Newsroom
NEMI Team Recommends Three Test Conditions
for Evaluating Tin Whisker Growth
Download SMTA International presentation
HERNDON, Va. - September 26, 2003 - The National Electronics Manufacturing Initiative's (NEMI's) Tin Whisker Accelerated Test Project presented its recommendations for tin whisker testing at the SMTA International conference in Chicago yesterday. The NEMI project team has submitted a test method document to JEDEC that recommends two storage conditions and one temperature cycling condition to evaluate the propensity of tin-based plating finishes to grow whiskers.
As the electronics industry moves toward lead-free soldering, manufacturers are considering replacing tin-lead solder with pure tin or tin with low levels of alloying elements (other than lead). However, pure tin electrodeposits may grow "whiskers" - tiny metal filaments emanating from the surface finish - which have the potential to short circuit or break off and interfere with other devices in certain applications. Currently, there are no widely accepted test conditions for determining the predilection of a specific plating finish or plating process to grow tin whiskers.
"Standard test methods are needed to help the industry move more quickly in the evaluation and development of tin-based lead-free finishes," said Nick Vo senior process engineer for the Motorola Final Manufacturing Technology Center (Austin, TX) and chair of the NEMI Tin Whisker Accelerated Test Project. "Standard tests will permit meaningful comparison of whisker propensity for different plating systems and processes, provide a consistent inspection protocol, and provide a standard method to compare and report results."
Recommended Test Methods
Vo says NEMI's Tin Whisker Accelerated Test Project was organized in 2001 to try to identify and/or develop such test methods. The team - which has involved more than 40 organizations - began with a benchmark study to collect all existing methods for growing whiskers. They then evaluated those test methods reported to successfully grow whiskers on some, but not all, tin-plated samples.
The NEMI project tested eight-lead small outline integrated circuit (SOICs) packages with varying thicknesses and different tin-based finishes. These samples were subjected to different environment combinations.
Based on the results of the tests conducted, the NEMI group has recommended three test conditions:
- temperature cycling (-55°C to + 85°C, approximately 3 cycles/hour)
- temperature humidity tests at 60°C/93% RH
- ambient storage (air-conditioned facility)
All three tests are required and each test must be performed independently.
In addition to the test specification submitted to JEDEC, NEMI is collaborating with JEITA (Japan Electronics & Information Technology Industries Association) and E3 (a European semiconductor consortium) to develop a test protocol to submit to the IEC (International Electrotechnical Commission). The three groups are working to establish unified whisker test methods worldwide, including pass/fail criteria and fields of application.
Tin Whisker Definition
Along with its recommendation for test conditions, the Tin Whisker Accelerated Test Project has proposed the following definition of a tin whisker:
A spontaneous columnar or cylindrical filament, which rarely branches, of monocrystalline tin emanating from the surface of aplating finish. Furthermore, tin whiskers may have the following characteristics:
- an aspect ratio (length/width) greater than 2
- can be kinked, bent, twisted
- generally have consistent cross-sectional shape
- may have striations or rings around it
Future Work
"More work needs to be done to further define tin whisker test methods," said Vo, "but the parameters we are recommending serve as a starting point for an industry-standard test that will provide a consistent protocol for tin whisker examination. It will also allow industry to compare various test methods to something that we know will grow whiskers. Additionally, we hope that this work will help advance the understanding of tin whisker growth mechanisms.
"The NEMI project team plans to conduct further evaluations to demonstrate that the three recommended tests can consistently produce whiskers with similar samples," continued Vo. "We also hope to confirm that they are applicable for other tin-based finishes and, possibly, for other sample types (connectors, special design coupons, etc.)."
Vo notes that the mechanism for whisker growth is not known and that further testing may provide some correlation to application life (risk) in applications where these current tin-plated products are used. Work is ongoing in a second NEMI project - the Tin Whisker Modeling Project - to develop a better understanding of whisker formation, which may lead to better test methodology or validate that these are the best tests that can be carried out.
For additional information about NEMI's tin whisker activities, including reports from the Tin Whisker Modeling Project and Tin Whisker User Group, visit: http://www.nemi.org/projects/ese/tin_whisker.html
About NEMI
The National Electronics Manufacturing Initiative's mission is to facilitate leadership of the North American electronics manufacturing supply chain. Based in Herndon, Va., the industry-led consortium is made up of approximately 70 electronics manufacturers, suppliers, industry associations and consortia, government agencies and universities. NEMI roadmaps the needs of the North American electronics industry, identifies gaps in the technology infrastructure, establishes implementation projects to eliminate these gaps (both business and technical), and stimulates standards activities to speed the introduction of new technologies. The consortium also works with government, universities and other funding agencies to set priorities for future industry needs and R&D initiatives. For additional information about NEMI, visit www.nemi.org.
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For further information:
Cynthia Williams
Director of Communications
207-871-1260
cynthiaw@maine.rr.com.com