Could you tell me if there is a minimum tin plating thickness fo whisker growth?
Do you know if electroless tin plating as a finish for PCB's is safe?
Gaby
jong s kadesch wrote:
> 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.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
>
> 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
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