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78 SMT Magazine • February 2015 to model the actual lead geometry rather than 'fooling' it with artificial values for some as- pects of the lead definition. Also, further testing of components should be conducted to better define the actual whisker densities and length distributions that could be expected in these parts. Despite these limitations, the model does appear capable of providing reasonable insight into whisker shorting risks, even with minimal test data. discussion The maximum allowable tin whisker length values specified in the JESD201 specification are shown in Table 1. Rockwell Collins utilizes the Class 2 values to assess tin whisker risk. The maximum tin whisker length permissible for the high temperature humidity conditioning sequence is 40 microns after 4000 hours. The authors consider the risk of a tin whis- ker issue for the Samtec SEARAY connector to be very low based on the following investigation results and observations: • The 85ºC/85%RH testing produced two leads on which tin whiskers were observed, both with lengths of less than 40 microns being recorded. In total, two leads out of a to- tal of 14,580 leads on the parts included in the test (0.014%) exhibited whiskers in this inves- tigation. • The two instances of recorded tin whis- kers that were observed on non-soldered com- ponents occurred in a region of the connector that would be covered with solder as a result of the reflow soldering process. Industry publica- tions [4, 6] have shown that tin whiskers do not initiate and grow in regions where there is sol- der coverage. • The portion of the exposed connector lead that is considered at risk for tin whisker initia- tion/growth (because it is not covered with sol- der by the reflow process) has an underplate of nickel. The SEM assessment revealed that a por- tion of the pure tin in this region of the lead forms intermetallic phases during the reflow soldering process. The reduction of the pure tin thickness and the use of nickel as an underplate are both accepted industry tin whisker risk miti- gation reduction methodologies [4] . • In addition, the majority of the lead area that is considered at risk for tin whisker ini- TIN WHISKer rISK aSSeSSMeNT OF a TIN SurFaCe FINISHed CONNeCTOr continues Feature Table 1: JeSD201 maximum tin whisker length criteria [10] .