Transmission electron microscopy characterization of the porous structure induced by high current density in the flip-chip solder joints

There are several identified failure mechanisms such as void formation and propagation, the local melting mechanism and the underbump metallization (UBM) dissolution in flip chip solder joints. In our previous studies Ni(V) UBM consumption was also found to cause the failure. The flip chip solder joint used in this study was with Al/Ni(V)/Cu UBM on the chip side and an Au/Ni surface finish on the substrate side. The solder was used by eutectic PbSn. The aging temperature was constant 150?C and a 0.32A current stressing was applied to make a nominal current density of 5 x 10³A/cm². Owing to the combined effects of current crowding and local Joule heating, the microstructure of the Ni(V) UBM near the entrance of the electrons into the joints was transformed to a porous structure after 550 hours. Afterwards the porous structure propagated all over the UBM to make the solder joints fail since the porous structure was non-conductive. The microstructure was firstly observed by field emission scanning electron microscopy (FE-SEM) and subsequently focused ion beam (FIB) was used to fabricate specimens for transmission electron microscopy (TEM) observation. The porous structure was composed of many voids near the interface of Ni(V)/(Cu,Ni)₆Sn₅ and a void-free area near the interface of Ni(V)/Al. Energy dispersion X-ray (EDX) analysis on TEM was performed for composition analyses. In the porous structure almost no Ni signal was detected except for the region near the interface of Ni(V)/Al. Pb-rich dark patches observed in the porous structure revealed that a severe diffusion behavior was occurred during electromigration. Selected area diffraction patterns (SADPs) were derived to identify phases. The results showed that the matrix of the porous structure was amorphous. Fine grains of Cu₆Sn₅ and V₂Sn₃ were randomly distributed in the matrix of the porous structure and a thin layer of Ni_{- 3}Sn₄ was located at the interface of Ni(V)/Al.