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 Cu₃Sn/Cu interface and Bi crystallites dispersed in Cu₃Sn layer in 42Sn58Bi/Cu microelectronic interconnect, when it was aged at 120 °C for 7 days. The mechanism for Bi redistribution like this was discussed. Cu₆Sn₅ turned into Cu₃Sn by Cu diffusion that is dominant in Sn/Cu inter-diffusion during the aging process. Bi precipitation occurred in Cu₃Sn due to lower Bi solubility in Cu₃Sn than that in Cu₆Sn₅. The Bi precipitates can traverse the formed Cu₃Sn quickly toward the Cu₃Sn/Cu interface, attributed to the Kirkendall effect. They stayed and nucleated there to form particles, owing to their unwettability on Cu. The formed Cu₃Sn got oversaturated with Bi, when the joint cooled from 120 °C to room temperature. Then Bi crystallites precipitated dispersedly in Cu₃Sn layer.