Effects of Ni addition to high-lead solders on the growth of Cu3Sn and micro voids

It is of importance for the reliability of lead-free and lead-bearing solder joints to have better understanding and control of the solder/metallization interactions during soldering. In the reactions between solders and Cu substrate, the formation of micro voids within the Cu3Sn layer had been report by many research groups. Because the Cu3Sn growth had been linked to the formation of micro voids, which in turn increased the potential for a brittle interfacial fracture, thinner Cu3Sn layers might translate into better solder joint strength. Main thrust of retarding Cu3Sn is the minor alloy additions. One of the more note-worthy alloying elements is Ni. In order to investigate the effects of Ni on Cu3Sn, the solders used for this study are 10Sn90Pb and 5Sn95Pb doped with 0, 0.06, and 0.2 wt.% Ni. These solders were prepared from 99.999 % purity elements. For the investigation of microstructure evolution of the solder joint, Cu plates with 99.99 % purity were used. Reaction conditions included one reflow at 350 °C for 2 min and solid-state aging at 160 °C for up to 2000 h. In reflow study, Cu3Sn was the only reaction product observed for all the different solders used. In solid state aging study, both Cu3Sn and Cu6Sn5 formed in 10Sn90Pb-xNi solders, but only Cu3Sn formed in 5Sn95Pb-xNi solders. These phenomena can clearly know by using the ternary SnPbCu phase diagrams. The objective of this study is to investigate the influences of Ni on Cu3Sn growth and micro voids in different kinds of solders. Emphasis is placed on a systematic comparison study on the effects of Ni addition. The effect of Ni on the Cu3Sn and micro voids were discussed in detail based on the experimental results. The results of this study can be summarized as below: (1) Minor Ni addition to high-lead solder can't retard Cu3Sn thickness. However, the Ni addition to lead-free solder can retard Cu3Sn. (2) The Sn flux from the solder towards the Cu substrate would be reduced by thick Cu6Sn5. The shrinkage of Cu3Sn attribute to the decrease of Sn flux. (3) Ni retards the Cu3Sn growth through Cu6Sn5. (4) Micro voids formed after aging at 160 oC for more than 500 h in 10Sn90Pb-xNi and 5Sn95Pb-xNi solders.