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

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.