A phenomenon that can result in electromigration suppression in solder bumps on Cu pads is reported. Under certain current stressing conditions, Ag migrates against the electron flow and forms a Ag3Sn compound layer at the cathode Sn5Ag/Cu interface. This phenomenon implies that it is the electrostatic force rather than the typical wind force that is the dominant electromigration force acting on the Ag solute atoms in the Sn matrix. The Ag3Sn compound layer formed at the Sn5Ag/Cu joint interface could effectively retard electromigration-induced consumption on the Cu bump pad. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3236787] All rights reserved.