Chien-Hsun Chuang

Effects of Annealing Twins on the Grain Growth and Mechanical Properties of Ag-8Au-3Pd Bonding Wires

An innovative Ag-8Au-3Pd bonding wire containing a large amount of annealing twins has been produced. In contrast to the apparent grain growth in a conventional Ag-8Au-3Pd wire during aging at 600°C, the grain size of this annealing-twinned Ag alloy wire remains almost unchanged. The high thermal stability of the grain structure leads to a smaller heat-affected zone near the free air ball of this twinned wire. The annealing twins in this material also result in the dual merits of increased tensile strength and elongation with aging time, which is beneficial for the reliability of wire-bonded packages.

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

Wire bonding on Al pad packages with an innovative annealing twinned Ag-8Au-3Pd alloy wire results in a sufficient interfacial intermetallic layer at the initial as-bonded stage, while its growth during the further temperature cycling test and pressure cooker test is very slow. Even after prolonged high-temperature storage at 150°C for 500 h, the thickness of its intermetallics is only around 1.7 μm. In contrast, a very thin CuAl₂ intermetallic layer appears at the interface of the Pd-coated Cu wire bonded package, and a thick layer of Au₈Al₃ intermetallic compounds forms in the Au wire package, which grows to a thickness of around 4.0 μm after high-temperature storage at 150^°C for 500 h. Energy dispersive X-ray spectrometry analyses indicate that AgAl₂, Au₈Al₃, and CuAl₂ are the main intermetallic phases formed in the packages bonded with Ag-8Au-3Pd, and Au and Pd-coated Cu wires, respectively. However, an additional Pd-containing Ag₂Al layer found at the AgAl₂/Al interface of Ag-8Au-3Pd wire bonded package can interrupt its intermetallic reaction, and the annealing twins can further slow the intermetallic growth.

Effect of Annealing Twins on Electromigration in Ag-8Au-3Pd Bonding Wires

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.

Mechanism of Electromigration in Ag-Alloy Bonding Wires with Different Pd and Au Content

The mechanism of electromigration in Ag-alloy wires containing different amounts of Pd and Au has been studied. Thinning and thickening accompanying grain growth were observed in worn bonding wire after current stress. The mean time-to-failure of bonding wires stressed with different current densities is highly dependent on their electrical resistivity, and wire temperature increases during current stress, owing to the Joule effect. An indirect method is proposed for in situ assessment of the temperatures of these fine wires under current stress. A mode of failure of these bonding wires was deduced by kinetic analysis. This mode can be correlated with atomic diffusion in the wire.

Materials Characteristics of Ag-Alloy Wires and Their Applications in Advanced Packages

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.

Enrichment of annealing twins in an Ag-4Pd bonding wire for electronic packaging

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.