Corrosion Behavior of Cu-Al Intermetallic Compounds in Copper Wire Bonding in Chloride-Containing Accelerated Humidity Testing
Accelerated reliability testing is widely used in the microelectronic packaging to evaluate the long-term reliability of electronic devices. Although a lower intermetallic growth rates in Cu-Al joint improved its performance in thermal aging comparing with Au wirebonds, it is a great challenge to pass through accelerated humidity tests for Cu wirebonds, because Cu-Al intermetallics are easily subjected to chloride-induced corrosion. Unfortuniately, no literature has ever given direct evidences on IMC corrosion process, thus fundamental corrosion mechanism is still obscured. In this paper, we systematically investigated the corrosion behavior of Cu-Al intermetallic compounds (IMCs) under accelerated humidity environment. A high resolution transmission electron microscopy (HRTEM) was used to identify interfacial intermetallics in thermal aged Cu-Al ball bonds, in which Cu<sub>3</sub>Al<sub>2</sub> was confirmed between Al<sub>2</sub>Cu and Cu<sub>9</sub>Al<sub>4</sub>. Selective Cu-Al IMC corrosion occurs when chloride was present in the accelerated humidity environment: Cu<sub>3</sub>Al<sub>2</sub> and Cu<sub>9</sub>Al<sub>4</sub> were continuously corroded while the Al<sub>2</sub>Cu remained immune. Microstructure analysis using HRTEM revealed the corrosion process of corroded IMCs (Al<sub>2</sub>Cu and Cu<sub>9</sub>Al<sub>4</sub>) and the root cause of corrosion immunity for Al<sub>2</sub>Cu layer.