Effect of Au Addition on the Microstructure and Properties of Ag-4Pd Bonding Wires
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INTRODUCTION
WIRE bonding is widely used for the chip-to-die interconnection of lead–frame, integrated circuits (ICs), RF microwaves, and optoelectronic packages because of such connection’s simplified process and cost effectiveness.[1] In the past, gold (Au) was the primary bonding wire utilized in the packaging industry because of its excellent bondability and reliability. In recent years, the demand for cost reduction has driven the development of alternative materials, such as copper and its alloys with superior properties. Given the substantial cost savings along with the high electrical and thermal conductivities achieved with the use of Cu wires, Cu wires have been applied as an excellent substitute for Au. However, the use of Cu wires encounters several BING-HAU KUO, DU-CHENG TSAI, YEN-LIN HUANG, and FUH-SHENG SHIEU are with the Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan. Contact e-mail: [email protected] PO-CHUN HSU is with the Department of Materials Engineering, KU Leuven, Leuven, Belgium. TUNG-HAN CHUANG is with the Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan. HSING-HUA TSAI is with the Wire Technology Co. Ltd., Taichung, Taiwan. Manuscript submitted July 25, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
challenges.[2,3] For example, Cu wires are prone to oxidation and sulfurization, which cause considerable reliability concerns. Pd-coated Cu wires and bonding in the protective forming gas are used to overcome the inherent oxidation and sulfurization problems of pure Cu.[4,5] However, the Pd-coated Cu wires exhibit greater hardness than those of the Cu or Au wires, which require higher bonding forces to achieve similar bonding strength. As a result, the risk of damage is increased for the metal pads and/or silicon substrate. The rise in failure probability and price has limited the application of Pd-coated Cu wires.[6] Moreover, the use of forming gas during wire bonding and the need for a special tooling machine added to the cost issue. Recently, Ag and its alloys have become new alternative wire bonding materials because of their excellent electrical and thermal conductivities relative to those of Cu and Au. The packaging cost of Ag wires is also similar to that of Pd-coated Cu wires. Nevertheless, problems such as oxidation and sulfurization, low tensile strength, unstable free air ball (FAB) shape, and sensitivity to electrolyte ion migration of the Ag wires easily deteriorate the bonding properties and reliability of electronic packages.[7] To solve these problems associated with the Ag wires, different alloying elements have been applied to Ag alloy wires. Ag alloy wires alloyed with Au and Pd are considered as potential
cost-effective and high-performance materials.[8–12] Ag, Pd, and Au possess similar crystal structures and atomic radii; hence, they tend to form a solid solution of face-centered cubic (FCC) structure across the entire composition range. The three metallic constitu
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