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 Cu
₃
Sn layer had been report by many research groups. Because the Cu
₃
Sn growth had been linked to the formation of micro voids, which in turn increased the potential for a brittle interfacial fracture, thinner Cu
₃
Sn layers might translate into better solder joint strength. Main thrust of retarding Cu
₃
Sn is the minor alloy additions. One of the more note-worthy alloying elements is Ni. In order to investigate the effects of Ni on Cu
₃
Sn, 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, Cu
₃
Sn was the only reaction product observed for all the different solders used. In solid state aging study, both Cu
₃
Sn and Cu
₆
Sn
₅
formed in 10Sn90Pb-xNi solders, but only Cu
₃
Sn 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 Cu
₃
Sn 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 Cu
₃
Sn 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 Cu
₃
Sn thickness. However, the Ni addition to lead-free solder can retard Cu
₃
Sn. (2) The Sn flux from the solder towards the Cu substrate would be reduced by thick Cu
₆
Sn
₅
. The shrinkage of Cu
₃
Sn attribute to the decrease of Sn flux. (3) Ni retards the Cu
₃
Sn growth through Cu
₆
Sn
₅
. (4) Micro voids formed after aging at 160 oC for more than 500 h in 10Sn90Pb-xNi and 5Sn95Pb-xNi solders.