Investigation of Cr/Cu/Cu/Ni under bump metallization for lead-free applications

The elimination of lead in the solder flip-chip attachment material has necessitated many new studies on the reliability of the resultant systems. There are many lead-free solder material systems. However, most of them contain a large amount of tin. The tin in the solder reacts with the copper layer of the conventional UBM, depleting the UBM of copper, and causing loss of adhesion and a weak interface. In this work, a reliability comparison of Cr/Cu/Cu and Cr/Cu/Cu/Ni underbump metallization systems was studied. The UBM systems were deposited with electroplated Sn-3.5Ag lead-free solder alloy. The results conclusively showed that the Cr/Cu/Cu/Ni UBM system is a better choice for the Sn-3.5Ag lead-free solder. This is primarily because Ni has a slow dissolution rate into the solder as compared with Cu. The former avoids excessive intermetallic formation between the joint metallization interfaces, which is a factor for the reliability of the joints. In this Cr/Cu/Cu/Ni UBM system, the thickness of the nickel layer was found to be an important parameter. The thickness of the nickel must be thick enough to inhibit copper diffusion and suppress the copper intermetallic formation with the tin in the solder. Multiple reflow and high temperature storage test results showed that serious depletion of UBM layer can occur if the UBM layers are not optimized. Both UBM systems were tested for temperature cycling. Thermal-mechanical reliability of Ni-based UBM bumps showed promising results even up to 1500 cycles of temperature cycling.