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March 2014 • SMT Magazine 15 that the presence of intermetallic compounds in the tin coating layer or at the interface be- tween tin layer and substrate is not a require- ment for tin whisker formation. Intermetallic compounds It should be emphasized that inter- metallic compounds at the inter- face of the tin coating and the substrate or in the bulk of the tin coating is not a necessary condition for the occurrence of tin whisker. However, in- termetallic compounds may exert additional effects in grain structure, as these com- pounds can form in various geometries and morphologies ranging from small, more- rounded particles to long nee- dles. This formation creates either high localized stress or well-distributed stress or both in the tin lattice structure. It should also be noted that the critical difference between SnPb and SnAgCu alloy is that SnPb does not (should not) form intermetallics in the bulk matrix, but SnAgCu alloys intrinsically contain intermetallics. The presence of intermetallics in SnAgCu and the absence of such in SnPb account for most of the 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 level, the dif- fusion rate of substrate atoms into the tin coat- ing layer with or without the companion of the formation of intermetallics may skew the linear relationship between CTE mismatch and the whisker propensity. Plating Process vs. coating Surface morphology Tin plating process parameters control the lattice defects incorporated in the tin layer. They also determine the thickness of the coat- ing layer. The organic content, grain size, and surface morphology highly depend on the coating chemistry and process parameters, including the type of electrolyte, additive/bright- eners, current density, process temperature, and the process control. For instance, high current density allows faster rate of plating, and a faster rate may impede the tin at- oms' ability to rearrange to a low-energy state, which con- tributes to subsequent whis- kering conditions. Take bright tin as an ex- ample, which is reportedly the most susceptible to tin whiskers. Its high susceptibil- ity is largely attributable to the high residual stresses with- in the tin plating caused by the plating chemistry and process. The added brighteners in making tin bright may serve as nucleation sites and may prevent tin from settling into the low energy state to form large grains. The resulting small grains provide more grain boundaries that in turn offer diffu - sion paths for tin. Plating Process vs. coating crystal Structure The effect of microstructure in terms of grain size on whiskers has been observed. It is hypothesized that as grain size reduces below 1 micron, the internal stress and the driving force for recrystallization will be built up. This condition creates high whisker pro- pensity. Thickness of Tin coating It is postulated that it takes a proper thick- ness for whisker to grow. To make a statement on the correlation between the thickness of tin layer and the whisker propensity is indeed over- Take bright tin as an example, which is reportedly the most susceptible to tin whiskers. Its high susceptibility is largely attributable to the high residual stresses within the tin plating caused by the plating chemistry and process. " " SMT proSpeCTS & perSpeCTiveS TIN WhISkErS, ParT 4: caUSES aND cONTrIBUTING FacTOrS continues