| dc.description.abstract | Owing to the ban of lead, the conventional lead-bearing solder has been replaced by lead-free solder. The drive for lead-free solders in the microelectronics industry presents some reliability challenges. Examples include package compatibility, creep, and Kirkendall’s voids. Along the Cu3Sn/Cu interface, we can find a series of Kirkendall’s voids. These Kirkendall’s voids were the true culprit responsible for the weakening of the interface. It is widely accepted that the formation of these Kirkendall’s voids 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 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’s voids 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’s 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 is 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. | en_US |