Journal of SMT Article


Authors: W. T. Chen, R. Y. Tsai, Y. L. Lin, and C. R. Kao
Company: National Central University
Date Published: 10/1/2002   Volume: 15-4

Abstract: The eutectic 99.3Sn/0.7Cu solder (wt.%, Sn/0.7Cu) is the most promising lead-free replacement for the eutectic SnPb solder in wave soldering applications, and Ni is used in several commercially important surface finishes, such as AuNi and PdNi. It had been reported in the literature that, during reflow of SnCu solders over the Ni-bearing surface finishes, a slight variation in Cu concentration produced completely different reaction products [1, 2]. With increasing Cu concentration, the reaction product switched from (Ni1-xCux)3Sn4 to (Cu1-yNiy)6Sn5+(Ni1-xCux)3Sn4, then to (Cu1-yNiy)6Sn5.

In this study, we extended our earlier effort to investigate the effect of a small perturbation in the Cu concentration on the solid-state aging reaction between the SnCu solders and Ni [2]. Specifically, four Sn/xCu solders (x=0.2, 0.4, 0.6, and 0.7) were reacted with Ni at 160, 180, and 225 degrees C. It was found that the strong sensitivity to 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.

Key words: SnCu, interfacial reaction, surface finish, lead-free solder.

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