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12 SMT Magazine • March 2014 by Dr. Jennie S. hwang h-TeChnologieS group SmT PrOSPEcTS & PErSPEcTIvES coluMn Tin Whiskers, Part 4: causes and contributing Factors In this installment of the tin whisker series, we'll take a look at causes and factors that have been found or are considered to contribute to the appearance of tin whiskers. In the remain- ing installments, we will address "Tin Whis- kers—Plausible Theory," "Tin Whiskers—Impact of Testing Conditions," and "Tin Whiskers—Preven- tive and Mitigating Measures." As all-encompassing tests to confirm or deny the culprits that cause tin whiskers are prohibi- tively costly and time-consuming, my thoughts focus on the logical causes and contributors. Fundamentally, the tin whisker follows the ba- sic physical metallurgy in its principles on nu- cleation and crystal growth through the classic theories of dislocation dynamics and of other lattice defects in tin crystal structure. Thus, for whiskers to appear from the tin-plated (or tin- coated) surface, the causes and contributing fac- tors should be intimately related to the nucle- ation sites creation and the subsequent growth paths after the coating process. However, for tin whisker due to tin's in- trinsic characteristics, the actual processes of nucle- ation and grain growth are dauntingly complex. Nucleation and growth can be encouraged by stresses introduced during and after the plat- ing process. The sources of these stresses come from multi-fronts. This includes residual stresses caused by electroplating and/or additional stress- es imposed after plating, and/or the induced stress- es by foreign elements, and/or thermally-induced stresses. Specific causes and contributing factors are outlined below. Organic Inclusions Organic inclusions af- fect the tin crystal struc- ture by distorting or crowd the crystal lattice,

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