Tin Whiskers Studied by Focused Ion Beam Imaging and Transmission Electron Microscopy

George T. T. Sheng and C. F. Hu

Macronix International Co., Hsinchu, Taiwan, ROC

W. J. Choi and K. N. Tu

Dept. of Materials Science and Engineering, UCLA, Los Angeles, CA 90095-1595

Y. Y. Bong and Luu Nguyen

National Semiconductor Corporation, Santa Clara, CA 95051

Abstract:

In surface mount technology of electronic packaging, the leadframes are finished with a layer of Pb-free solder. A large number of Sn whiskers are found on the surface of the finish, especially that of eutectic SnCu or pure Sn. The whiskers have a faster growth rate on eutectic SnCu finish than on pure Sn finish. Some of the whiskers on SnCu are long enough to short the neighboring legs of the leadframe. We report here the study of spontaneous growth of Sn whiskers on these finishes by using focused ion beam imaging and transmission electron microscopy. Cross-sectional samples, both normal and parallel to the growth direction of a whisker, were prepared. Precipitates of Cu₆Sn₅ in the grain boundaries of the finishes have been found. The growth of these grain boundary precipitates due to the chemical reaction between Cu and Sn at room temperature provides the driving force for spontaneous Sn whisker growth. Many more of these grain boundary precipitates exist in the SnCu finish than in the pure Sn finish. This is the main reason why Sn whiskers grow faster on the SnCu finish.

[Published in J. Appl. Phys., 92, 64-69 (2002).]