Interfacial reactions between electroplated Ni–Zn alloy films and lead-free solders

Abstract

Because of the miniaturization trend in electronic devices in recent years, the issue of reliability of solder joints in these miniaturized devices becomes very critical. Studies have shown that a thin layer of Ni is effective in reducing the interfacial IMC growth between Cu and Sn-based solder. This is because the reaction kinetics of Ni–Sn compounds are much slower than that of Cu–Sn IMCs. In this study, zinc is incorporated into the nickel barrier film in the form of Ni–Zn alloy by electrodeposition. The effects of the presence of Zn on the interfacial reactions between nickel barrier film and Sn–3.8Ag–0.7Cu (SAC) and Sn–3.5Ag (SA) solders are investigated. Ni–Zn alloy films with 1.73 wt% Zn were prepared from ammoniacal diphosphate baths. Elemental composition of the alloy film was determined by energy dispersive X-ray spectroscopy while X-ray diffraction method was used to determine the phases present in the alloy film. Solders’ spreading rate was characterized with the use of optical microscope. Reflows were done for 1 and 12 cycles to investigate the effect of multiple reflows on the IMC growth and morphology. Results have shown that the IMC formed at the interface of SA/Ni and SAC/Ni was Ni₃Sn₄ and (Cu,Ni)₅Sn₆, respectively. (Ni,Cu)₃Sn₄ IMC was formed at the interface of SA/Ni–Zn alloy film. No spalling was detected at the SA/Ni–Zn solder joint. On the other hand, it has been observed that (Ni,Cu)₃Sn₄ and (Cu,Ni)₆Sn₅ layer with continuous non-uniform morphology were formed on the SAC/Ni–Zn alloy film after 1× reflow. As the number of reflow increased, (Cu,Ni)₆Sn₅ layer spalled from the interface leaving only (Ni,Cu)₃Sn₄ IMC at the interfacial region.