Effect of Ni layer thickness and soldering time on intermetallic compound formation at the interface between molten Sn-3.5Ag and Ni/Cu substrate

The binary eutectic Sn-3.5wt.%Ag alloy was soldered on the Ni/Cu plate at 250°C, the thickness of the Ni layer changing from 0 through 2 and 4 µm to infinity, and soldering time changing from 30 to 120 s at intervals of 30 s. The infinite thickness was equivalent to the bare Ni plate. The morphology, composition and phase identification of the intermetallic compound (IMC, hereafter) formed at the interface were examined. Depending on the initial Ni thickness, different IMC phases were observed at 30 s: Cu6Sn5 on bare Cu, metastable NiSn3 + Ni3Sn4 on Ni(2 µm)/Cu, Ni3Sn4 on Ni(4 µm)/Cu, and Ni3Sn + Ni3Sn4 on bare Ni. With increased soldering time, a Cu-Sn-based η-(Cu6Sn5)1−xNix phase formed under the pre-formed Ni-Sn IMC layer both at 60 s in the Ni(2 µm)/Cu plate and at 90 s in the Ni(4 µm)/Cu plate. The two-layer IMC pattern remained thereafter. The wetting behavior of each joint was different and it may have resulted from the type of IMC formed on each plate. The thickness of the protective Ni layer over the Cu plate was found to be an important factor in determining the interfacial reaction and the wetting behavior.