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14 SMT Magazine • September 2014 2. Control coating grain size Tin plating process parameters control the lattice defects incorporated in the tin layer. The initial coating grain size is also determined by the coating process. For pure tin, the objective is to avoid the formation of ultra-fine grains, producing a grain size in the range of 1–10 mm. 3. Control coating thickness To achieve the proper stress management ability, the thickness of the tin coating plays an important role, which is associ- ated with the plating process. The tricky thickness to be desired falls either thin, at around or below 2 mm, or thick (more than 8 mm). 4. Direct crystal (phase) structure of the coating layer The crystal or phase struc- ture also depends upon the coating process. Albeit with delicate control, the desired structure can be accomplished. Its practicality varies with the manufacturer. The target struc- ture must be equivalent to an equi-axed structure, as exhib- ited in tin-lead coating. 5. modulate substrate surface topology Physically maneuvering the surface rough- ness of the substrate on which the coating is to be deposited to a rougher state can improve the properties of the interface between the coat- ing and the substrate, which in turn contributes to the reduction of the overall driving force of whiskering. 6. Use of underlying barrier for Cu substrate An Ni layer with nominally 0.5 to 2 micron thickness is found to be effective in reducing (not eliminating) the whiskering propensity. 7. minimize mechanical deformation Avoid imposing external mechanical force on the coating surface. 8. minimize heat excursion Although the intermetallic compounds at the interface of the tin coating and substrate, or in the bulk of the tin coating, are not neces- sary for the occurrence of tin whiskers, inter- metallic compounds may exert additional ef- fects in grain structure and stress distributions. The presence of intermetallics in SnAgCu and the absence of such in SnPb account for most of the phenomenal and property differenc- es between SnAgCu and SnPb, including tin whiskers. 9. Keep coating surface intact Make sure that the exposed surface is free of contami- nation and corrosion. Also, avoid creating surface notch- es, scratches, and grooves. 10. monitor metallic or non-metallic particulate inclusions As metallic particles en- ter into the tin lattice (or tin matrix), they may or may not lead to the formation of inter- metallic compounds or other phases, depending on the metallurgy of the elements involved. These particles can change or distort the lattice spacing in tin structure, potentially serving as nucleation sites. 11. minimize Cte mismatch of the system The relative coefficient of thermal expan- sion (CTE) between the tin plating and the lead material (e.g., alloy 42 vs. Cu) is a factor. But the whisker propensity is not necessarily pro- portionate to the degree of CTE mismatch. 12. Add additional stress-relief steps This can be done by melting the coating through fusion or reflow process. A heat treat- ment, annealing at an elevated temperature (e.g., 150°C, for one hour) can also achieve the goal. The caveat is that this is not a permanent cure. The uncertainties associated with tin whisker growth make it difficult to predict if or when tin whiskers may appear, nonethe- less, this list of tactics provides options and approaches for reducing the risk of tin whisker-induced failures. " " SMT ProSPeCTS & PerSPeCTIveS tin Whiskers, part 6: preventive and mitigating measures—Strategy and tactics continues