Effects of Ce and La Additions on the Microstructure and Mechanical Properties of Sn-9Zn Solder Joints |
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Authors: | Hsiu-Jen Lin Tung-Han Chuang |
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Affiliation: | (1) Institute of Materials Science and Engineering, National Taiwan University, Taipei, 106, Taiwan |
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Abstract: | The effects of rare-earth elements on the microstructure and mechanical properties of Sn-9Zn alloys and solder joints in ball
grid array packages with Ni/Au(ENIG) surface finishes have been investigated. Metallographic observations showed that (Ce0.8Zn0.2)Sn3 and (La0.9Zn0.1)Sn3 intermetallic compounds appeared in the solder matrix of Sn-9Zn-0.5Ce and Sn-9Zn-0.5La alloys, respectively. Both fiber-
and hillock-shaped tin whiskers were inhibited in the Sn-9Zn-0.5Ce solder, while tin fibers were still observed on the surface
of oxidized (La0.9Zn0.1)Sn3 intermetallics in Sn-9Zn-0.5La after air exposure at room temperature. Mechanical testing indicated that the tensile strength
of Sn-9Zn alloys doped with Ce and La increased significantly, and the elongation decreased, in comparison with the undoped
Sn-9Zn. The bonding strengths of the as-reflowed Sn-9Zn-0.5Ce and Sn-9Zn-0.5La solder joints were also improved. However,
aging treatment at 100°C and 150°C caused degradation of ball shear strength in all specimens. During the reflowing and aging
processes, AuZn8 intermetallic phases appeared at the interfaces of all solder joints. In addition, Zn-rich phases were observed to migrate
from the solder matrix to the solder/pad interfaces of the aged specimens. |
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