High temperature reliability of Pd-alloyed Au wire/Al bonding interface

Abstract

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°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 Au <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</inf> Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> intermetallic compound (IMC) layer. Au <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> Al IMC did not detected in HP-wire TVs. In LP-wire TVs, we could not observe the ‘Pd-rich’ layer at bonding interface but Au <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> Al formed at the interface of LP-wire and Au <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</inf> Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> 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 Au <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> Al IMC and, therefore, the Au-Al bonding interface becomes vulnerable to this kind oxidation.