Herndon, Va. The National Electronics Manufacturing Initiative's (NEMI's) Tin Whisker User Group is offering the electronics industry a revised set of proposed recommendations for tin whisker acceptance test requirements and acceptance criteria for evaluating devices with tin finishes. Members of the group 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 for high-reliability applications and for design life of more than five years.
NEMI's Tin Whisker User Group says 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, and if the whiskers grow to critical lengths in service, they could cause electrical shorts, disruption of moving parts, and/or degraded RF/high-speed performance, said the group.
Since there isn't a set of standard tests that can accelerate whisker formation and growth, which can be applied to field conditions with any reasonable degree of certainty, it's difficult for developers to safely specify tin-based lead-free finishes for components used in their
products, said NEMI.
Key changes to the recommendations include:
- Change significantly the "qualification by similarity" matrix, which will dramatically reduce the number of tests (and associated cost) required by an individual supplier.
- Eliminate the annual re-qualification requirement, and replace it with a simpler, supplier-defined, whisker-monitoring requirement.
- Eliminate requirements for control samples that produce whiskers.
- Provide options in the specification if SnPb reference samples are not used or available for baseline whisker length comparisons.
- Provide guidelines for optical inspection qualification as a possible alternative to SEM.
- Eliminate 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.
- Allow an option for bias testing (when required) to be conducted in socket.
The revised acceptance test requirements have been submitted to both IPC and JEDEC to pursue the 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
Three NEMI projects are currently addressing the issue. The Tin Whisker Accelerated Test project is working to identify accelerated test(s) to predict whisker formation, while the Tin Whisker Modeling project is focusing on the root cause(s) of whiskers. The Tin Whisker User Group is in the midst of defining guidelines for minimizing risk of failure from tin whiskers in high-reliability electronic applications.