Effect of surface finish on interfacial reactions of Cu/Sn-Ag-Cu/Cu(ENIG) sandwich solder joints

The interaction between Cu/Sn-Ag-Cu and Sn-Ag-Cu/Cu(ENIG) interfacial reactions has been studied during isothermal aging at 150 °C for up to 1000 h using a Cu/Sn-3.5Ag-0.7Cu/Cu(ENIG) sandwich solder joints. A typical scallop-type Cu-Sn intermetallic compound (IMC) layer was formed at the upper Sn-Ag-Cu/Cu interface after reflow. On the other hand, a (Cu,Ni)6Sn5 IMC layer was observed at the Sn-Ag-Cu/ENIG interface. The Cu in the (Cu,Ni)6Sn5 IMC layer formed on the Ni side has to be contributed from the dissolution of the opposite Cu metal pad or Cu-Sn IMC layer and the original Cu atom in the Sn-Ag-Cu solder. When the Cu arrived at the interface of the Ni pad, the (Cu,Ni)6Sn5 IMC layer was formed on the Ni interface, preventing the Ni pad from reacting with the solder. Although a long isothermal aging treatment at 150 °C was performed, any Ni was not detected in the Cu-Sn IMC layer formed on the upper Cu side. Compared to the single Sn-Ag-Cu/ENIG solder joint, the formation of the (Cu,Ni)6Sn5 IMC layer at the lower Sn-Ag-Cu/ENIG interface of the Cu/Sn-Ag-Cu/ENIG sandwich joint retarded effectively the consumption of the Ni from the electroless Ni-P layer, resulting in the formation of the very thin Ni3P layer. The coupling effect of the Cu/Sn-Ag-Cu/ENIG sandwich joint is helpful to prevent the excessive consumption of Ni substrate and the formation of the brittle Ni3P and Ni2SnP layers.