The reaction sequence between liquid Sn-3Ag-0.5Cu solder and solid Au/Pd(P)/electrolytic-Ni films was carefully examined using a field-emission transmission electron microscope at different exposure times (15 s, 30 s, and 120 s). After 15 s of exposure, the uppermost layer of Au was removed from the interface and a portion of the Pd(P) film remained. At this stage of the reaction, the predominant products were PdSn3 and Pd3P. After 30 s of exposure, Pd(P) was completely exhausted, and three additional intermetallic species, including Pd-Sn-P, Pd6P, and Pd15P2, nucleated. After 120 s of exposure, the aforementioned species were destroyed, and Cu and Ni were involved in the reaction. The predominant product became (Cu,Ni)(6)Sn-5, and the nucleation of a nanocrystalline Ni2SnP layer in the middle of (Cu,Ni)(6)Sn-5 resulted. These results suggest that Pd and P play a vital role in the early stage of soldering reaction, even though the Pd(P) film is only a few submicrons thick and its P content is quite low (2-5%). (c) 2012 Elsevier B.V. All rights reserved.