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14 SMT Magazine • March 2014 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 with similar grain sizes generated the following results: For the coating containing 0.2% carbon, 235 µm whisker was formed while 12 µm whisker was formed for the coating containing 0.05% carbon content [1] . Surface Physical condition Surface conditions, such as notches and scratches in the coating surface, can be a source of atomic irregularity, which could contribute to the driv- ing force of tin whisker forma- tion. Substrate Surface morphology Physically maneuvering the surface morphology of the substrate in the level of roughness was found to alter the tin whisker propensity— a rougher surface being less prone to tin whiskers [2] . It is be- lieved 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 the tin crystal structure, oxygen can serve as nuclei and can also restrain grain boundary mobility and diffusion. When the lat- tice structure is oriented in such a way that is favorable to the protruding crystal growth, tin whiskers will occur. Some studies found that surface oxide (commonly tin oxide) promotes tin whiskers [1] . And studies also found that con- tamination contributes to tin whisker [3] . External mechanical Stresses Externally applied forces such as those intro- duced by the lead-forming, bending or torque after plating process may affect tin whisker for- mation. In studying the effect of external me- chanical force that is imposed on the coating on tin whisker growth, the relative whisker growth under different levels of organic inclusions with and without an exter- nal mechanical force was performed. Under each level of organic inclusions, an ex- ternal mechanical force (by means of bending) created an increased rate of whisker growth as shown in Table 1 [4] . Substrate Base material It was found that there is a difference in tin whisker propensity between bronze and brass and between Cu- based and Alloy 42 leads, re- spectively. The differences are primarily attributed to relative inter-diffusion between the sub- strate material and the tin coating, as well as to the relative abundance of intermetallic com- pounds. metallic Impurities As metallic particles enter into the tin lat- tice, they may or may not lead to the formation of intermetallic compounds, depending on the metallurgy of the elements involved. These me- tallic particles can change or distort the lattice spacing in the tin structure. It should be noted Organic Impurity No mechanical Bend mechanical Bend 0.2%, 4 months 245 microns 312 microns 0.004%, 7 months 6 microns 6 microns Table 1. In studying the effect of external mechanical force that is imposed on the coating on tin whisker growth, the relative whisker growth under different levels of organic inclusions with and without an external mechanical force was performed. " " SMT proSpeCTS & perSpeCTiveS TIN WhISkErS, ParT 4: caUSES aND cONTrIBUTING FacTOrS continues