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July 2014 • SMT Magazine 21 TIN WHISKERS: CAPSulIzATIoN continues fEATURE tin whiskers will occur. Other studies found that surface oxide promotes tin whiskers [1,2] and surface corrosion and contamination also contribute to tin whiskers [4,5] . It was also found that whisker growth occurred on SAC305 solder joints on either the copper or the alloy 42 leaded components, and the al- loy 42 leads exhibited a delay in long whisker growth [4,5] . External Mechanical Stresses Externally applied forces such as those introduced by the lead-forming, bending or torque after plating process may affect tin whisker formation. In studying the effect of external mechanical force that is imposed on the coating on tin whisker growth, the rela- tive whisker growth under different levels of organic inclusions with and without an exter- nal mechanical force were performed. Under each level of organic inclusions, an external mechanical force (by the means of bending) created an increased rate of whisker growth as shown in Table 1 below [1] . Substrate Base Material It was found that there is a difference in tin whisker propensity between bronze and brass [6] and between Cu-based and alloy 42 leads, re- spectively. The differences are primarily attrib- uted to relative inter-diffusion between the sub- strate material and the tin-based materials, as well as to the relative abundance of intermetal- lic compounds. Metallic Impurities As metallic particles enter into the tin lat- tice, there may or may not lead to the forma- tion of intermetallic compounds, depending on the metallurgy of the elements involved. These metallic particles can change or distort the lat- tice spacing in the tin structure. Intermetallic Compounds It should be emphasized that intermetallic compounds at the interface of tin coating and the substrate or in the bulk of the tin-based ma- terial is not necessary for tin whiskers to form. However, intermetallic compounds may ex- ert additional effects in grain structure, as these compounds can form in various geometries and morphologies ranging from small, more-round- ed particles to long needles. This formation cre- ates either high localized stress or well-distribut- ed stress or both in the tin lattice structure. It should also be noted that the critical dif- ference between SnPb and SnAgCu alloy is that SnPb does not (should not) form intermetallics in the bulk matrix, but SnAgCu alloys intrin- sically contain intermetallics. The presence of intermetallics in SnAgCu and the absence of such in SnPb account for most of phenomenal and property differences between SnAgCu and SnPb, including tin whisker. CTE Mismatch Between Tin Coating and Substrate The relative coefficient of thermal expan- sion between the tin plating and substrate can contribute to the occurrence of tin whisker as the result of additional global stress as well as localized stresses. In this regard, the lead mate- rial (e.g., alloy 42 vs. Cu) is a factor. Although the larger mismatch between the tin layer and the substrate causes higher stress levels, the dif- fusion rate of substrate atoms into the tin-based material layer with or without the companion of the formation of intermetallics may skew the linear relationship between CTE mismatch and whisker propensity. Plating Process vs. Coating Surface Morphology Tin plating process parameters control the lattice defects incorporated in the tin layer. It Table 1. organic Impurity No Mechanical Bend Mechanical Bend 0.2%, 4 months 245 microns 312 microns 0.004%, 7 months 6 microns 6 microns