Recrystallization and grain growth of cold-drawn gold bonding wire
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ion in semiconductor packaging. As packaging technology continues to advance, improved properties in bonding wire are needed. In particular, the ball shape, breaking load, elongation and the homogeneity of microtexture and microstructure are important. These characteristics are related to the purity of the original materials, the drawing process and the annealing process. The homogeneity of the microtexture and the microstructure can affect the swing or looping of bonded wires and theses are major factors for failure of packaging. Most bonding wires also undergo a final annealing before the packaging process. During final annealing, breaking load decreases and elongation increases. Therefore it is necessary to optimize annealing processes in order to obtain optimum bonding wire. High purity gold (99.9999%wt Au) is too soft and unstable for obtaining good properties for bonding when it is drawn and annealed. Generally, the annealing and recrystallization temperature for pure gold is in the range of 150-200 °C and it has been reported that highly deformed pure gold will show recovery and recrystallization at room temperature [1]. Therefore bonding wire commonly has various dopants at J.-H. CHO, Researcher, Y.W. KIM, Research Associate Professor, and K.H. OH, Professor, are with the School of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea. Contact e-mail: [email protected]. J.-S. CHO, Senior Researcher, J.-T. MOON, Principal Researcher, and J. LEE, R&D Head, are with MKE Electronics, Kyunggi-do 449-810, Korea. Y.W. CHO, Principal Researcher, is with the Nano-materials Research Center, Materials Science and Technology Division, KIST, Seoul 130-650, Korea. A.D. ROLLETT, Professor, is with the Materials Science and Engineering Department, Carnegie Mellon University, Pittsburgh, PA 15213-3890. Manuscript submitted January 31, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
parts per million (ppm) level in order to control annealing response and to obtain better thermal and mechanical properties. Impurities, even at these low levels, are important for controlling the final mechanical properties and microstructures of gold wire by raising the recrystallization temperature and preventing grain growth.[2,3] Such small concentrations of impurities are known to strongly affect the migration rate of grain boundaries in many materials.[4,5] Recrystallization, recovery and grain growth all occur during annealing, and they affect the microstructures, microtextures and mechanical properties of gold wires.[6-9] The evolution of textures during wire drawing has been investigated by many researchers. The wire drawing textures of fcc metals typically have 具111典 and 具100典 fiber texture components. The textures of aluminum, copper, and brass wires have been investigated for cyclic symmetry.[10] The texture of drawn silver wires have radial symmetry, which is related to twinning.[11] Recrystallized grains are formed near the surface due to frictional heating in the die. Heizmann et al. have reported that
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