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26 SMT Magazine • July 2014 • Organic content – <0.05% (a typical military requirement) • Coating grain size – 0.5 to 5 mm – (matte Sn 1 -10 mm) • Coating thickness – < / = 2 mm or > 8 mm • Coating surface morphology – Semi-bright • Coating crystal orientation • Additional process, e.g., – Fusion – Reflow – Annealing (150°C, one hour) • Surface intactness – Absence of surface corrosion – Free of surface notches, scratches, grooves… • Minimize deformation – Avoid external mechanical force imposed on the coating surface • Use of underlying barrier for Cu substrate – Ni layer with nominally 0.5 to 2 micron thickness • Minimize CTE mismatch of the system • Minimize heat excursion • Choice of conformal coating • Change to a composition that is less prone to whiskering when needed • Dipping process • Use of alloying tactics (vs. SnPb) • Most effective elements include Bi, In • SnCu is not a good in whisker-resistance In order to prevent and retard tin whisker growth, it is highly recommended to exercise the good practice by using aggregate tactics to suppress its driving forces to the level that is be- low the threshold. Relative Effectiveness— use of Alloy Tactic My anticipated effectiveness of tin-based materials in preventing and mitigating tin whis- ker formation and growth in descending order is depicted here: 1. SnBi, SnPb 2. SnZn 3. SnAg, SAC 4. SnCu 5. Sn Plausible Theories Tin whiskers occur by science. What are the driving forces that initiate the formation of whiskers? What sustain the growth? Can these driving forces be controlled practically and eco- nomically? These are million dollar questions and de- serve a deliberate treatment. Overall, disparities in theories and reports abundantly exist. Thus far, there is not a uniform conclusion on the theory and mechanism behind tin whisker occurrence. Discussion of plausible postulation will ap- pear in the future publication of my column se- ries on tin whiskers. Below outlines some key points to be addressed. Whisker involves an intricate and complex process. Under accelerated test conditions or in real life services, the understanding of tin whis- ker calls for a deeper atomic level treatment con- sidering crystal structure, crystal orientation, grain size, grain boundaries, grain boundaries mobility, atomic mobility, and lattice structural changes to foreign elements. This goes to the heart of physical metallurgy theories in crystal nucleation and grain growth, by normal growth and by abnormal (protruding) growth, from a high energy state to a low energy or to a stress- free state. Driving to the stress-free state involves sev- eral stages: • Forming nuclei • Nucleation • Grain and sub-grain growth • Impingement of grains • Classical grain growth Tin crystal structure (body-centered tetrago- nal, Figure 6) differentiates tin from other met- als that are less prone to whiskering. The aniso- tropic properties of tin result in different surface energies of grains exposed at the surface. This difference and the immobility of grain bound- aries pinned by surface grooves is expected to favor "abnormal" grain growth. Relatively speaking, the energy to drive grain growth is very low and so it tends to oc- TIN WHISKERS: CAPSulIzATIoN continues fEATURE