The Role of Trace Elements in Tin Whisker Growth
WP-1751
Objective
The propensity of pure tin (Sn) platings to form Sn whiskers has been known since the 1940s. Sn whiskers have been found to form on a wide variety of Sn-plated substrates under a wide range of environmental conditions. The use of pure Sn platings as a surface finish in electronics is increasing due to recent European legislation banning the use of lead (Pb) in electronics. As it becomes increasingly difficult for aerospace and military programs to procure electronics made with SnPb surface finishes, these programs will be under pressure to use Sn-plated components. The increased use of Sn-plated components will increase the risk of electrical shorting and plasma events due to Sn whiskers, and these risks may become unacceptable for those that require high reliability electronics. Current mitigation strategies for dealing with Sn whiskers pose their own set of problems. For example, dipping Sn-plated component leads into molten SnPb has been shown to prevent whisker formation, but it is expensive and may damage some components.
The addition of small amounts of Pb to Sn platings greatly reduces or eliminates whisker formation and growth; however, the mechanism by which Pb prevents whisker formation is not understood. Preliminary data suggests that other elements besides Pb can also reduce or eliminate whisker formation. The objective of this project is to quantify the effectiveness of these alternative elements.
Technical Approach
Boeing has identified six elements (in addition to elemental Pb) that can be added to Sn platings in small amounts that will reduce or prevent the formation of Sn whiskers--antimony, cobalt, nickel, gold, germanium, and copper. In this project, researchers will develop electrochemical methods for incorporating these candidate elements into Sn platings and then quantify the effectiveness of the dopants in reducing whisker formation and growth. For some of these elements, electrochemical co-deposition of the element and Sn may not be practical or even possible. For those elements, the effectiveness of thin layers deposited over or under the Sn platings will be evaluated. Data suggests that a thin plating of certain elements applied over Sn platings can totally prevent whisker formation. The propensity of the doped and undoped Sn platings will be correlated with the morphology, grain size distribution, and grain boundary misorientations as measured using focused ion beam microsections and electron backscatter diffraction. Grain boundary misorientations have been shown to correlate with the free energy and the mobility of grain boundaries, which in turn may play a role in whisker formation.
Benefits
Identification of elements besides Pb that can suppress Sn whisker growth when added to Sn platings (or when deposited over the platings) would offer Department of Defense users new mitigation strategies for preventing electrical shorting due to Sn whiskers growing on military/aerospace circuit assemblies. The use of these new mitigation strategies will ensure the reliability of military/aerospace hardware in the field. In addition, if the properties of the platings (morphology and grain boundary misorientations) can be correlated to whisker growth, the understanding of why whiskers form will be advanced and better mitigation strategies can be developed. (Anticipated Project Completion - 2013)
Points of Contact
Principal Investigator
Ms. Jean Nielsen
The Boeing Company
Phone: 425-965-1342
Document Types
- Fact Sheet - Brief project summary with links to related documents and points of contact.
- Final Report - Comprehensive report for every completed SERDP and ESTCP project that contains all technical results.
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