[tinwhiskers] Re: NASA experiments: conformal coat vs. tin whiskers
- From: Steve Smith <steve@xxxxxxxxxxxxxxxxxxx>
- To: tinwhiskers@xxxxxxxxxxxxx
- Date: Fri, 14 Mar 2008 11:11:42 -0700
Jay,
What was the ambient humidity in your storage tests?
Humidity can affect the mechanical properties of polyurethanes,
sometimes severely.
I have no data on high-humidity mechanical properties of 5750. Do
you?
Steve Smith
-----------------------------------------------------------------------
> Thank you for that data. It was interesting that you feel 2 mils of
> Polyurethane prevents whisker penetration.
> Dewey
>
> ________________________________
> From: tinwhiskers-bounce@xxxxxxxxxxxxx
> [mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of Brusse, Jay A.
> (GSFC-560.0)[QSS]
> Sent: Friday, March 14, 2008 10:55 AM
> To: tinwhiskers@xxxxxxxxxxxxx
> Subject: [tinwhiskers] NASA experiments: conformal coat vs. tin whiskers
>
> 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
--
Best regards,
Steve Smith <steve@xxxxxxxxxxxxxxxxxxx>
www.woodrestoration.com www.fiveyearclear.com
www.smithandcompany.org, and especially www.smithandcompany.org/mwp/
http://www.lignu.com/lignu/tech_info/tech_info.php
www.consultingscientist.us
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