Kinetic study of eutectic Sn-3.5Ag and Electroplated Ni metallization in flip-chip solder joints

It is well known that Ni and its alloys possess a lower reaction rate with Sn than Cu and Cu alloys. Therefore Ni-based under bump metallization have attracted attention in recent years. In this study, the diffusion-controlled reaction between eutectic Sn-3.5Ag flip chip solder joints between electroplated-Ni (EP-Ni) and electroless-Ni (EL-Ni) has been investigated. Morphology and growth kinetics of the formed Ni
₃
Sn
₄
are study under different reflow temperatures, durations and times. The growth rates and the formation activation energy of Ni
₃
Sn
₄
IMC were estimated with EP-Ni and EL-Ni, respectively. The obtained data showed that the Ni
₃
Sn
₄
in the Sn-3.5Ag filp chip solder grow much faster with EP-Ni than EL-Ni UBM under the same condition. In summary, we can conclude that the slow reaction rate of EP-Ni provided a well-attached contact at the interface after 20 minutes reflow process and is better compare with EL-Ni UBM at the substrate.