Effects of the gold thickness of the surface finish on the interfacial reactions in flip-chip solder joints

Abstract

The effects of Au thickness on the flip-chip solder joints with Cu/Ni/Al underbump metallurgy (UBM) on one end and the Au/Ni surface finish on another was studied. Two different thicknesses, 0.1 µm and 0.65 µm, were used for the surface finish. After assembly, the joints were subjected to thermal aging at 150°C. The difference in Au thickness had a strong effect on the consumption rate of the Ni layer in the UBM as well as on the failure mode of the solder joints. When the Au layer was thin (0.1 µm), the dissolved Cu from the Cu/Ni UBM was able to inhibit the formation of AuSn4. When the Au layer was thick (0.65 µm), the dissolved Cu was not able to inhibit the formation of AuSn4. These AuSn4 enhanced the Ni consumption rate of the UBM. The presence of a large amount of AuSn4 inside the solder also weakened the solder because of the Au embrittlement effect. In view of these observations, the gold thickness on the Au/Ni surface finish must be kept to the minimum controlled in order to prolong the service life of flip-chip packages.