Effects of the Gold Thickness of the Surface Finish on the Interfacial Reactions in Flip Chip Solder Joints

Chip Solder Joints Y. L. Lin, W. C. Luo, Y. H. Lin, C. E. Ho, and C. R. Kao Department of Chemical & Materials Engineering National Central University Jhongli City, Taiwan (*E-mail: [email protected] Phone/Fax: +886-3-4227382) Abstract The effects of Au thickness on the flip chip solder joints with the Cu/Ni/Al UBM on one end and the Au/Ni surface finish on another was studied. Two different thicknesses, 0.1μm 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 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 large amount of AuSn4 inside the solder also weakened the solder due to the Au-embrittlement effect. In view of these observations, the gold thickness on Au/Ni surface finish must be kept to the minimum controlled in order to prolong the service life of the flip chip packages.