Investigation of interfacial phenomena of alloyed Au wire bonding

Solid solution-type alloying in Au wire matrix is one of the well-known methods for improving the mechanical properties of Au wire as well as the reliability of bonding interface. Palladium (Pd) uses as a typical alloying element of Au bonding wire manufacturing which makes solid solution hardening effect in Au matrix. The behavior of Pd at the Au wire-Al pad bonding interface during the thermal aging, and the effect of Pd on Au/Al interfacial reactions were investigated. Two types of Au wires, Au-0.25wt%Pd (Low Pd content) and Au-0.95wt%Pd (High Pd content), were used for the fabrication of wire-bonded test vehicles (TVs). The wirebonded TVs were thermally aged at 175°C up to 1200hours, and the formation of a Pd-accumulation layer was investigated at Au-Al bonding interface by using a cross-sectional scanning electron microscope (SEM) and an electron probe microanalysis (EPMA). The accumulation of Pd atoms was confirmed at the TVs fabricated with the Au wire with high Pd content. According the results of a transmission electron microscope (TEM), the thickness of Pd accumulation layer was about 500nm and it located between Au wire (Au-0.95wt%Pd) and Au₈Al₃ intermetallic compound (IMC) layer. Au₄Al IMC did not detected in Au-0.95wt%Pd wire TVs. In TV with Au-0.25wt%Pd wire, the phenomenon of Pd accumulation was not observed at bonding interface after thermal aging but Au₄Al formed at the interface between Au-0.25wt%Pd 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.