Formation of Whiskers and Hillocks on the Surface of Sn-6.6RE Alloys
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INTRODUCTION
THE addition of rare-earth (RE) elements into solders has been reported to have beneficial effects for their wettability,[1] tensile strength,[2] creep resistance,[2,3] and ductility.[4] Such an alloying design has been considered a promising method for the development of Pb-free solders.[4] However, we found by accident that amazingly rapid growth of tin whiskers occurred on the surface of Ce-doped Sn3Ag0.5Cu solders.[5–7] It is known that whisker formation can cause short circuits in solder joints and is risky for the reliability of electronic devices in applications.[8] According to our observations,[5–7] fiber-shape whiskers grow to a length of more than 100 lm after storage at room temperature for 10 days in these alloys. If they are heated at 150 C, coarse hillocks appear in as soon as 30 minutes. It has also been shown that such abnormal whisker and hillock growth is attributed to the predominant oxidation of RE-containing intermetallic phase formed in the matrix of these solder alloys. In order to further confirm this mechanism, binary tin alloys with various RE elements such as La, Ce, and Lu added were investigated. It is known that the electron configurations of La57, Ce58, and Lu71 atoms are 4f05d16s2, 4f15d16s2, and 4f145d16s2, respectively. The chemical activities of these RE elements have the tendency of La > Ce > Lu, which can influence their intermetallic phases and oxidation reactions. This study focused on the morphology and growth of whiskers and hillocks on the surface of these Sn-6.6RE alloys and the
T.H. CHUANG, Professor, C.C. CHI, Doctoral Candidate, and H.J. LIN, Doctoral Candidate, are with the Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan R.O.C. Contact e-mail: [email protected] Manuscript submitted June 15, 2007. Article published online January 26, 2008 604—VOLUME 39A, MARCH 2008
correlation with the chemical activity and oxidation behavior of various RE elements. In addition, the volume expansion of oxide layers on the RE-containing intermetallics leads to a compressive stress that extrudes the tin atoms out to form the whiskers and hillocks. The size of these intermetallic phases should affect the behavior of whisker and hillock growth, a factor which will also be considered in this study. II.
EXPERIMENTAL
For the preparation of various Sn-6.6 wt pct RE alloys, pure Sn (99.9 pct) and pure RE elements (Ce, Lu, and La) were melted at 1000 C under a 10-4 Pa vacuum. After solidification, the ingots were cut with a diamond saw. The surfaces of the specimens were ground with 2000-grit SiC paper and polished with 0.3-lm Al2O3 powder. After storage at room temperature and 150 C for various time periods, the morphology of the tin whiskers was observed by scanning electron microscopy (SEM). The chemical composition of the intermetallic phase formed in the alloy matrix was analyzed via an electron probe microanalyzer (EPMA). For the study of the oxidation behaviors of thesealloys, weight gain the weight gain percentage original w
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