Effects of Ni on the nucleation and growth behavior of Cu6Sn5 in Sn–8.5Cu alloy: An in situ observation

Abstract Cu6Sn5 and Cu3Sn are important intermetallic compounds, greatly influencing the reliability of the Sn-Cu solder system. Micro-alloying Ni has been proved to be effective in suppressing Cu3Sn, but its influence on the formation of Cu6Sn5 is still unclear. In this paper, the nucleation and growth behavior of Cu6Sn5 in Sn-8.5Cu-xNi (x = 0, 0.05, 0.1, 0.5, 1) alloys are in situ investigated using synchrotron X-ray radiography, X-ray diffraction, and electron probe micro-analyzer. The results show that Ni can significantly increase the nucleation temperature of (Cu, Ni)6Sn5 since (Cu, Ni)6Sn5 has a more negative heat of formation. When Ni ≥ 0.05 wt%, the nucleation temperature of (Cu, Ni)6Sn5 will be higher than that of Cu3Sn. As a result, the peritectic reaction Cu3Sn + L → Cu6Sn5 doesn't occur and instead (Cu, Ni)6Sn5 nucleates from the melt before cooling down to the nucleation temperature of Cu3Sn. During the nucleation and growth of (Cu, Ni)6Sn5, Ni-rich clusters preferentially form stable crystal nuclei, and then low Ni parts grow on them. Raising Ni content increases the number of Ni-rich clusters, so the grain density of (Cu, Ni)6Sn5 increases with Ni addition. With the growth of (Cu, Ni)6Sn5, some Ni-rich clusters are in contact with (Cu, Ni)6Sn5-liquid interface and promote the rapid growth of (Cu, Ni)6Sn5 around the newly formed Ni-rich region. The excessively enlarged parts break down at the weak points because of gravity, further refine the grain. The results based on this study provide an opportunity to optimize the structure of Cu6Sn5 in the Sn-Cu system.