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70 SMT Magazine • February 2015 ceptible to the potential long term threat of tin whiskers. Industry studies have shown [7, 8 ] that pure tin surfaces can be "poisoned" with tin/lead solder provided the tin plating is con- sumed/converted during the soldering process. With many short components such as resistors or capacitors, the soldering process consistently and repeatedly eliminates any tin whisker risk. However, the component lead geometry, solder paste deposit and component pad dimensions all factor into whether a component can be suc- cessfully poisoned by the soldering process. Fig- ure 2 illustrates two components, with one be- ing successfully poisoned and one inadequately poisoned. Connectors are typically not an issue, in terms of solder poisoning, as connector suppli- ers utilize the connector housing design/con- figuration to eliminate potential shorting from a tin whisker or ensure that the areas with pure tin surface finishes will be poisoned as part of the soldering process. Previously, Rockwell Collins completed a connector technology qualification that in- cluded the Samtec SEARAY solder charge con- nector[9]. The connector had very robust ther- mal cycle solder joint integrity and utilized a matte tin (i.e., pure tin) surface finish on the non-contact pin surfaces. During metallo- graphic cross-sectioning phase of the connec- tor's qualification effort, it was observed that a region of matte tin was not poisoned during the soldering process nor was the tin whisker shorting potential in this region mitigated by the connector design/configuration. Figure 3 illustrates this region of the connector after as - sembly. An investigation was conducted to de- termine if this region of the connector posed a tin whisker risk threat. Feature Figure 2: Solder process poisoning (left) acceptable and (right), inadequate. Yellow arrow defines highest point of solder flow. Figure 3: Samtec SeArAY Connector after assembly soldering (yellow arrows indicate the exposed area of un-poisoned tin plating that represents potential tin whisker risk). TIN WHISKer rISK aSSeSSMeNT OF a TIN SurFaCe FINISHed CONNeCTOr continues