Studies on the Interfacial Reactions and Whisker Growth in Ce-Doped Sn3Ag0.5Cu Pb-free Solder Ball Grid Array Package

• Main Authors: Shiu-Fang Yen, 顏秀芳
• Other Authors: 莊東漢
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/34044837821290791613

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 95 === Recently, the addition of a trace amount of rare earth elements is reportedly able to considerably improve the wetting and mechanical properties of mast of these Pb-free solder, related researches have just gotten underway is very recent years and China government are willing to arrange this Ce-doped solder into China ROHS. However, to apply this Ce-doped solder into a real package production, a lot of these efforts are still to be clarified through further experiments so that this Ce-doped solder will be confirmed as the Pb-free solder replacer. In the first part of this study is concerned with the Sn3Ag0.5Cu-XCe(X=0.1, 0.25, 0.5, 1.0wt%) Pb-free solder joints in Ball Grid Array (BGA) Package, while focusing on the melting point, interfacial intermetallic reactions and shear test after high temperature aging. The second part of this study is to clarify the mechanism of tin whisker formation through the whisker growth properties. From the experimental results, it is evidenced that the Ce-dope is effectively to slow down the coarsening of Ag3Sn precipitates in the solder matrix and the degradation of bonding strengths after high temperature aging. For the solid interfacial reactions, Ce-dope increases the growth activation energy of Cu6Sn5 and Cu3Sn, which implies that the growth reaction of Cu6Sn5 and Cu3Sn-intermetallics is much more sensitive to the aging temperature. In the microstructure of solder matrix, Ce-dope results in the formation of precipitated CeSn3 clusters in the reflowed solder matrix and the oxidation of CeSn3 clusters makes the tin whisker grow. However, this Ce-doped solder will be restricted by the reliable issue of tin whisker growth. In this study, the morphology of whiskers observed can be summarized as fiber, column, flower-cluster and hillock. Different diameter fiber whiskers are Types I and II, which can coexist on the same CeSn3 oxide layer. In addition, the morphology of whiskers will transfer from fiber into flower-cluster whisker with increasing related humidity and into hillock whisker with increasing temperature. The growth rate and diameter of fiber whisker will be increased by increasing temperature, but the average length decreased. Lower temperature at 5℃ and 10-2 torr vacuum can inhibit the whisker formation. Through the relationship between cross section observation and oxidation behavior at various temperatures, the mechanism of abnormal tin whisker formation in this Ce-doped solder can be attributed to the compressive stress induced by the diffusion of oxygen into the CeSn3 precipitate clusters in the solder, which squeezes the Sn atoms in the Ce-depleted region of CeSn3 phase out of the oxide layer.