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16 SMT Magazine • September 2016 THE THEORY BEHIND TIN WHISKER PHENOMENA, PART 4 to both types of intermetallics—at the interface between the tin coating and the substrate, and in the bulk of the tin coating. When comparing Sn coating between Cu substrate and Ni substrate, Ni substrate tends to retard the whisker formation. This phenom- enon related to inter-diffusion rate correlates well with the relative diffusion rates between Cu and Sn vs. Ni and Sn. Cu has a higher diffusion rate into tin than the tin into Cu. As a result, tin lattice is distorted and the tin lattice spac- ing is altered. The change in lattice spacing may impart stresses to the tin plating layer, and the stress may then seek to be relieved, which con- tributes to the driving force for the formation of tin whiskers. It also relates to the relative reac- tivity between Cu and Sn vs. between Ni and Sn for the formation of intermetallic compounds. The mobility of tin atoms and the level of net stress imposed by IMCs in bulk matrix appear to be a trade off to the propensity of whisker growth. It should be noted that the critical difference between SnPb and SnAgCu alloy is that SnPb does not (should not) form intermetallics in the bulk matrix, but SnAgCu alloys intrinsically contain intermetallics. The fact of the presence of intermetallics in SnAgCu and the absence of such in SnPb, in conjunction with micro struc- tural and other metallurgical distinctions be- tween SnPb and SnAgCu systems, account for the differences in most phenomena and prop- erties between SnAgCu and SnPb, including tin whisker. It is not an exaggeration to state that such differences in most phenomena and properties between SnAgCu and SnPb, including tin whis- ker, were expected even without conducting tests. Part 5 will address the last process behind tin whisker phenomena—crystal structure and defects. SMT Dr. Jennie S. Hwang is a for- ward thinker, an international businesswoman and speaker, and a business and technology advi- sor. She is a pioneer of and long- standing contributor to SMT man- ufacturing since its inception, as well as to the lead-free electronics implementation. Among her many awards and honors are induction into the International Hall of Fame—Women in Tech- nology, election to the National Academy of En- gineering, YWCA Women Achievement Award, and being named an R&D-Stars-to-Watch (In- dustry Week). Having held senior executive posi- tions with Lockheed Martin Corp., Sherwin Wil- liams Co., Hanson, plc, IEM Corp., she is cur- rently CEO of H-Technologies Group, providing business, technology and manufacturing so- lutions. She serves as Chairman of Assessment Board of DoD Army Research Laboratory, Na- tional Institute of Standards and Technology (NIST), National Materials and Manufacturing Board, Board of Army Science and Technology, Commerce Department's Export Council, vari- ou s 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 author on trade, business, education, and social issues. Her formal education includes four academic de - grees (Ph.D., M.A., M.S., B.S.) as well as Harvard Business School Executive Program and Colum- bia University Corporate Governance Programs. For more information, click here. " The mobility of tin atoms and the level of net stress imposed by IMCs in bulk matrix appear to be a trade off to the propensity of whisker growth. "