Spalling phenomenon is one of the current urgent reliability issues for the Pb-free solder implementation in flip chip technology. Essentially, spalling was caused by high interfacial energy between intermetallic compound and Cr, which dewetted the layer compound into a spherical compound. In this paper, we report that spalling of Ni thin UBM (Under Bump Metallization) can be prevented during the soldering reaction, if a Cu reservoir is introduced into the structure of C4 (Controlled Collapse Chip Connections) 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. Cu-Sn compound layer served as a reaction barrier to retard the consumption of Ni thin UBM. So, spalling is retarded. After prolonged reflowing, Ni thin UBM was converted to ternary Cu-Sn-Ni compounds. Remarkably, we found that Cu-Sn-Ni compound still resided on the Cr surface without spalling. Unlike interfaces of Cu-Sn compound/Cr or Ni-Sn compound/Cr, the interface of Cu-Sn-Ni compound/Cr is very stable and has a low interfacial energy. Hence, spalling effect on Ni thin UBM is prevented.