Effect of interfacial strength between Cu6Sn5 and Cu3Sn intermetallics on the brittle fracture failure of lead-free solder joints with OSP pad finish

The brittle failure mechanism of solder joints has gained more and more attention due to the introduction of lead-free solders and also the extensive use of different kinds of portable electronic products. The excessive growth of the IMC is blamed to increase the propensity of the brittle failure. Typically the thickness of the IMC layer is used to indicate the risk of failure of solder joints. Cu₆Sn₅ and Cu₃Sn are the most common IMCs found in Sn-Ag-Cu (SAC) lead-free solder joints with OSP pad finish. Previous studies revealed that the brittle failure mostly occurred in the IMC region. However, the respective role of Cu₆Sn₅ and Cu₃Sn in the solder joint brittle failure still remains unclear. In the present study, different amounts of Nickel (Ni) were doped in the Sn-Cu based solder. Its effects on the growth rate of (Cu, Ni)₆Sn₅/Cu₆Sn₅ and Cu₃Sn were characterized at first. Subsequently, the results of characterization were utilized to produce different growth rates of (Cu, Ni)₆Sn₅ and Cu₃Sn under thermal aging. The thicknesses of (Cu, Ni)₆Sn₅/Cu₆Sn₅ and Cu₃Sn after different aging hours were measured using SEM (with EDX and BSE). High speed ball pull/shear tests were performed. The correlation between IMC strength and IMC layer thicknesses was established. An investigation on the effect of Ni on the micro-hardness of solder is also included in this study.