., <0.2 mils) the whiskers CAN escape outward
through the coating. (See slides discussing Euler Buckling strength of the
whiskers --> buckling strength decreases as inverse square of whisker length)
- We believe having conformal coat on adjacent conductors is a God-send against
whisker induced shorting.
- Shorting distances in MOST (not all) circuits is usually large enough such
that the buckling strength of a whisker capable of bridging that gap is so low
that the whiskers will not be able to RE-ENTER the distant coated surface to
make a physical short. However, be cognizant of dielectric breakdown of the
thin coating in the event your circuits use high voltage. Direct physical
contact may not be necessary to cause harm in such cases.
- Any detached (loose) whiskers or other conductive debris will not present a
shorting hazard for properly conformal coated surfaces. Once detached the
whiskers do NOT continue to grow
- Warning: Conformal coating processes often suffer from incomplete coating.
For example, shadowing effects during spray application, thinning of the
coating from surface tension and other forces during curing can leave you with
"less coverage" than the process documentation tells you that you have. Good
idea to INSPECT and verify YOUR coating processes ability to coat all areas of
interest to you.
- There is a low but non-zero risk of electrical shorting from one whisker to
another whisker if they are growing from surfaces at different electrical
potentials - Electrostatic attraction can impose a "come hither" force bending
whiskers towards surfaces at different electrical potential. We do not think
this electrostatic force has an appreciable effect on whether or not whiskers
will grow nor how fast.
Tom Woodrow at Boeing has published work on conformal coat vs. whiskers using a
variety of different coatings. His results show whisker escapes outward for
certain coatings acrylics, silicones ESPECIALLY when stored in VERY HUMID
CONDITIONS. However, despite some escapes, the coatings are still blocking
MANY MANY whiskers and providing a benefit. You may read his work directly:
"Evaluation of Conformal Coatings as a Tin Whisker Mitigation Strategy, Part 2"
<https://mail01.ndc.nasa.gov/exchweb/bin/redir.asp?URL=http://nepp.nasa.gov/whisker/reference/tech_papers/2006-Woodrow-Conformal-Coating-PartII.pdf>
, T. Woodrow, SMTAI, Sept. 2006
http://nepp.nasa.gov/whisker/reference/tech_papers/2006-Woodrow-Conformal-Coating-PartII.pdf
<https://mail01.ndc.nasa.gov/exchweb/bin/redir.asp?URL=http://nepp.nasa.gov/whisker/reference/tech_papers/2006-Woodrow-Conformal-Coating-PartII.pdf>
T. Woodrow, "Evaluation of Conformal Coatings as a Tin Whisker Mitigation
Strategy", IPC/JEDEC 8th International Conference on Pb-Free Electronic
Components and Assemblies, San Jose, CA, April 18-20, 2005
http://nepp.nasa.gov/whisker/reference/tech_papers/2005-woodrow-tin-whisker-conformal-coat-study.pdf
http://nepp.nasa.gov/whisker/reference/tech_papers/2005-Woodrow-tin-whisker-conformal-coat-presentation.pdf
<https://mail01.ndc.nasa.gov/exchweb/bin/redir.asp?URL=http://nepp.nasa.gov/whisker/reference/tech_papers/2005-Woodrow-tin-whisker-conformal-coat-presentation.pdf>
Sincerely,
Jay Brusse
Sr. Components Engineer
Perot Systems at NASA Goddard
Jay.A.Brusse@xxxxxxxx
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