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The cross-interaction of the under-bump metallurgy (UBM)/solder interface and the solder/surface-finish interface in flip-chip solder joints was investigated. In this study, the UBM on the chip side was a single layer of Cu (8.5 µm), and the surface finish on the substrate side was a 0.2-µm Au layer over 5-µm Ni. It was shown that, after two reflows, the Ni layer of the surface finish had been covered with (Cu 1Ϫx Ni x ) 6 Sn 5 . This shows that the effect of cross-interaction of the two interfaces is important even during the reflow stage. During subsequent solidstate aging at 115°C, 135°C, and 155°C, the formation of (Cu 1Ϫx Ni x ) 6 Sn 5 over the Ni layer was found to have the effect of reducing the Ni consumption rate. At the same time, the Cu consumption rate of the UBM was accelerated. The results of this study show that the selection of the UBM and the surface finish has to be considered together because the cross-interaction of the two interfaces plays an important role.

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. 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 AuSn 4 . When the Au layer was thick (0.65 µm), the dissolved Cu was not able to inhibit the formation of AuSn 4 . These AuSn 4 enhanced the Ni consumption rate of the UBM. The presence of large amount of AuSn 4 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.

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Summary In this paper, the innovative phase control active tuned mass damper (PC‐ATMD) is developed so that the PC‐ATMD can achieve a 90° phase lag of structure to induce maximum power flow. The equation of motion and proposed phase control algorithms are derived in detail. By utilizing the ATMD velocity relative to structure and different structural measurement feedback, two phase control algorithms are, thus, proposed individually. The PC‐ATMD, which uses structural displacement feedback, is denoted as the phase control displacement feedback active tuned mass damper (PCD‐ATMD), whereas the use of structural absolute acceleration feedback is the phase control acceleration feedback active tuned mass damper (PCA‐ATMD). The control of PC‐ATMD is smooth, proportional, and neither full‐state system measurement nor estimation required. This paper also describes the optimal control gains of the PCD‐ATMD or PCA‐ATMD which can be determined by direct output feedback optimization method. The shear building structures that apply PC‐ATMDs are subjected to earthquakes. The results indicate that, after applying the PCD‐ATMD or PCA‐ATMD, the peak displacement and absolute acceleration of the primary structure are both well suppressed. Moreover, the PCD‐ATMD has comparable effectiveness than the traditional linear quadratic designed ATMD which uses full‐state feedback, while the PCA‐ATMD is only slightly less performance than the PCD‐ATMD but more feasible for the application related to tall building structures. Finally, the stability of the proposed algorithms is also verified. It concludes that the PCD‐ATMD and PCA‐ATMD are both quite stable systems in a certain range of control gains.