Redistribution of Pb-rich phase during electromigration in eutectic SnPb solder stripes

The microstructural evolution occurring within the eutectic SnPb solder during electromigration is investigated utilizing Blech specimens. Solder stripes of about 3 mu m in thickness were fabricated on Cu/Ti metallization. It was found that the Pb-rich phase ripened and aligned along the direction of the electron flow following the current stressing of 9.7x10(3) A/cm(2) at 80 degrees C for 24 h. As the stressing time or current density increased, the redistribution of the Pb-rich phase became more significant. Three-dimensional simulation was performed to examine the change in resistance and current-density distribution during electromigration. It was found that when the Pb-rich phase aligned along the direction of the electron flow, the total resistance of the solder stripe decreased. This reduction in total resistance may provide the driving force for the redistribution of the Pb-rich phase during electromigration. In addition, current crowding occurred in the vicinity of the interface of the Pb-rich and Sn-rich phases, creating a gradient of current density. This gradient might trigger the resistive Pb atoms to align along the direction of the electron flow.