The mutual interaction between Sn/Ni and Sn/Cu interfacial reactions in a Ni/Sn/Cu sandwich sample has been studied. The major interfacial reaction product on the Cu side was Cu 6Sn 5, while on the Ni side, a ternary (Cu,Ni) 6Sn 5 compound layer was formed. We found that the growth kinetics of the interfacial compound layers on both sides reached a steady state in the late reflow stage. The interfacial compound layer on the Cu side retained a constant thickness. On the other hand, the interfacial compound layer on the Ni side grew at a relatively fast rate, which was found to be linear with time. Our results indicate that the growth of the ternary (Cu,Ni) 6Sn 5 compound layer was controlled by the Cu dissolution flux at the solder/Cu 6Sn 5 compound interface. The dissolution constant of the Cu 6Sn 5 compound into the molten Sn was determined to be 0.13 μm/s.
- Flip chip
- Pb-free solder