Effects of Pd addition on Au stud bumps/Al pads interfacial reactions and bond reliability

Abstract

The main purposes for developing low-alloyed Au bonding wires were to increase wire stiffness and to control the wire loop profile and heat-affected zone length. For these reasons, many alloying elements have been used for the various Au bonding wires. Although there have been many studies reported on wire strengthening mechanisms by adding alloying elements, few studies were performed on their effects on Au bonding wires and Al pad interfacial reactions. Palladium has been used as one of the important alloying elements of Au bonding wires. In this study, Au-1wt.%Pd wire was used to make Au stud bumps on Al pads, and effects of Pd on Au/Al interfacial reactions, at 150°C, 175°C, and 200°C for 0 to 1200 h thermal aging, were investigated. Crosssectional scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electron probe microanalysis (EPMA) were performed to identify intermetallic compound (IMC) phases and Pd behavior at the Au/Al bonding interface. According to experimental results, the dominant IMC was Au 5 Al 2 , and a Pd-rich layer was at the Au wire and Au-Al IMC. Moreover, Au-Al interfacial reactions were significantly affected by the Pd-rich layer. Finally, bump shear tests were performed to investigate the effects of Pd-rich layers on Au wire bond reliability, and there were three different failure modes. Cracks, accompanied with IMC growth, formed above a Pd-rich layer. Furthermore, in longer aging times, fracture occurred along the crack, which propagated from the edges of a bonding interface to the center along a Pd-rich layer.

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

“…The data are in agreement with results given in literature [2,3,6] for Pd-doped and 4N wires if the higher annealing temperature of 175°C used in [6] is considered. As a result the parabolic rate constants of the doped wire contacts are lower compared to the 4N reference (k p…”

“…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 suppressing the formation of one or more of the regularly observed IMPs. A critical analysis of existing literature [1][2][3][4][5][6][7] reveals that no such detailed picture is yet available due to limitations in the applied analytical techniques and their resolution capabilities.…”

“…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.…”

Formation and Growth of Intermetallics in an Annealing-Twinned Ag-8Au-3Pd Wire Bonding Package During Reliability Tests

“…The chemical composition of intermetallics at the Pd-coated Cu wire/Al pad is Cu:Al = 33.9:66.1, which corresponds to the CuAl 2 phase. Kim et al [9] reported that a Pd-rich layer formed at the interface between in Au-1Pd wire bonded specimen, can neither be observed in this Ag-8Au-3Pd package, the Pd content in the Ag-Al intermetallic layer is indeed higher than that in the matrix of the Ag-8Au-3Pd alloy wire, which decelerates the Ag-Al intermetallic growth and prevents the overgrowth of brittle intermetallic compounds that occurs in Au wire bonded packages. On the other hand, the higher diffusivity of Ag atoms in comparison to that of Cu atoms in Al pads leads to a more sufficient intermetallic layer in wire bonding packages using Ag-8Au-3Pd wire than in those using Cu wire.…”