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February 2015 • SMT Magazine 65 tries where reliability is top priority. Mitigation of pure-tin plated components is essential. Em- pirically, minimum 3% lead addition into the pure tin composition is currently the only way to ensure severe tin whisker growth does not oc- cur. As research to mitigate tin whisker growth on PCBs continues in chemical, coating, and processes, there are two main approaches for conversion of 100% tin terminal finishes. 1. Leaded active or passive devices Pure-tin plated active devices can be dipped in standard tin-lead solder. Preheating can pre- vent thermal shock, which can lead to cracking and delamination. Robotic handling can im- prove precise dipping angle and travel. Howev- er, it is very difficult to dip inside the meniscus at the lead egress points of leaded devices. 2. SMT chip-size active or passive devices Pure tin plated chip-size devices (01005 case size and larger) are too small to dip in tin-lead solder. To do so would either invalidate the physical size specification or risk thermal shock, microstructural cracking of certain material sys- tems due to the hot solder bath "plunge." The ideal mitigation would convert pure-tin into the typical, and previously universally used "solder plate" with at least 3% lead content. Conclusions Tin whisker growth is an unpredictable, high-reliability risk when pure tin is present anywhere in the electronics assembly. For elec- tronic components where no source of tin-lead plated terminations are available, solder dip- ping appears to be the best short term solution for leaded components. SMT Feature Figure 4: base metal base construction MlCC (ni electrodes, Cu termination, ni & Sn). Scott Sentz is the marketing and business development director/export compliance manager at aeM inc. THe uNPredICTabILITy OF TIN WHISKerS eNdureS continues