Orientation distribution, morphology, and size distribution of copper-tin intermetallic compound in reactions between molten solder and copper in flip chip solder joints

Metallic bonding between copper and solder is achieved through the formation and growth of Cu-Sn intermetallic compound (IMC) at the interface between them. Many reliability issues of solder joints are related to the formation and growth of the IMC since failure tends to occur at solder joint interfaces. Thus understanding on IMC formation and growth can help to improve reliability of solder joints. In this study, a systematic study on formation, growth and morphology of Cu6Sn5 IMC has been conducted.;To understand formation mechanism of the Cu6Sn 5, structural relationship between the Cu6Sn5 and substrate Cu was investigated. The orientation distribution of the Cu 6Sn5 scallops and its relationship with the orientation of Cu was studied using synchrotron radiation based micro x-ray diffraction. The orientation of the Cu6Sn5 scallops had a strong dependence on that of Cu. The [-101] direction of Cu6Sn5 is always parallel to [110] of the Cu. The misfit was 0.24%. The strong correlation in orientation suggests that Cu6Sn5 forms prior to the Cu3Sn in solder reaction. The orientation relationship was still valid when pure Sn reacted with Cu, suggesting that Pb-free solders will show same results. A (001) single crystal Cu was used as substrate to simplify experimental conditions because the low misfit directions between Cu 6Sn5 and Cu lie on (001) plane of Cu. The morphology of Cu 6Sn5 showed a dramatic change, indicating that ripening and growth mechanism of Cu6Sn5 may be different from the usual case.;Once Cu6Sn5 has formed, both ripening and growth of Cu6Sn5 takes place at the solder/metal interface. It was found that morphology of Cu6Sn5 scallops depends on composition of solder. The Cu6Sn5 scallops showed round scallop-type morphology when the SnPb solder composition was changed from eutectic (63Sn37Pb) to about 40Sn60Pb. In other compositions, the IMCs showed faceted morphology. The 55Sn45Pb solder was chosen to measure the size distribution and growth rate of scallops. Average radius of scallops was proportional to cube root of time, and size distribution was in good agreement with the Flux-Driven-Ripening (FDR) theory, which assumed nonconservative ripening under constant total interfacial area between the scallops and solder.