In this study, we fabricated eutectic Au–Sn (Au–20 wt% Sn) flip-chip solder bumps from a single electroplating bath. After reflowing, the average diameter of the solder bump was approximately 80 μm. The (Ni,Au)3Sn2 phase was initially formed when the liquid Au–Sn solder reacted with the Ni UBM (under bump metallization). After aging at 150 °C, the (Ni,Au)3Sn2 intermetallic compound (IMC), which formed at the interface during reflow, was fully transformed into the (Au,Ni)Sn IMC due to the restricted supply of Ni atoms from the UBM to the interface. On the other hand, after aging at 250 °C for 1000 h, two IMC layers, (Au,Ni)Sn and (Ni,Au)3Sn2, were formed at the interface. The lower (Ni,Au)3Sn2 phase was formed when the (Au,Ni)Sn phase reacted with the Ni UBM. The interfacial (Au,Ni)Sn IMC grew with the preferential consumption of the available δ-phase in the solder matrix. Eventually, the ζ-phase covered most of the interfacial layer. In the bump shear tests, the Au–Sn/Ni joint aged at 150 °C fractured through the bulk of the solder, confirming the mechanical reliability of the interface. In contrast, the Au–Sn/Ni joint aged at 250 °C fractured along the interface, thereby demonstrating brittle failure, possibly a result of the brittle IMC layer at the interface.

As an alternative to the time-consuming solder pre-forms and pastes currently used, a co-electroplating method of eutectic Au-Sn alloy was used in this study. Using a co-electroplating process, it was possible to plate the Au-Sn solder directly onto a wafer at or near the eutectic composition from a single solution. Two distinct phases, Au 5 Sn (f-phase) and AuSn (d-phase), were deposited at a composition of 30 at.%Sn. The Au-Sn flip-chip joints were formed at 300 and 400°C without using any flux. In the case where the samples were reflowed at 300°C, only an (Au,Ni) 3 Sn 2 IMC layer formed at the interface between the Au-Sn solder and Ni UBM. On the other hand, two IMC layers, (Au,Ni) 3 Sn 2 and (Au,Ni) 3 Sn, were found at the interfaces of the samples reflowed at 400°C. As the reflow time increased, the thickness of the (Au,Ni) 3 Sn 2 and (Au,Ni) 3 Sn IMC layers formed at the interface increased and the eutectic lamellae in the bulk solder coarsened.

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