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November 2015 • SMT Magazine 15 sMT prospecTs & perspecTIves A LooK AT THe THeory beHIND TIN WHISKer PHeNomeNA, PArT 3 researchers at the university of cambridge, in collaboration with novalia, have developed a low-cost, high-speed method of printing graphene inks using a conventional roll-to-roll printing process. The method allows graphene and other electrically conducting materials to be added to conventional water-based inks and printed using typical commercial equipment. "We are pleased to be the first to bring graphene inks close to real-world manufac - turing. There are lots of companies that have produced graphene inks, but none of them has done it on a scale close to this," said Dr Tawfique Hasan of the cambridge graphene centre (cgc), who developed the method. "being able to produce conductive inks that could effortlessly be used for printing at a com - mercial scale at a very high speed will open up all kinds of different applications for graphene and other similar materials." "This method will allow us to put electronic systems into entirely unexpected shapes," said chris Jones of novalia. Graphene-based Inks for High-speed Manufacturing of Printed Electronics For an impure metal, as the temperature is raised, the solubility of impurities increases. When a fine-grained sample is heated just below the solubility temperature of precipitates and as the particles dissolve and coarsen, a non-uni- form grain growth may occur. This may result in a few coarse grain sizes due to a few boundar- ies (without restrictions) that are released and sweeping quickly through the matrix. However, if the same fine-grained sample is quickly heat- ed to a temperature above the solubility limit, all grain boundaries are released together, lead- ing to a more uniform grain growth across the structure. Under the environment that is prone to whiskers, the nucleation rate can affect the growth pattern. As temperature increases, the nucleation rate increases, leading to many short and nodular whiskers. Similarly, when impuri- ties (vs. pure tin) are present in the tin coat- ing layer, whiskers grow in a large number, but short and nodular. Comparing pure tin with SnCu coating, with all other conditions being equal, pure tin exhibits whiskers many times in length than SnCu coating. As the temperature is lower, nucleation rates decrease, and long and needle-like whiskers will form if there is suffi- cient stored energy (threshold strain). Discussions of the key processes engaged in tin whisker growth will continue and be com- pleted in Part 4, and Part 5 will summarize the theory behind tin whisker phenomena. SmT Dr. Jennie Hwang is an inter- national businesswoman and speaker, business and technology advisor, and a pioneer and long- standing contributor to SMT man- ufacturing since its inception, as well as to the lead-free electronics implemen- tation. among her many awards and honors, she is inducted to the WIT International Hall of fame, elected to the national academy of engineering, and named an r&D-Stars-to- Watch. Having held senior executive positions with lockheed Martin corp., Sherwin Williams co., Hanson, plc, IeM corp., she is currently ceo of H-Technologies group, providing business, technology and manufacturing solu- tions. She serves as chairman of Assessment board of DoD army research laboratory, national Materials and Manufacturing board, commerce Department's export council, vari- ous national panels/committees, international leadership positions, and the board of fortune 500 NYSe companies and civic and university boards. She is the author of 450+ publications and several textbooks, and a speaker and au- thor on trade, business, education, and social issues. Her formal education includes four academic degrees as well as Harvard business School executive program and columbia uni- versity corporate governance programs. for further info, go to www.JennieHwang.com.