The effect of Ni on the interfacial reaction between Sn-Ag solder and Cu metallization

Lead-bearing solders, such as the Sn37Pb (wt.%), have been used by microelectronics industry for connecting electronic components to print circuit boards in the past 50 years. Concerns about lead as being toxic for the environment cause the microelectronics industry and academic circles to search for Pb-free replacements. The Sn-Ag series of solders are among the most promising Pb-free candidates. During the soldering operation, solders will melt and react with the Cu substrate of the PCB. In the electronic products, Ni is one of the most common elements used in interconnections in electronic products, and Ni will inevitably dissolve into solders during the soldering. The dissolved solder maybe influence the interfacial reaction between the solders and Cu substrate. The objective of this study is to understand the effect of Ni on the interfacial reaction between Sn-Ag solder and Cu metallization. Four different Sn-Ag-Ni solders, with the Ag concentration fixed at 3.5 wt.% and Ni concentration varied between 0.0∼1.0 wt.%, were used to react with Cu disk at 240°C for 30 s-9 hr. The reaction between the Sn-3.5 Ag solder and Cu disk only formed a continuous Cu₆Sn₅ layer. However, two Ni-bearing Cu₆Sn₅ layers with different morphologies formed at the Sn-3.5 Ag-xNi solder/Cu interface. For long reaction time, all Sn-Ag solders, with Ni or without, formed Cu₃Sn between (Cu_1-yNi_y)₆Sn₅ and Cu disk. The thickness of the interfacial metallization of the Sn-3.5 Ag-xNi solders decreased with the increasing Ni concentration for the reaction when the reaction times were shorter than 1 hr, but it was still thickener than that for the Sn-3.5 Ag solder. The intermetallic thickness (Cu₃Sn plus (Cu_1-yNi_y)₆Sn₅) vs. the square root of time is linear. This implies that the growth of the intermetallic is diffusion-controlled at 240°C. The results of the study show that the Ni concentration in solders must be controlled carefully.