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12 SMT Magazine • November 2016 In this installment of the series, we will complete the discussion of key processes likely engaged in tin whisker growth. These key pro- cesses include: 1. Grain boundary movement and grain growth 2. Energy dynamic of free surface 3. Role of recrystallization 4. Solubility and grain growth in response to external temperature 5. Lattice vs. grain boundary diffusion 6. Reaction and dynamic of intermetallic compounds 7. Crystal structure and defects In Parts 2, 3, and 4, we discussed the first six; now we will outline the last processes for crystal structure and defects. Crystal Structure and Defects Crystal structure is the bedrock of properties and behavior of materials. Crystal structure defects and the dynam- ics of defects including dislocation structure and its movements account for the character- istics and performance of materials. Because the stress needed to move dislocations increas- es with the spacing between the planes, dislo- cations are expected to move along the densest planes of atoms in a material. Let's take two pri- mary parameters of crystal lattice into consider- ation: type of unit cell and atomic packing fac- tor (APF). Face-centered cubic (FCC) and body-cen- tered cubic (BCC) metals have more dense planes than other crystal structures, so dislo- cations move relatively easy and the materi- als possessing these crystal structures impart by Dr. Jennie S. Hwang H-TECHNOLOGIES GROUP The Theory Behind Tin Whisker Phenomena, Part 5 SMT PROSPECTS & PE RSPECTIVES

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