Nanohardness study of CoSn2 intermetallic layers formed between CO UBM and Sn flip-chip solder joints

The possibility to use Co as UBM for Pb-free solder joints is related to the reliability of the formed intermetallic layers at the solder-UBM interface. The interdiffusion reaction results in a layer of CoSn₂ intermetallic, which was tested by means of nanoindentation. A nanohardness of 2Gpa was measured, which is lower then the one of Ag₃Sn, Cu₃Sn or Cu₆Sn₅, reported in the literature. This makes the chances of brittle behaviour of this intermetallic very low, which is confirmed by SEM examinations. The measured low hardness of the CoSn₂ intermetallic layer and the gradual change of the hardness in the stack Co-CoSn₂-Sn suggest that this interface will be very stable and reliable during the lifetime of the solder joint.