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14 SMT Magazine • November 2016 high ductility. Lattice structures with close- ly packed planes allow more plastic deforma- tion than those that are not closely packed. It is easier for planes of atoms to slide by each oth- er if those planes are closely packed. For exam- ple, lead (Pb) with FCC lattice structure exhibits higher ductility than tin (Sn) with tetrahedral lattice. However, when obstacles are introduced into the lattice structure, such as interstitial at- oms or grain boundaries, dislocations can be pinned and their movements hindered. In ad- dition, if more dislocations are produced they will get into each other's way and impede their own movements. Within cubic lattice, a FCC crystal structure will exhibit more ductility (deform more readily under load before breaking) than a BCC struc- ture. The BCC lattice, although cubic, is not closely packed and forms strong metals (e.g., al- pha-iron and tungsten). The FCC lattice is both cubic and closely packed and forms more duc- tile materials (e.g., silver, gold, and lead). Comparing between cubic-lattice (FCC, BCC) and non-cubic lattice (HCP, tetragonal, or- thorhombic, monoclinic) structures, cubic-lat- tice structures allow slippage to occur more eas- ily than non-cubic lattices because their sym- metry provides closely packed planes in several directions. In comparison, hexagonal close packed (HCP) lattices are closely packed, but not cubic. HCP metals (e.g., cadmium, cobalt and zinc) are not as ductile as the FCC metals. The FCC and HCP structures both have an APF of 0.74 and a coordination number of 12, consisting of closely packed planes of atoms (vs. APF of BCC = 0.68). The difference between the FCC and HCP is the stacking sequence. The HCP lay- ers cycle among the two equivalent shifted po- sitions whereas the FCC layers cycle between three positions. So how does crystal structure affect tin whis- ker? Tin possesses a non-cubic crystal structure (tetragonal), thus it does not allow agile slip- page to readily occur and cannot proceed de- formation easily. Indium also has a tetragonal crystal structure. This "inconvenient slippage" contributes to the driving forces in forming whiskers. Zn has a HCP structure. Comparing Sn with Zn, Sn's lower APF (0.54) further facili- tates whisker process as the result of more free diffusion distance, thus it is expected that Sn is even more prone to whisker than Zn. From crystal structural perspective, among the common metals used in electronics, what is the relative whisker propensity? If the role of crystal structure is a pure play in whisker process, tin and indium are more prone to whisker than zinc. In turn, Zn is more prone to whisker than Pb, Ag, Au and Cu. Part 6 will conclude the series by summa- rizing the theory behind tin whisker phenom- ena. SMT Dr. Hwang is a forward thinker, an international businesswoman, inter- national speaker, and a business and technology advisor. She is a pioneer of and long-standing contributor to SMT manufacturing since its inception, as well as to the lead-free electronics implementation. Among her many awards and honors are induc - tion into the International Hall of Fame—Women in Technology, election to the National Academy of Engineering, YWCA Women Achievement Award, and being named an R&D-Stars-to-Watch (Industry Week). Having held senior executive positions with Lockheed Martin Corp., Sherwin Williams Co., Han - son, plc, IEM Corp., she is currently CEO of H-Tech- nologies Group, providing business, technology and manufacturing solutions. She serves as Chair- man of Assessment Board of DoD Army Research Laboratory, National Institute of Standards and Technology (NIST), National Materials and Manu- facturing Board, Board of Army Science and Tech- nology, Commerce Department's Export Council, various 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 sever- al textbooks, and a speaker and author on trade, business, education, and social issues. Her formal education includes four academic degrees (Ph.D., M.A., M.S., B.S.) as well as Harvard Business School Executive Program and Columbia University Corpo- rate Governance Programs. For more information, click here. THE THEORY BEHIND TIN WHISKER PHENOMENA, PART 5

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