Electromigration failure of SnAgCu lead-free BGA package assembled with SnPb solder paste

Electromigration damage in Sn-3.0Ag-0.5Cu (SAC305) lead-free solder joints of Ball Grid Array (BGA) package assembled with eutectic SnPb paste was investigated after current stressing at 100°C with a density of 1×10⁴ A·cm^-2 for up to 180 h. The under bump metallization (UBM) for the mixed solder joints on the substrate side was Cu/electroplated Ni, while the pad on the printed circuit board (PCB) side was Cu plate. The results show that open failure occurred in the Cu trace at the upper-right corner of the solder joint with a downward current flow in the thermo-eletromigration test. As the current crowding occurred at that point and induced the Ni and Cu atoms excessive migration. The Pb atoms were found to move in the same direction as with the electron current flow, and accumulated at the anode. The two new factors, the initial concentration gradient of Pb atoms and current density distribution in the solder bump, for the redistribution of Pb-rich phases to be considered to clarify that phenomenon. Meanwhile, the polarity effect of electro-migration on the interfacial reaction of mixed solder bump was obvious. Electric current enhance the growth IMC layers at anode, and the interfacial microstructure of triple layers ((Cu, Ni)₆Sn₅, Cu₆Sn₅, and Cu₃Sn) was observed. While the growth of IMC at anode, Cu₆Sn₅, was retarded.