Microstructural evolutions of Cu(Ni)/AuSn/Ni joints during reflow

Interfacial reactions of the Ni/AuSn/Ni and Cu/AuSn/Ni joints are experimentally studied at 330 °C for various reflow times. The microstructures and mechanical properties of the as-solidified solder joints are examined. The as-solidified solder matrix of Ni/AuSn/Ni presents a typical eutectic ζ-(Au,Ni)₅Sn+δ-(Au,Ni)Sn lamellar microstructure after reflow at 330 °C for 30 s. After reflow for 60 s, a thin and flat (Ni,Au)₃Sn₂ intermetallic compound (IMC) layer is formed, and some needle-like (Ni,Au)₃Sn₂ phases grow from the IMC layer into the solder matrix. On the other hand, a cellular-type ζ(Cu) layer is found at the upper AuSn/Cu interface in the Cu/AuSn/Ni joint after reflow for 30 s, and a (Ni,Au,Cu)₃Sn₂ IMC layer is also formed at the lower AuSn/Ni interface. For both joints the IMC layer grows significantly with the increase of reflow time, but the growth rate of (Ni,Au,Cu)₃Sn₂ IMC in the Cu/AuSn/Ni joint is smaller than that of the (Ni,Au)₃Sn₂ layer in the Ni/AuSn/Ni joint. The comparisons of the shear strength and fracture surface between the Ni/AuSn/Ni and Cu/AuSn/Ni joints suggest that the coupling effect of the Cu/AuSn/Ni sandwich joint is helpful to prevent the excessive growth of (Ni,Au)₃Sn_2, which in turn enhances the mechanical reliability of the solder joint.