Optimal Ag addition for the elimination of voids in Ni/SnAg/Ni micro joints for 3D IC applications

•We found the optimal Ag addition for the elimination of voids in Ni/SnAg/Ni micro joints.•In solid-state reactions, the optimal weight percentage of Ag is between 2.4 and 3.5wt.%.•In solid–liquid reactions, void-free structures can be obtained after Sn was fully consumed.•The addition of Ag is found to have no effect on the growth kinetics of Ni3Sn4. In micro joints, in which the height of solder is smaller than 10μm, the reaction between Sn and Ni is known to produce microvoids. Although it was recently shown that the addition of Ag can inhibit the formation of such microvoids, the optimal concentration of Ag has not yet been established. This study systematically investigates the effects of Ag concentration in the range of 0–8wt.%, with the objective of identifying the optimal Ag addition. It is found that the optimal weight percentage of Ag is between 2.4 and 3.5wt.%; when it is lower than 2.4wt.%, not all of the microvoids are eliminated, while when it is substantially higher than 3.5wt.%, Ag3Sn becomes the primary solidification phase and large Ag3Sn plates form. Furthermore, the addition of Ag is found to have no effect on the growth kinetics of Ni3Sn4.