Field-emission transmission electron microscopy study of the reaction sequence between Sn–Ag–Cu alloy and an amorphous Pd(P) thin film in microelectronic packaging

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)6Sn5, and the nucleation of a nanocrystalline Ni2SnP layer in the middle of (Cu,Ni)6Sn5 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%).

► Reaction sequence between an amorphous Pd(P) film and Sn–Ag–Cu alloy.
► Solder reaction assisted the crystallization of amorphous Pd(P) into Pd–P phase(s).
► Direct proof of the Pd(P)-induced Ni2SnP nucleation.
► Pd and P both played a central role in the early stage of soldering reaction.