HERNDON, VA, Aug. 30 — A group of technology experts today issued new recommendations for testing assemblies for tin whiskers and evaluating devices with tin finishes. The recommendations by the National Electronics Manufacturing Initiative's Tin Whisker User Group update previous guidance released in May.

In a press statement NEMI said the new guidelines include input from suppliers that wasn't in the May version.

Joe Smetana, principle engineer, advanced technology for Alcatel and group chairman, said, "[T]here are a number of options for reducing the risk of tin whiskers, and we recognized the need for supplier feedback on the practicality of implementing the various approaches. Our goal was to address a complex issue in a practical manner with a justifiable approach while limiting the testing requirements in an effort to save time and money but still reducing the risk of tin whisker failures."

Among the updates:

• Changed significantly the "qualification by similarity" matrix, dramatically reducing the number of tests (and associated cost) required by an individual supplier.
• Eliminated the annual requalification requirement, replacing it with a simpler, supplier-defined, whisker monitoring requirement.
• Eliminated requirements for control samples that produce whiskers.
• Provided options in the specification if tin-lead reference samples are not used or available for baseline whisker length comparisons.
• Provided guidelines for optical inspection qualification as a possible alternative to SEM (scanning electron microscopy).
• 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.

"As long as the fundamentals and various contributing factors to tin whisker growth are not fully understood, acceptance of lead-free tin finishes always carries some risk, particularly when relating testing to field life," said Smetana. "However, we believe that the combination of mitigation practices, testing and plating process control reduces the risk associated with tin whiskers."

The revised requirements have been submitted to IPC and JEDEC to consider for standardization.

Not everyone agrees that new efforts are needed, however. Several solutions were devised in Japan as much as 20 years ago; see Jan Vardaman's column in Circuits Assembly in September for details.

The Tin Whisker User Group is comprised of 11 large manufacturers of electronics 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.

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