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Due to small bond size, short bonding time, especially slight interface reaction, bonding details cannot be recognized using scanning electron microscope and energy density X-ray spectrum. In order to understand the physical mechanism of ultrasonic wedge bonding, in this paper, bond interface of ultrasonic AlSi wire wedge bonding on Au/Ni/Cu pad was investigated under high resolution transmission electron microscope. Au8Al3 intermetallic compounds were identified by convergent beam electron diffraction, thickness of which was about 200 nm and its lattice images were captured. Solid-state diffusion theory cannot be used to explain why such thick compound formed within milliseconds at room temperature. Ultrasonic effects attributed to formation of the metallurgical bonds.