Prevention of spalling by the self-formed reaction barrier layer on controlled collapse chip connections under bump metallization

The spalling phenomenon on under bump metallization (UBM) is one of the current urgent reliability issues for the Pb-free solder implementation in flip chip technology. In this paper, we report that spalling of Ni thin UBM can be prevented during the soldering reaction, if a Cu reservoir is introduced into the structure of controlled collapse chip connections (C4) solder joints. Once molten Sn-3.5Ag solder was saturated with Cu atoms, Cu precipitated out as a layer of Cu-Sn compound on Ni thin UBM. The Cu-Sn compound layer served as a reaction barrier to retard the consumption of Ni thin UBM. So, spalling was retarded. After prolonged reflowing, Ni thin UBM was converted to ternary Cu-Sn-Ni compounds. Unlike interfaces of the Ni-Sn compound/Cr, the interface of the Cu-Sn-Ni compound/Cr was very stable and no spalling was found.