The effects of minor Fe, Co, and Ni additions to lead-free solders on the thickness of Cu3Sn at the interface

Owing to the ban of lead, the conventional lead-bearing solder has been replaced by lead-free solder. Most lead-free solders are Tin-based. The drive for lead-free solders in the microelectronics industry presents some reliability challenges. Examples include package compatibility, creep , and Kirkendall void. Along the Cu₃Sn/Cu interface, we can find a series of Kirkendall void. These Kirkendall void were the true culprit responsible for the weakening of the interface. It is widely accepted that the formation of these Kirkendall void is related to the growth of Cu₃Sn. In order to promote the quality of lead-free solder, minor elements addition can reduce the Cu3Sn thickness. Recently, our research group showed that a 0.1 wt.% Ni addition to SnAg could reduce the Cu₃Sn thickness during the solder/Cu reaction. We want to extend this past result to find out the minimum level of Ni addition that still retains this beneficial effect. In addition, we will also investigate whether the elements, Fe and Co will have a similar effect. The experimental solder alloys were fabricated from 99.999% purity Sn, Ag, Cu, Fe, Co, and Ni. The objective of this study is to investigate the effects of minor Fe, Co, and Ni on the soldering and aging reactions between lead-free solders and Cu. The experimental result shows that the presence of Ni can in fact reduce the growth rate of Cu₃Sn but increase the formation of Cu₆Sn₅. Moreover, the presence of Fe and Co can have the some effect. We can find the Kirkendall void in the reaction between Sn2.5Ag-xNi (x=0~0.1wt. %) and electroplated Cu at 160 ^oC for excess 1000 hr. The observation of Kirkendall void formation near the Cu₃Sn/Cu is direct evidence of Cu diffusion since we can use the voids to serve as diffusion markers. On the side, we didn´t find voids in the reaction between Sn2.5Ag0.8Cu-xNi (x=0~0.1wt. %) and electroplated Cu. The growth of voids i- - s complicated. We consider that the Cu concentration in the solders is the factor to control the void formation. In the Sn2.5Ag-xNi solders, the addition of Ni also produces two distinct Cu₆Sn₅ regions at the interface. The outer region contains more Ni, and the inner region contains less Ni. Cooling conditions changed the Ni content of the Cu₆Sn₅ formed at the interface. Besides, the Sn2.5Ag0.8Cu-xNi solders didn´t have two different Ni content in the Cu₆Sn₅. This is because there are more Cu₆Sn₅ precipitated in the Sn2.5Ag0.8Cu-xNi than in Sn2.5Ag-xNi solders. A part of Ni could be dissolved in the Cu₆Sn₅. Therefore, a few Ni could come back to interface.