Chih-Hsin Tsai

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.

Growth and photoluminescence study of ZnTe quantum dots

ZnTe quantum dots embedded in ZnS were grown successfully by controlling the flow duration in a metalorganic chemical vapor deposition system. Blueshift as large as 250 meV was observed in photoluminescence measurement, and the emission persists up to room temperature. The amount of blueshift decreases with increasing quantum dot size and for large quantum dots, no photoluminescence could be detected. From studying the temperature-dependent integrated intensity of the emission spectra, it is found that the activation energy for the quenching of photoluminescence increases with decreasing quantum dot size, and is identified as the binding energy of exciton in ZnTe quantum dot.

An innovative annealing-twinned Ag-Au-Pd bonding wire for IC and LED packaging

An innovative annealing twinned Ag-(8-30%)Au-(0.01-6%) Pd wire for IC and LED packaging has been developed. It exhibits high thermal stability during aging at 600°C and a small heat affected zone after wire bonding. The mean failure time of this annealing twinned Ag-Au-Pd wire when stressed with a current density of 1.23 × 105A/cm2 is about double that of the conventional grained Ag-alloy wire. For packaging on Si chips with Al pads, it possesses sufficient intermetallic compounds at the initial as-bonded stage but a slow growth rate during further reliability tests. The excellent reliability of this new bonding wire has been verified in a DDRII BGA IC package and a 0605 LED package. In the LED package, this Ag-alloy wire provides the extra benefit of increasing the light output power (LOP) by about 3.2%.

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.

High performance Ag-Pd alloy wires for high frequency IC packages

Binary Ag-Pd alloy wires designed for both requirements of high reliability and low electrical resistivity have been developed and patented by Wire Technology Co. in Taiwan. The electrical resistivity of these Ag-Pd bonding wires are 1.98 to 3.5μΩ·cm. After stressing with various current densities, their mean times to failure are much higher than those of Au wire and Pd coated Cu wire. For the wire bonding of IC packages using these Ag-Pd binary alloy wires with forming gas, the units per hour (UPH) are similar to those using Au wire and better than those using Pd coated wire. In addition, these Ag-Pd alloy wires show sufficient intermetallic layers at the initial as-bonded stage on Al pads but slow growth rates during further aging at 150°C. The Au wire bonded interfaces reveal an overgrowth of intermetallic compounds in contrast to the diminutive intermetallics growth in Cu-wire bonded packages. The high reliability of such Ag-Pd wire-bonded IC products has been verified in a high frequency DDR III package.

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.

Evaluation of Corrosion Resistance of Ag-Alloy Bonding Wires for Electronic Packaging

The corrosion resistances of various Ag-alloy wires for the interconnection of integrated circuit and light-emitting diode packages were compared with those of traditional Au, Al, Cu, and pure Ag bonding wires through potentiodynamic polarization tests in 3.5 wt% NaCl aqueous solution. The results indicated that alloying the Ag bonding wires with Pd and Au elements decreases their thermodynamics corrosion tendency. However, the kinetics analyses of the corrosion rates from polarization curves showed that the corrosion current density of Ag-alloy wires decreases with increasing Pd and Au contents. The beneficial effects of Pd and Au alloying similarly occur in the passive region and passive current density. The results indicated that adding Pd and Au can effectively reduce the corrosion of Ag-alloy wires. Aging treatment at 600 °C for 3 h in ambient air increased the corrosion tendencies and corrosion rates of both Ag-3Pd and Ag-8Au-3Pd wires. In comparison to the corrosion of traditional Au, Al, and Cu wires, the experimental results showed that the corrosion potential of Ag-8Au-3Pd alloy wire was near that of Pd-coated Cu wire, and more noble than those of Cu wire and Al wire. The corrosion current densities increased on the order: Au wire < Ag-8Au-3Pd wire < Pd-coated Cu wire < Cu wire < Al wire. The sequence of pitting corrosion potentials was: Al wire < Cu wire < Pd-coated Cu wire < Ag-8Au-3Pd wire < Au. After corrosion tests, CuCl2 appeared on the surfaces of Cu wire and Pd-coated Cu wire, while AlCl3 was found on the corroded Al wire. The corrosion product of Ag-alloy wires was found to be AgCl, which was reported to possess high ohm resistance and could inhibit the further dissolution of Ag wire.

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.

Applications of Ag-alloy stud bump for IC and LED packages

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.