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 A degrees 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 A degrees C to room temperature. Then Bi crystallites precipitated dispersedly in Cu3Sn layer.