Growth of CuAl intermetallic compounds in Cu and Cu(Pd) wire bonding

In this study, Cu wire bonds are aged at 150, 175, 250, and 350°C for time ranging from 5 min to 2000 h. The top temperature of 350°C is chosen to have the fastest kinetics, while maintaining the same types of stable intermetallics (IMCs) according to the Al-Cu binary phase diagram. Two types of Cu wire are used, including bare Cu wire and Pd coated Cu wire. To avoid possible damages from conventional mechanical polishing processes, all cross-sections are carried out by using an Ar ion beam cross-section polisher. Observations are carried out using a field-emission scanning electron microscope. The compositions of IMCs are determined by using a state-of the-art field-emission electron microprobe. According to the results of this study, in the as-bonded condition, the IMC formed is CuAl₂. Its morphology is island-type and covers only a small percentage of the interface. During aging, the growth directions of IMCs are both lateral and perpendicular to the interface. At the lowest temperature (150°C), the growth of IMCs is very slow. Even after 2000 h of aging, a large portion of Al remains un-consumed, and the IMCs formed are CuAl₂ and CuAl. The growth rate increases very rapidly with temperature. At the highest temperature (350°C), most of the Al is consumed after only 30 min of aging, and a thick layer of Cu₃Al₂ form over CuAl and CuAl₂. Detailed morphological evolutions of IMCs for all temperatures are described. The Pd atoms in Pd-coated Cu wire do not participate in the interfacial reaction, and have no marked effect on the growth rate of IMCs. There is no detectable Pd signal in the IMCs. With the formation of IMCs, the Pd atoms are rejected into Cu in front of the IMCs.