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74 SMT Magazine • February 2015 3000 Hour/Cycle Inspection Interval Several small tin whiskers were observed at the 3000 hours inspection interval. The tin whiskers observed were approximately 20 mi- crons long and had very contorted shapes. The tin whiskers were observed in a region where solder would flow during the automated reflow process (Figure 9). Their location was in the same region as the single tin whisker observed at the 1500 hours inspection interval. SeM examination of reflowed Connectors In response to the two samples that were ob- served to have tin whiskers after 85°C/85%RH testing, a series of connectors that had been re- flow soldered were examined using SEM analy- sis. Figures 10 and 11 illustrate the regions of focus during the SEM examination. During the 4000 hours of testing, tin whiskers were not observed in the region of whisker risk concern (Figure 11, Region A) but instead in the region where solder would wet/flow during the reflow process (Figure 11, Region C). Verification of solder wetting in this region would demon- strate that the tin whiskers that were found dur- ing the 85°C/85%RH testing in Region C would not present a tin whisker risk in products. High magnification examination revealed good solder wetting and coverage in Region C of the reflow soldered connectors (Figure 12). The connector plating was also confirmed dur- ing the SEM examination, with Region A being comprised of tin plating over nickel plating on a copper based lead material. Diffusion of the tin plating into the nickel plating, as a result of the exposure to reflow soldering temperatures, was found. Simplified Short Circuit risk Modeling A simplified model to predict the risk a tin whisker creating an electrical short between two adjacent leads on a component has recent- ly been developed [11] . This model accounts for the geometry of leads, the effect of voltage be- tween the leads, the effects of conformal coat- ings and the view factor between surfaces (the TIN WHISKer rISK aSSeSSMeNT OF a TIN SurFaCe FINISHed CONNeCTOr continues Feature Figure 8: Magnified view of single tin whisker observed at 1500 hours. Figure 9: Tin whiskers observed at 3000 hours.