Identification of IMCs at SAC/Fe-Ni interface during solid-state aging at 125°C

The high temperature storage test (HTST) was conducted on the SnAgCu/Fe-Ni solder joints. The microstructural evolution during aging at 125°C was observed by both Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). During the reflow process, FeSn
₂
layer and rod-like (Cu,Ni)
₆
Sn
₅
grains were formed. During the aging at 125°C, dispersed (Cu,Ni)
₆
Sn
₅
with two distinct morphologies were newly formed at the outer side of FeSn
₂
layer which have island shape and small rod-like shape respectively. Through TEM analysis, it was confirmed that the newly formed (Cu,Ni)
₆
Sn
₅
IMCs also has hexagonal primitive crystal structure. It was proved that the hexagonal structure was formed even below 189°C with the existence of Ni element. As the aging time increased, rod-like grains started to interconnect with each other and the island shape grains were progressed into a continuous IMC layer. The formation and the size of η-(Cu,Ni)
₆
Sn
₅
grains were determined by the diffusion flux of Cu and Ni elements. At the SAC/Fe-Ni interface during the reflow process, the Cu element was mostly supplying from the solder, thus the (Cu,Ni)
₆
Sn
₅
were mostly floating within the solder. Inversely, during the long-time aging, diffusion of Cu element came mostly from Cu pad at the bottom of Fe-Ni UBM while partly from the solder. Due to the restriction of substrate, island shape grains grew next to the FeSn
₂
layer and its size decreased giving to the diffusion barrier effect of Fe-Ni UBM. Moreover, the small rod-like grains were usually growing upon the island shape grains, so the decreased supply of Cu and Ni element would further reduce the dimension of the grain size of formed (Cu,Ni)
₆
Sn
₅
.