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NEMI Issues Revises Tin Whisker Acceptance Test Requirements
September 1, 2004 |Estimated reading time: 2 minutes
Herndon, Va. — The National Electronics Manufacturing Initiative's (NEMI) Tin Whisker User Group has released revised recommendations for tin whisker acceptance test requirements.
The group published a draft set of recommendations in May of this year and, after actively soliciting input from suppliers, revised its recommendations based on the feedback received.
The NEMI Tin Whisker User Group is comprised of 11 large manufacturers of electronic assemblies that agree that pure tin finishes (and other high-tin content lead-free alloy finishes) present a risk of tin whisker-related failures in electronics, particularly those demanding high reliability. Seeing the need for industry-wide guidelines and criteria for accepting lead-free components in the face of uncertain technical risk, the group put together proposed recommendations that included tin whisker testing requirements and acceptance criteria for evaluating devices with tin finishes.
The NEMI Tin Whisker User Group sponsored a supplier meeting at the IEEE Electronic Components and Technology Conference (ECTC) in early June, specifically to solicit feedback on its proposal.
Key changes made to the recommendations include:
- Changed significantly the "qualification by similarity" matrix (Appendix A), dramatically reducing the number of tests (and associated cost) required by an individual supplier.
- Eliminated the annual requalification requirement, replacing it with a much simpler, supplier-defined whisker monitoring requirement.
- Eliminated requirements for control samples that produce whiskers.
- Provided options in the specification if SnPb reference samples are not used or available for baseline whisker length comparisons.
- Provided guidelines for optical inspection qualification as a possible alternative to SEM.
- Eliminated preconditioning requirements for components with nickel underplating, which cuts sample sizes for components using nickel as a mitigation practice and also reduces the test time required for these components.
- Allowed an option for bias testing (when required) to be conducted in sockets.
The revised acceptance test requirements have been submitted to both IPC and JEDEC to pursue release of a formal standard or guideline for tin whisker acceptance testing. This document is available on the NEMI Web site at www.nemi.org/projects/ese/tin_whisker_activities.html.
As the electronics industry moves toward lead-free assemblies, the simple manufacturing solution is to use pure tin, or alloys with high tin content, as coatings on lead-frames. However, tin-based plating is known to be susceptible to the formation of needle-like protrusions, or whiskers under certain conditions. If whiskers grow to critical lengths in service, they could cause electrical shorts, disruption of moving parts, and/or degraded RF/high-speed performance.
Despite five decades of research, there is no scientific consensus on whisker formation and growth fundamentals. In addition, 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. As a result, it is difficult for developers to safely specify tin-based lead-free finishes for components used in their products, particularly those concerned with high-reliability and/or design life of more than five years.
Three NEMI projects are currently addressing the issues: Tin Whisker Accelerated Test, which is working to identify accelerated test(s) to predict whisker formation; Tin Whisker Modeling, which is focusing on the root cause(s) of whiskers; and Tin Whisker User Group, which is defining guidelines for minimizing risk of failure from tin whiskers in high-reliability electronic applications.
The National Electronics Manufacturing Initiative's mission is to assure leadership of the global electronics manufacturing supply chain. For more information, visit www.nemi.org.