Synchrotron radiation Scanning X-ray micro-diffraction study of Sn whiskers on Pb-free surface finish


W. J. Choi, T. Y. Lee, and K. N. Tu

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

N. Tamura, R. S. Celestre, and A. A. MacDowell,

Advanced Light Source, LBNL, Berkeley, CA 94720

Y. Y. Bong, and Luu Nguyen,

National Semiconductor Corporation, Santa Clara, CA 95051,




Tin (Sn) whisker growth is a surface relief phenomenon.  A large number of Sn whiskers have been found on the Pb-free solder finish on leadframes used in consumer electronic products.  Some of the whiskers on eutectic SnCu finishes are long enough to short the neighboring legs of the leadframe.  We have performed synchrotron radiation micro-diffraction analysis to measure the local stress level, the orientation of the grains in the finish around a whisker, and the growth direction of the whisker.  The  compressive stress in the solder finish is quite low, less than 10 MPa, nevertheless there exists a stress gradient from the root of a whisker.   From the orientation map and pole figure, we found that the growth direction of whisker is [001] and a preferred orientation of [321] grains exists on the solder finish. The grain orientation of the grain just below the whisker is different; it is [210].