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It is well known that intermetallic compounds (IMCs) is the controlling factor on the reliability since the IMC is the brittle part in a solder joint. Alloy elements into solders can change the reaction products or eliminate the formation of Kirkendall voids at the solder joints. In this study, four different Sn-Ag-Al solders, with the Ag concentration fixed at 3.5 wt% and Al concentrations varied between 0.5-1.5 wt%, were used to study the interfacial reactions between Sn- 3.5Ag-xAl (x=0.5, 1.0, 1.5 wt%) solders with Cu and ENIG (electroless nickel immersion gold) metal pads. The effects of Al concentration in solders on the interfacial microstructures were investigated. The results showed that the interfacial reaction products depended on the Al concentration in the reactions between Sn-Ag-xAl and Cu. When the Al concentration was 0.5 wt% and 1.0 wt%, a CugSn5 layer formed at the interface and CuAl2 IMCs were observed in the solder matrix, and CuAl2 IMC developed at the solder joints during the aging. An addition of 1.5 wt% Al introduced a layer-type CuAl2 layer at the interface and eliminated Cu-Sn IMCs, and the thickness of CuAl2 was much thinner than that of CuSn IMC. For electrloss Ni(P)/Au UBM, 0.5Al and 1.0Al solders suppressed the formation of Ni3SnP layer and IMC spalling at the solder joints, but Ni3P layer was still observed. The beneficial effect of Al was clearly revealed in the case of 1.5Al solder ; a mixed phase, Al3Ni2 and Ni3Sri4, developed during the reflow and subsequent aging and Ni3P layer was effectively suppressed, which would increase the solder joint reliability significantly.