Interfacial reactions and formation of intermetallic compound of Sn-ball/Sn-3.0Ag-0.5Cu-paste/Cu joints in flip-chip on BGA packaging

The interfacial reactions and formation of intermetallic compound of Sn-ball/Sn-3.0Ag-0.5Cu-paste/Cu joints in flip-chip on BGA packaging were studied by using a differential scanning calorimeter. Results show that a thin circular Cu₆Sn₅ layer forms first on the surface of Cu substrate due to the diffusion of Sn atoms dissolved in the soldering flux, subsequently the planar-like Cu₆Sn₅ layer forms and covers over the first layer, and many Ag₃Sn particles form in the grain boundaries of Cu₆Sn₅. Then, a slight increase of the soldering temperature from 217°C to 218°C has a significant influence on the morphologies of the interfacial Cu₆Sn₅ and Ag₃Sn, that is, the morphologies of Cu₆Sn₅ and Ag₃Sn are changed into scallop-like and large plate-like, respectively. When the soldering temperature is increased to 221°C, a large amount of plate-like Ag₃Sn phase dissolves into the molten solder and the small network-like Ag₃Sn phase forms, and the grain size of Cu₆Sn₅ is increased. Furthermore, there is almost no trace of Ag₃Sn at the interface layer of Cu₆Sn₅ at the soldering temperature of 227°C. The morphology changes of the interfacial Cu₆Sn₅ and Ag₃Sn are attributed to the increase of interfacial energy and the eutectic reaction between Sn and Ag in the solder matrix, respectively.