Effect of Ag concentration on Ni/Sn-xAg/Ni micro joints under space confinement

Previous studies have shown that voids were observed in 3D IC-scaled micro joints of Ni/Sn/Ni solid state reaction under severe space confinement and Ag addition can effectively eliminate the void formation. Therefore, the present study is aimed at investigation of the lower limit of Ag concentration which prevents void formation and also the effect of Ag concentration when primary Ag₃Sn appears. The solid state diffusion couple, Ni/Sn-xAg/Ni, was prepared by thermal compression with six different Ag concentrations from 0 to 8.0 wt.%. After thermal compression bonding, samples were placed at 200°C for isothermal aging process. Subsequent observation of cross-section showed that as-bonded Ag₃Sn initially formed as fine particles dispersed in Sn matrix. Then, Ag₃Sn particles coarsened during aging and were likely heterogeneously located at the Ni₃Sn₄ layer. After Sn was completely consumed, Ag₃Sn particles were located at the middle of sandwich structure and microstructure of various Ag concentrations were distinct. A nearly void-free sandwich structure could be established by adding more than 3.5 wt. % Ag in solder when Sn was fully consumed. In addition, the relation between the area fraction of continuous Ag₃Sn layer and Ag concentration was found linear.