In this study, the microstructure and performance of different Ag-based conductive wires were investigated. Ag-based wires that contain 8, 15, 20, and 28 wt% Au were produced by multiple drawing and rapid annealing processes to substitute commercial gold wires in electronic packaging industries. The cross-sectional gain structures observed using focused ion beam showed the formation of a slender granular structure at the center, and the outer portion changed from equiaxed to elongated grains with the increase in Au content. High-resolution transmission electron microscopy showed a remarkable decrease in the twin thickness and stacking defect of 9R structures dissociated from incoherent twin boundaries. Electron backscatter diffraction analysis revealed that slender grains at the center showed strong < 001 > crystallographic orientation, and the equiaxed grains near the surface were in the < 111 > preferred orientation. The high Au solute concentration in Ag-based wires induced strong interaction on dislocations, thereby resulting in the formation of low-angle grain boundaries (LAGBs) and nano-twins. Ternary Ag-based alloy wires exhibited high strength and hardness but low conductivity and elongation with the increase in Au content because relatively the high LAGB density and Au solute atoms caused strengthening and electron scattering.

The materials characteristics of annealing-twinned Ag-alloy wires with various Au and Pd contents were evaluated in this paper. The results indicated that both Ag-8Au-3Pd and Ag-15Au-3Pd have higher strength and corrosion resistance than do pure Ag and binary Ag-Pd wires. On the other hand, the pure Ag, Ag-0.5Pd, Ag-3Pd, and Ag-4Pd wires possess the merits of lower material cost, higher electrical conductivity, and higher electromigration durability than do the ternary Ag-Au-Pd wires. However, the breaking load and elongation of pure Ag wire are inferior to those of Ag-0.5Pd, Ag-3Pd, and Ag-4Pd wires. In addition, the corrosion resistances of pure Ag and Ag-0.5Pd wires are far inferior to those of Ag-3Pd and Ag-4Pd wires. Based on these performances, the ternary Ag-8Au-3Pd wire is an ideal substitute for the traditional Au wire due to its high strength, corrosion resistance, and reliability, while the Ag-3Pd and Ag-4Pd are cost-friendly bonding wires for high-frequency integrated circuit devices.

Annealing twins have been reported to have beneficial effects on the mechanical properties of structural materials. For electronic applications, it has also been reported that electromigration can be retarded by one order of magnitude in a Cu thin film under current stressing. Due to its low stacking fault energy, it is expected that Ag alloy can be produced as a bonding wire with a high percentage of twinned grains for IC and LED applications. This paper presents a new method of multiple drawing and annealing procedures for further enhancing the formation of annealing twins. The experimental results indicate that the twinned grain percentage of an Ag-4Pd wire increases from 15.8% to 35.3% during multiple drawing and multiple annealing treatments as the wire diameters decrease from 30 μm to 17.6 μm, and further increases to values over 50% after aging at 600 °C or current stressing at 1.23×105 A/cm2 for 3 hrs. In comparison to the conventional grained Ag-alloy wires with the same alloy compositions, such twin-rich bonding wires exhibit higher thermal stability of the grain structure and longer mean failure time under current stressing.

Ag-4Pd binary alloy wire has been produced as an alternative to a previously developed Ag-8Au-3Pd ternary alloy wire to meet requirements for high electrical conductivity and low cost. The electrical resistivity of this Ag-4Pd bonding wire, manufactured with a conventional method, is 3.7 μΩ cm, close to the values of traditional 3N Au wire (3.5 μΩ cm) and Pd-coated Cu wire (1.8 μΩ cm). To further improve the performance of this bonding wire, a large amount of annealing twins were introduced in this Ag-4Pd alloy wire through an innovative concept of sequential drawing and multiple annealing processes. The resulting electrical resistivity of this annealing-twinned Ag-4Pd wire is 3.5 μΩ cm. In contrast to the apparent increase in grain size in the conventional Ag-4Pd wire under electrical stressing with a current density of 1.23 × 105 A/cm2 for various times, the grains in this annealing-twinned wire grow much more slowly. The breaking load and elongation of this annealing-twinned Ag-4Pd wire are also higher than those of conventional wire. Furthermore, annealing twins increase the durability to electromigration of this Ag-4Pd wire under electrical stressing with various current densities.

Recently, 3-D IC and 2.5-D IC packages become more popular and Cu pillars are often employed as their conductive bumps. However, since the Cu pillars are hard and rigid, it is difficult to plastically deform for the solid contact between the Cu pillars and bonding pads during the assembly process utilizing hot pressing. In this case, a coplanar problem may occur, which leads to voids or even failure at the bonding interfaces. The Ag-alloy wires previously developed by Wire Technology Co., LTD., have been further manufactured as stud bumps for flip-chip interconnections. This innovative Ag alloy stud bump is softer, so it is able to plastically deform during hot pressing to be in solid contact with the on-substrate bonding pad, preventing the coplanar problem of Cu pillars in 3D- or 2.5D-IC packages. The Ag-alloy stud bumps also exhibit many other advantages for applications in advanced IC and high power flip chip packages as comparing with the conventional Au- or Cu-stud bumps.

The durability against electromigration of an annealing twinned Ag-8Au-3Pd wire is about double that of the conventional grained wire under electrical current stressing of 1.23 × 105 A/cm2. During electromigration, a particular morphology of surface reconstruction comprising a stepwise structure and hillocks can be observed in this annealing twinned wire. The stepwise structure, which has been correlated to longer electromigration life, is postulated to result from dislocation slips driven by electron wind collisions and thermal diffusion of metallic atoms. The simultaneous processes of primary and secondary slips in crossing directions cause hillocks to form at the intersections of both slips. The results also indicated that the electrical current could enhance the grain growth in both wires but had an insignificant effect on the formation of annealing twins.

An innovative Ag-8Au-3Pd bonding wire with a high twin density has been produced. The grain size of this annealing-twinned wire changes moderately during electrical stressing, unlike that of the conventional grained wire, which increases drastically and even leads to a bamboo structure. In addition, the durability against electromigration of the annealing-twinned Ag-8Au-3Pd alloy wire is higher than that of the conventional grained wire. This higher durability can be ascribed to the surface reconstruction of a stepwise morphology and slow grain growth resulting from the abundance of annealing twins in this wire.

In a Ag-8Au-3Pd wire, a large number of annealing twins can be observed. In contrast to the rapid grain growth in Au and Cu wires during aging at 600 degrees C, the grain size of this Ag alloy wire remained almost unchanged. The annealing twins in this material also possess the dual merits of increasing strength and elongation with aging time, while the electrical resistivity remained constant. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.