High Temperature Reliability of Pd-Alloyed Au Wire/Al Bonding Interface

Palladium (Pd) is an important alloying element of Au bonding wire which plays as solid solution-type alloying effect in Au matrix. The behavior of Pd at Au/Al bonding interface during the thermal aging, and its effect on Au/Al interfacial reactions were investigated. Two types of Pd-alloyed Au wires, Au-0.25wt%Pd (Low-Pd-Au wire: LP-wire) and Au-0.95wt%Pd (High-Pd-Au wire: HP-wire), were used for the fabrication of wire-bonded test vehicles (TVs). The TVs were thermally aged at 175 degrees C up to 1200hours, and the formation of a Pd-rich layer was investigated at Au-Al bonding interface by using a cross-sectional scanning electron microscope (SEM) and an electron probe microanalysis (EPMA). The Pd-rich layer was confirmed only at the TVs fabricated with the HP-wire. According the results of a transmission electron microscope (TEM), the thickness of Pd-rich layer was about 500nm and it located between HP-wire and Au8Al3 intermetallic compound (IMC) layer. Au4Al IMC did not detected in HP-wire TVs. In LP-wire TVs, we could not observe the Pd-rich layer at bonding interface but Au4Al formed at the interface of LP-wire and Au8Al3 IMC. After long-term thermal aging, the bonding interface was degraded by the oxidation phenomena. According to the cross-section analysis, it was mainly due to the oxidation of Au4Al IMC and, therefore, the Au-Al bonding interface becomes vulnerable to this kind oxidation.