Symposium Conference Proceedings


Authors: Karl Seelig and Tetsuro Nishimura
Company: AIM and Nihon Superior
Date Published: 6/21/2007   Conference: Symposium

Abstract: Tin whiskers are of great concern for long term reliability of lead free assemblies. Many theories and tests have been established to filter out potential whisker growth and to include similar growth environments for tin whisker, while also concentrating on tin plating and not alloys.

Several groups and organizations including iNEMI and NASA have in depth studies of tin whiskers. The International Electronics Manufacturing Initiative (iNEMI) states that they (iNEMI) have “found it possible to get conflicting data on the same finishes and also on the same mitigation practices from different users”. This is very typical of tin whiskers. Conformal coating, annealing (reflowing tin) at 150°C have all been suggested to mitigate tin whisker growth.

One point that is not tested in all the studies reviewed has been mechanically stressing the finish coat. In prior studies reviewed, there was similar testing running thermal cycling -40 or -55 to +85, ambient soak 30c 60Rh, and high temperature soak 60°C 87 RH. All the studies tests were on as processed finishes, or in an annealed state. Some tests have been conformal coated to mitigate whisker growth. Additionally, all tests have looked at surface finishes for lead coatings.

Alloys are believed to be acceptable since they contain enough containments and additives not allowing the growth of whiskers. In the alloy testing that has been run, the criterion involves bulk casting, not as a coating over copper. In real life usage of lead-free solder it will end up as a thin coating due to wicking on leaded devices. It will also become a thin coating on wire that is soldered into a circuit board. In both of these applications the overall life of the product will be mechanically stressed. Many studies state that the whisker growth is from the stress of intermetallic growth or compression due to scratches. Other studies seem to refer to whisker growth resulting from stress during compression.

Based on the research reviewed from many large consortiums that studied whisker growth on lead finishes it was determined to investigate lead free alloy affect on whisker growth. Nihon Superior requested a study be run comparing lead free alloys. The initial thought was to assemble boards and observe whisker growth. After careful review this seemed to be the wrong direction to take, noting that the whisker growth would have been intermittent and may not grow in all areas due to solder volume or joint geometry. With the assistance of Trace Labs in Maryland, and consulting with John Rodman of Trace Labs, we came to the conclusion that a solder coated and stressed copper wire would be the best medium in growing whiskers. A suggestion to make a “U-shaped” bend in the copper wire after coating was also agreed upon.

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