Effects of Cu and Pd addition on Au bonding wire/Al pad interfacial reactions and bond reliability

Abstract

Finer pitch wire bonding technology has been needed since chips have more and finer pitch I/Os. However, finer Au wires are more prone to Au-Al bond reliability and wire sweeping problems when molded with epoxy molding compound. One of the solutions for solving these problems is to add special alloying elements to Au bonding wires. In this study, Cu and Pd were added to Au bonding wire as alloying elements. These alloyed Au bonding wires-Au-1 wt.% Cu wire and Au-1 wt.% Pd wire-were bonded on Al pads and then subsequently aged at 175°C and 200°C. Cu and Pd additions to Au bonding wire slowed down interfacial reactions and crack formation due to the formation of a Cu-rich layer and a Pd-rich layer at the interface. Wire pull testing (WPT) after thermal aging showed that Cu and Pd addition enhanced bond reliability, and Cu was more effective for improving bond reliability than Pd. In addition, comparison between the results of observation of interfacial reactions and WPT proved that crack formation was an important factor to evaluate bond reliability.

Interface microstructure effects in Au thermosonic ball bonding contacts by high reliability wire materials

“…More detailed understanding on how wire doping can obstruct atomic and defect diffusion processes in bond contacts would not only contribute to understand the background of the lifetime enhancements as reported in [1][2][3][4][5][6][7] as well as their further potential and limitations within applications. In particular, it would also support deriving improved strategies for further knowledge-based material developments in wire bonding and reliability optimization, also for alternative wire materials such as Cu, Cu/Pd or Ag in addition to Au.…”

“…by applying ball pull or shear tests after high temperature storage and cross section analyses of the ball contacts (e.g. [1][2][3][4][5][6][7]). Comparing the high reliability wires with ''pure'' (4 or 5N) reference wires, a significant increase in lifetime was found to be correlated with a deceleration in growth of the intermetallic Au/Al phases (IMP) formed within the contact interface.…”

Phase diagram of Au-Al-Cu at 500 °C

“…A low-temperature Au-Al-Cu phase diagram could provide essential information to understand the reactions between the Au-Cu alloy wire and the Al pad. The stability of intermetallic phases is closely related to the reliability of wire bonding [1,2].…”

“…Al-Au binary systems exhibit five intermediate phases at 500°C, namely, AuAl 2 , AuAl, Au 2 Al, Au 8 Al 3 (Au 5 Al 2 ), and Au 4 Al [6]. Al-Cu binary systems exhibit five intermediate phases at 500°C, namely, θ (Al 2 Cu), η 2 (AlCu), ζ 2 (Al 9 Cu 11 ), δ (Al 2 Cu 3 ), and γ 1 phases [7].…”

Phase diagram of Au–Al–Pd at 500 °C

“…A lowtemperature Au-Al-Pd phase diagram could provide the information required for understanding the reactions between the Au-Pd alloy wire and the Al pad. The stability of the intermetallic phases is closely related to the reliability of wire bonding [1,2].…”