Microstructure and mechanical properties of Sn-Ag-xNi composite solder

SnPb solder joints are an essential part for microelectronic packages manufacturing industrial for a long time. In recent year, environmental concerns and RoHS (Restriction of Use of Hazardous Substances) has demanded elimination of lead from electronics products by July 1, 2006. However, base on increasing pressures to achieve environmentally friendly electronic materials and processes, and indeed, growing governmental regulations around the word, the drive is strong to use lead-free solders in electronic assemblies. Among the various lead-free solder alloys, eutectic Sn-Ag solder has attracted particular interest. Composite solders offer improved the mechanical property as compared to non-composite solder. Ni reinforcement particles were incorporated to the Sn-Ag solder paste to formed composite solder. In this study, composite solders were prepared by mechanically added Ni reinforcement particles with 0.5, 1, and 3 wt% into the Sn- Ag solder paste were studies. The microstructure of Sn-Ag-xNi composite solder alloy and with Cu substrate joints were evaluated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The XRD peaks of Sn-Ag-xNi were observed for different x values of Ni concentration. That shows the X-ray diffraction spectrum of the prepared Sn-Ag-xNi solder, indicating that the Ag₃Sn and a few Ni₃Sn₄ were random mixed with the Sn-3.5Ag solder. The microstructure of Ni reinforcement addition will suppress the beta-Sn grain coarsened and an intermetallic compound of (Ni,Cu) 3Sn4 particle was dispersed throughout the eutectic matrix. The reaction between the molten composite solder and Cu substrate resulted in the formation of two IMCs, (Cu,Ni)6Sn5 at the interface and (Ni,Cu)3Sn4 in the matrix and nearby interface of solder joints. Additionally, the intermetallic compound layer of (Cu,Ni)6Sn5 compound will be thickened with increasing Ni content. The top morphology of IMCs was also characterized after the solder was selectively etched away. The IMCs on the Sn-Ag-xNi/Cu showed long rod-like shape. The experimental results reveal that the addition small amount of Ni reinforcement particles to the molten Sn-Ag solder will increase the tensile strength in the as-soldered specimens. For Ni added less than 1 wt% that presence of dimple-like structure on the fracture surfaces in the solder matrix which is suggested a ductile failure mode. For Ni additions exceed 1 wt%, the failure mode changes from ductile fracture to brittle fracture, and the specimens fail adjacent the interface of the solder matrix and the IMC layer.