Cross-interaction of Interfacial Reactions in Ni (au/ni/cu)-snag-cu Solder Joints during Reflow Soldering and Thermal Aging

The coupling effect of interfacial reactions between two pads in Ni-SnAg-Cu and Au/Ni/Cu-SnAg-Cu solder joints was investigated in this paper. After reflow soldering, a large amount of Cu can diffuse across the solder joint to the opposite pad to form (Cu,Ni)6Sn5 IMC on the Ni pad in Ni-SnAg-Cu solder joint. A little Ni can also be detected in Cu6Sn5 layer, which contains some trapped solder, on the opposite Cu pad. Some Cu6Sn5 was found in the solder bulk. For as-reflowed Au/Ni/Cu-SnAg-Cu solder joint, (Cu,Ni)6Sn5 and Cu6Sn5 were formed on the Au/Ni/Cu and Cu pad, respectively. A small amount of Au was involved in the (Cu,Ni)6Sn5 and Cu6Sn5 IMCs during the subsequent thermal aging. It is interesting to note that some (Cu,Au)6Sn5 with lower Au concentration was always surrounded by (Cu,Au)6Sn5 with higher Au concentration at the interface or in the solder bulk. Compared with Ni-SnAg-Cu solder joint, the IMCs formed on both interfaces in Au/Ni/Cu-SnAg-Cu solder joint were much thinner. Au plays an important role in inhibiting the growth of interfacial IMCs. At the same time, Cu from Cu pad at one side inhibits the redeposition behavior of (Au,Ni)Sn4.