Effect of Cu Concentration on the Solid-State Aging Reactions between SnAgCu Lead-Free Solders and Ni
L. G. Shiau, C. E. Ho, W. C. Luo, and C. Robert Kao
Department of Chemical & Materials Engineering
National Central University
Chungli City, Taiwan
The SnAgCu series of solders are considered very promising lead-free replacements for the Sn-37Pb solder, especially for reflow soldering applications. Recent studies revealed that, during soldering, the Cu concentration had a very strong effect on the reactions between SnAgCu solders and Ni. When the Cu concentration was low (x=0.3 and lower), the reaction product was (Ni1-xCux)3Sn4 at the interface of a solder joint. At high Cu concentrations (x=0.6 and higher), the reaction product was (Cu1-yNiy)6Sn5. When the Cu concentration was in-between (x=0.4 and 0.5), both (Ni1-xCux)3Sn4 and (Cu1-yNiy)6Sn5, formed. In other words, during soldering, the Cu concentration must be strictly controlled in order to obtain consistent results. In this study, we would like to extend the earlier studies to investigate whether this strong concentration dependency also occurs during the solid-state aging of the solder joints. We aged solder joints at the solid-state at several different temperatures for time as long as 1000 hours. The solder compositions studied include Sn-3.5Ag-0.2Cu, Sn-3.5Ag-0.3Cu, Sn-3.5Ag-0.4Cu, Sn-3.5Ag-0.5Cu, Sn-3.5Ag-0.6Cu, Sn-3.5Ag-0.7Cu, and Sn-3.5Ag-1.0Cu. Analysis techniques used include optical microscope, SEM, EPMA, and XRD. It is found that the strong sensitivity to the Cu composition disappeared after solid-state aging at high temperatures for a sufficient period of time. For all the Cu concentrations studied, the same type of intermetallic compounds formed at the interface after aging. A layer of (Cu1-yNiy)6Sn5 over a layer of (Ni1-xCux)3Sn4 was found at the interface. This study showed that the initial difference in the intermetallic compounds right after reflow can be aged out at high temperatures. The growth mechanisms for (Cu1-yNiy)6Sn5 and (Ni1-xCux)3Sn4 are different, and are pointed out in this study.