Bismuth redistribution induced by intermetallic compound growth in SnBi/Cu microelectronic interconnect

It was found that Bi particles, with a diameter of 100 nm, precipitated along Cu(3)Sn/Cu interface and Bi crystallites dispersed in Cu(3)Sn layer in 42Sn58Bi/Cu microelectronic interconnect, when it was aged at 120 A degrees C for 7 days. The mechanism for Bi redistribution like this was discussed. Cu(6)Sn(5) turned into Cu(3)Sn by Cu diffusion that is dominant in Sn/Cu inter-diffusion during the aging process. Bi precipitation occurred in Cu(3)Sn due to lower Bi solubility in Cu(3)Sn than that in Cu(6)Sn(5). The Bi precipitates can traverse the formed Cu(3)Sn quickly toward the Cu(3)Sn/Cu interface, attributed to the Kirkendall effect. They stayed and nucleated there to form particles, owing to their unwettability on Cu. The formed Cu(3)Sn got oversaturated with Bi, when the joint cooled from 120 A degrees C to room temperature. Then Bi crystallites precipitated dispersedly in Cu(3)Sn layer.