The National Electronics Manufacturing Initiative's (NEMI, Herndon, VA) Tin Whisker User Group has released a draft proposal of tin whisker acceptance test requirements. As part of its efforts to help suppliers and users minimize the exposure to problems associated with tin whiskers, the group has developed the document to provide testing requirements and acceptance criteria for evaluating devices with tin finishes in high-reliability applications. 

As the electronics industry moves toward lead-free assemblies, a simple manufacturing solution is to use pure tin, or alloys with high tin content, as coatings on lead-frames.  However, tin is known to be susceptible to the formation of needle-like protrusions, or whiskers, and tin whiskers are a reliability concern. They can cause electrical shorts, disruption of moving parts or degraded RF/high-speed performance. Tin whiskers may grow between adjacent conductors of different potentials and cause either a transient short circuit if the whisker is burned open, or a permanent short if the whisker remains intact. At shorter lengths, whiskers may create a stub-type effect that degrades performance of high-speed/high-frequency circuits. Additionally, whiskers can potentially break loose and, as debris, cause mechanical or other electrical problems.

"System developers face two key problems," said Joe Smetana, principle engineer, advanced technology for Alcatel and chair of the NEMI Tin Whisker User Group. "First, there is no scientific consensus on whisker formation and growth fundamentals, despite five decades of research. Second, there is no standard set of tests that can accelerate whisker formation and growth and relate these tests to field conditions with any reasonable degree of certainty. These issues make it difficult for developers to safely specify tin-based lead-free finishes for components used in their products, particularly those concerned with high-reliability or design life of more than five years."

NEMI has previously published recommendations for test conditions for evaluating tin whisker growth and for mitigation practices to help prevent whisker formation. The latest publication combines practices established by the previous documents and adds information such as test lengths, failure criteria, number of parts and additional bias voltage testing. It provides guidelines for post finishing inspection and preconditioning; outlines a flowchart for acceptance requirements; provides a framework for a qualification test report; and defines requirements for process controls and periodic testing.

The Tin Whisker User Group has scheduled a meeting on June 2, in conjunction with the IEEE Electronic Components and Technology Conference (ECTC), in Las Vegas, NV.  The group will review the proposed test requirements with suppliers and solicit feedback on the practicality of implementing the various recommended mitigation and testing approaches. Supplier inputs during the meeting will be considered by the User Group for possible modifications to the test requirements. 

The draft of the NEMI acceptance test requirements document is available at http://www.nemi.org/projects/ese/tin_whisker_activities.html.

www.nemi.org

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