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Microstructure evolution and shear strength of eutectic Sn-9Zn and Sn-0.7Cu lead-free BGA solder balls

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2 Author(s)
C. M. L. Wu ; Dept. of Phys. & Mater. Sci., City Univ. of Hong Kong, China ; C. M. T. Law

The eutectic Sn-9Zn (in wt%) solder alloy has been regarded as a possible substitute for eutectic Sn-37Pb (in wt%) alloy because its eutectic temperature (199°C) is very close to that of the eutectic Sn-37Pb (183°C). Also, the eutectic Sn-0.7Cu (in wt%) solder alloy is cheap and possesses good solderability, strength and creep properties. To demonstrate their feasibility to be used as ball grid array (BGA) solder balls, Sn-9Zn and Sn-0.7Cu solder balls were attached on Au/Ni/Cu BGA metallization pad to study the interfacial microstructure evolution at 150°C. In the as-reflowed Sn-9Zn solder ball, its bulk microstructure mainly contained the β-Sn matrix, with randomly distributed Zn-rich and AuSn4 compounds. The Au-Zn intermetallic compounds (IMCs) were found near the interface, at which a Ni-Zn-Sn ternary intermetallic layer (IML) of about 1 μm thick was detected. Upon aging, the NiZn3 γ-phase and Au-Zn IMCs were formed quite close to the Ni-Zn-Sn IML. Further aging provided a NiZn3 layer on top of the original Ni-Zn-Sn IML. The Zn content in the Ni-Zn-Sn ternary layer increased with aging time. In the as-reflowed Sn-0.7Cu solder ball, the bulk microstructure had Cu6Sn5 and AuSn4 segregated along the β-Sn boundary. At the interface, the Cu-Sn-Ni ternary IML was formed and was found to be (Cu,Ni)6Sn5. After aging, the AuSn4, IMCs were coarsened. The Cu-Sn-Au phase was detected in the bulk solder after thermal aging for 500 hr. The (Cu,Ni)6Sn5 layer at the interface was thickened due to aging. Both Pb-free systems showed good resistance to thermal aging with a ball shear strength maintained at about 1.5 to 1.6 kgf, which is higher than that of the Pb-bearing solder alloys.

Published in:

High Density Microsystem Design and Packaging and Component Failure Analysis, 2004. HDP '04. Proceeding of the Sixth IEEE CPMT Conference on

Date of Conference:

30 June-3 July 2004