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May 2014 • SMT Magazine 15 to the external temperature (test temperatures) will change. However, recrystallization is only a part of the tin whisker process, and far from the whole story. Accelerated tests (e.g., temperature cycling, elevated temperature storage, etc.) that induce and/or aggravate the internal strains offer a fer- tile bed for tin whisker growth. With a given tin coating, the real-life excursion right after the coating process all contributes to its energy state. For all these reasons—both theoretical and practical, the inclusion of a reference material is in order when testing a new system. A reference material is also the simplest way to fill the gap. Based on the scientific principles as well as the decades' field service performance, a tin-lead reference material containing lead in the range of 3–37% is indispensable. And this defines tin- whisker-resistance. Testing tin whisker propensity, due to its underlying mechanisms, is a more challenging endeavor than testing solder joint reliability. Not to over-test nor under-test is the gist of the effort. Upcoming appearances Dr. Hwang will present an extensive semi- nar series Preventing Manufacturing Defects and Product Failures and PoP and BTC Updates at NEPCON Malaysia 2014, June 10–12. SmT Dr. Hwang, a pioneer and long- standing contributor to SMT manufacturing since its incep- tion as well as to lead-free de- velopment and implementation, has helped improve production yield and solved challenging reliability issues. Among her many awards and honors, she is inducted to the WIT In- ternational Hall of fame, elected to the na- tional Academy of engineering, and named an r&D-Stars-to-watch. having held senior executive positions with lockheed Martin Corp., Sherwin williams Co., SCM Corp, ieM Corp., she is currently Ceo of H-Technologies group, providing business, technology and manufacturing solutions. She has served on u.S. Commerce Department's export Council, various national panels/committees, and the board of Fortune 500 nySe companies and civic and university boards. She is the author of 400+ publications and several textbooks, and an international speaker and author on trade, business, education, and social issues. Her formal education includes four academic degrees (ph.D., M.S., M.a., b.S.) as well as harvard business School executive program and Columbia university Corporate gover- nance Program. To read past columns, or to contact the author, click here. for further info, go to: engineers would love to create flexible electronic devices, such as e-readers that could be folded to fit into a pocket. one approach involves designing circuits based on electronic fibers known as carbon nano - tubes (CnTs) instead of rigid silicon chips. But reliability is essential. now a team at Stanford has developed a process to create flexible chips that can tolerate power fluctuations in much the same way as silicon circuitry. "This is the first time anyone has designed flexible CnT circuits that have both high immu- nity to electrical noise and low power consump- tion, " said Zhenan Bao, a professor of chemi- cal engineering at Stanford. The Bao lab reported its findings in the Proceedings of the national academy of Sciences. Flexible Carbon nanotube Circuits Made More Reliable SMT proSpeCTS & perSpeCTiveS TIN WHISKERS, PaRT 5: ImPaCT oF TESTING CoNDITIoNS continues

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