The influence of element-Ni on interfacial reactions between lead-free Sn-Ag-Cu and Cu substrate

The formation and growth of intermetallic compounds (IMC) is a key factor to the reliability of soldering joints in modern electronic mounting and packaging industry. In this work, Sn-3.8Ag-0.5Cu, Sn-3.8Ag-0.5Cu-0.1Ni lead-free solders were prepared using vacuum equipment. The influence of element-Ni on interfacial reactions between Sn-3.8Ag-0.5Cu and Cu during soldering as well as on IMCs behavior during aging was studied. When soldered, the Sn-3.8Ag-0.5Cu-0.1Ni/Cu IMC layer was much thicker than that of Sn-3.8Ag-0.5Cu/Cu; by calculating the metastable phase equilibria and comparing the driving forces of formation of individual IMC, the compounds sequence and reaction path at solder/copper interface were predicted through Thermo Calc software which is based upon CALPHAD method. The results showed that the compound which formed first was Cu₆Sn₅, then Cu₃Sn. The microstructure of the joint was also identified by means of Olympus, scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis methods, the addition of Ni made IMC of SnAgCu/Cu turn into (Cu, Ni)₆Sn₅. The results from thermodynamic calculation are in good agreement with the experiments. When 130°C aged, the total IMC layer of SnAgCu/Cu solder joint thickened with the increasing of aging time. The thickness of IMC layer has straight-line relation to square root of time, which accords with the rule of parabola; while (Cu,Ni)₆Sn₅ layer of the SnAgCu-0.1Ni/Cu solder joint appear greater stabilities during aging.