In IC packages, thermosonic wire bonding is the preferred method for making electrical connections between the die pad and lead frame. These interconnect are made using fine metal wires. On thermal aging (under 175 degreesC for 5 h) gold aluminide easily forms in gold ball bonds while formation of intermetallic compound is absent in case of copper ball bonds. An analysis of the atomic property of the elements bonded explains that atomic radii and electronegativity factors favor gold aluminide formation. (C) 2003 Elsevier Science Ltd. All rights reserved.