SMT007 Magazine

SMT-Sept2016

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36 SMT Magazine • September 2016 by David Pinsky, RAYTHEON INTEGRATED DEFENSE SYSTEMS; Tom Hester, RAYTHEON SPACE AND AIRBORNE SYSTEMS; Dr. Anduin Touw, THE BOEING COMPANY; and Dave Hillman, ROCKWELL COLLINS Abstract The risk associated with whisker growth from pure tin solderable terminations is fully mitigated when all of the pure tin is dissolved into tin-lead solder during SMT reflow. In order to take full advantage of this phenomenon, it is necessary to understand the conditions under which such coverage can be assured. A round robin study has been performed by IPC Task group 8-81f, during which identical sets of test vehicles were assembled at multiple locations, in accordance with IPC J-STD-001, Class 3. All of the test vehicles were analyzed to determine the extent of complete tin dissolution on a va- riety of component types. Results of this study are presented together with relevant conclu- sions and recommendations to guide high-reli- ability end-users on the applicability and limi- tations of this mitigation strategy. Background Manufacturers of high reliability electron- ics have been working for many years to mit- igate the deleterious effects of tin whisker for- mation. One highly effective means to suppress the growth of tin whiskers is to replace the pure tin plating with reflowed tin lead solder. (This approach is only available to manufacturers whose products are not subject to RoHS.) One approach to achieve total replacement of tin with tin lead solder is to perform a special hot solder dip process on the piece parts prior to as- sembly. Another approach is to fully consume the tin plating by tin lead solder during the SMT reflow process that occurs during circuit card assembly. This phenomenon of tin replace- ment during SMT reflow has been termed "self- mitigation," because the components mitigate by themselves without the need of any special additional processing. Self-mitigation has many advantages over other forms of tin mitigation because it is: highly effective, adds no addition- al cost, and subjects the components to no ad- ditional handling. The principal challenge to implementing self-mitigation as a standard practice is lack Mitigation of Pure Tin Risk by Tin-Lead SMT Reflow– Results of an Industry Round-Robin FEATURE

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