Effects of Ag concentration on the Ni-Sn interfacial reaction for 3D-IC applications

Ni/Sn solid-state reaction might induce the void formation with low-volume solder under a severe space confinement in 3D IC packaging, and Ag addition could effectively eliminate void formation. In the present study, the different Ag concentration in solder was carried out to figure out the lower limit of Ag concentration to avoid void formation. The solid-state diffusion couple, Ni/Sn-xAg/Ni (x = 0 wt.%, 1.0 wt.%, 2.4 wt.%, 3.5 wt.%, 4.5 wt.%, 8.0 wt.%), was prepared by thermal compression. The isothermal aging process was carried out at 200°C. There were only two IMCs being observed during the entire aging process: Ag₃Sn and Ni₃Sn₄. The Ag₃Sn initially formed as fine and eutectic network in the Sn matrix after cooling. The Ag₃Sn particles were found eventually located at the middle of sandwich structure. After Sn was totally consumed, there was very different microstructure in the sandwich structure in different Ag concentration.