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September 2015 • SMT Magazine 73 raytheon parts and Libraries tin Whisker self-mitigation project A. Overall objective: Validate printed wiring board design, component selection, and solder assembly guidelines that will assure mitigation of tin whisker growth risk when using electronic components with pure tin solder terminations. B. Objective for sample part selection: Test to see if component samples with solder termi- nations < 0.035 inch in height, under condi- tions optimized for introduction of lead into the solder surface, would be self-mitigating for tin whisker risk. C. Objective for sample PWB design: Test to see if use of Raytheon standard solder pads, multilayer printed wiring boards, and multiple grounding layers affects self-mitigation for tin whisker risk. D. Objective for multi-chip-module sam- ples: Validate self-mitigation of selected com- ponents in MCM for tin whisker risk. part selection Criteria for pWb Designs 1 and 2 The components selected for this project included a range of termination lengths and package heights that would both have some parts succeeding and some failing the pure tin mitigation process. These were purchased from commercial sources for pure tin containing electronic components. The package height factor in self-mitigation was understood from previous studies, but in hindsight the termi - nation length factor was not considered as it was not recognized to be important before this project. part selection Criteria for multi-Chip module The components analyzed for self-mitiga- tion on the MCM were limited to surface mount chip resistors size 01005 and surface mount capacitors ranging in body height up to 0.065 inch. No other components on the MCM were analyzed for self-mitigation. sample Designs Two sample PWBs and a multi-chip module were created. The first sample board was a sim- ple two-layer circuit card with standard manu- facturer's suggested solder pad sizes. A second sample board was developed with features se- lected to challenge the ability of components to self-mitigate, which included multiple layers, multiple ground planes, and Raytheon selected solder pad sizes, which are larger than the man- ufacturer's suggested sizes. Photographs of the test boards are shown in Figures 3 and 4. These factors were considered important in simulat- ing worst-case production conditions for self- mitigation. The MCMs are highly compact multilayer wiring boards with internal ground planes. Eight MCMs were made for this project. A pho- tograph of the MCM is shown in Figure 5. ArtiCle tIN WHIsKer seLF-mItIGAtION IN surFACe mOuNt COmpONeNts continues figure 3: Sample board design 1. figure 4: Sample board design 2.