Letter 26110

European directives vs. Tin Whiskers & Tin Pest  

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Europe is pushing removal of Pb from everything come 2006. We are examining the alternatives. Pure tin (>99.5%) appears to be what the consumer industry is going with. We in the industrial market keep hearing about whiskering and "pest" We have examples of whiskers, and while we are worried, apparently the component manufacturers are, in general, not. Pest is something not even discussed.

Solders will probably be something like Sn[92]Ag[4]Cu[4]. Platings are apparently going to Sn. Will the alloying during the soldering process be sufficient to keep whiskers and pest from being an issue? Solder probably forms 90% of the bulk, but I would expect some metallurgically pure tin at the interfaces.

You folk appear to have experience with this, and so I would ask the following:
1) Anyone have alloying percentages to keep whiskering and pest under control?
2) Anyone have rates of decomposition for pest?
3) Any anecdotal information that could help?

The entire electronics industry appears to be moving this direction, and to be perfectly honest, there is a mint to be made by discussing tin related issues at electronics conferences. We have no experience with it (except some from tin plated brass hardware). We need to become instantly knowledgeable, since, with 2.5 years until implementation, there will be no time to correct for errors.

David Hiltner
test & measurement electronic engineer - Beaverton, OR, USA

The (U. S.) National Institute of Standards & Technology has conducted considerable research on lead-free solders. See "Properties of Lead-Free Solders" at http://www.boulder.nist.gov/div853/lead%20free/props01.html

NASA Goddard Space Flight Center has conducted a lesser amount of research on tin whiskers. Interesting website (including photos and results from other sources): http://nepp.nasa.gov/whisker/experiment/index.html. A page devoted to MIL & NASA prohibitions on the use of pure tin shows that alloying with 3% lead is specified in many cases, so anticipate changes. A University of Maryland site (http://www.calce.umd.edu/lead-free/tin-whiskers/) offers "Mitigation Strategies for Tin Whiskers" for circuit designers and platers. See also "Whiskers" under "Plating Problems" in the finishing.com FAQs.

Apply for a free subscription to NASA Tech Briefs . . .

Other sites with considerable info on lead-free solders and tin whiskers are the National Electronics Manufacturing Initiative (http://www.nemi.org/), www.TinTechnology.com (formerly the International Tin Research Institute) and Tin Technology Ltd. (http://www.tintechnology.biz/soldertec/) [link no longer works] There is little research on Tin Pest Disease; its metallurgical cause has been known for many years. The rate of transformation from white tin to gray tin as a function of cooling temperature was determined by J. H. Becker, Journal of Applied Physics, vol. 29, p. 1110 (1958). See Letters #0740, 18849 and 25988 for more info on tin pesting. Alloying with as little as 0.26 % antimony prevents pesting. Research on preventing tin whiskers via alloying is also relevant to the prevention of Tin Pest Disease. There are numerous professional and subscription-based journal articles on these subjects. A free, online article on lead-free solders: http://www.tms.org/pubs/journals/JOM/0106/Frear-0106.html

Hope this helps,

Ken Vlach - Goleta, California

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