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20 SMT Magazine • July 2014 whisker follows the basic physical metallurgy in its principles on nucleation and crystal growth through the classic theories of dislocation dy- namics and of other lattice defects in tin crys- tal structure. Thus, for whiskers to appear from the tin-based (or coated) surface, the causes and contributing factors should be intimately related to the nucleation site creation and the subsequent growth paths. However, the actual processes of nucleation and grain growth of tin whisker are dauntingly complex. The nucleation and growth can be encour- aged by stresses introduced during and after the plating process. The sources of these stresses come from multiple fronts. This includes re- sidual stresses caused by electroplating and/or additional stresses imposed after plating, and/or the induced stresses by foreign elements, and/ or thermally-induced stresses. Specific causes and contributing factors are excerpted from my previous article (SMT Magazine, March 2014): organic Inclusions Organic inclusions affect the tin crystal structure by distorting or crowding the crystal lattice, thus creating the internal stress. It is found that tin whisker growth is correlated to the organic inclusions as represented in carbon content in the coating. A test conducted at 50°C for four months on coatings that have simi- TIN WHISKERS: CAPSulIzATIoN continues lar grain sizes generated the following results: 235 µm whisker was formed from the coating containing 0.2% carbon; 12 µm whisker was formed from the coating containing 0.05% car- bon content [1,2] . Surface Physical Condition Surface conditions, such as notches or scratches on the surface, are the source of atom- ic irregularity, which could contribute to the driving force of tin whisker formation. Substrate Surface Morphology Physically maneuvering the surface mor- phology of the substrate in the level of rough- ness was found to alter the tin whisker propen- sity—a rougher surface being less prone to tin whiskers [3] , as shown in Figure 3. It is believed that a relatively rougher surface facilitates the formation of an even interface between the tin coating and the substrate surface that contains a thinner and more uniform intermetallic layer. oxidation or Contamination level It is postulated that as the oxygen atoms diffuse into tin crystal structure, oxygen can serve as nuclei and can also restrain grain boundary mobility and diffusion. When the lattice structure is oriented in a way that is favorable to the protruding crystal growth, fEATURE figure 3: Tin whiskers—role of surface morphology (chemical micro-roughening to produce a set of surface roughness with specific Ra values).