This paper investigates the effect of 0.05 wt.% Ni on the formation and growth of primary Cu₆Sn₅ in Sn-0.7 wt.%Cu solder paste soldered on a Cu substrate, using a real-time synchrotron imaging technique. It was found that small additions of Ni significantly alter the formation and growth of the primary Cu₆Sn₅ intermetallics, making them small. In contrast, without Ni, primary Cu₆Sn₅ intermetallics tend to continue growth throughout solidification and end up much larger and coarser. The primary effect of the Ni addition appears to be in promoting the nucleation of a larger amount of small Cu₆Sn₅. The results provide direct evidence of the sequence of events in the reaction of Nicontaining Sn-0.7 wt.%Cu solder paste with a Cu substrate, and in particular the formation and growth of the primary Cu₆Sn₅ intermetallic
