Study On Joining Physical Mechanism Of Ultrasonic Wedge Bond Interface

Interfacial joining physical mechanism of the ultrasonic wedge bonding has become to be the key issue for severely limiting the innovation for the ultrasonic bonding equipments and techologies, and directly affecting the qualities and reliabilities of the electronic components and devices in chip-level packaging in the micro-electronic and optic-electronic systems. Therefore, clarifying the joining characteristics of the ultrasonic bond interface and understanding the effects of acoustic on the bonding process have important significance on guiding the upgrade of the interconnection equipments in electronic packaing, optimizing the ultrasonic bonding technologies and improving the functions and life of the components and devices.In this paper, the Al-1wt.%Si wire with 25μm diameter was ultrasonic wedge bonded on thin Au layer Au/Ni/Cu, thick Au layer Au/Ni/Cu and Cu pad at atmosphere temperature. The bond resistance was in-situ measured; the physical process and characteristics on the bond footprints were uncovered; also, the interfacial evolution in Au/Al and Al/Au systems was investigated based on high temperature aging method; at the same time, by using the focused ion beam-transmission electron microscopy (FIB-TEM), nano-scale constructions of the bond interface were presented and atomic-scale characterization of the bond interface was observed. According to the theories of the solid-state reaction diffusion, the function of the ultrasonic vibrations during the ultrasonic wedge bonding process for the formation of interfacial solid solutions added with discontinuous nano Au8Al3 particles was given.To evaluate the bond properties, the circuit of in-situ resistance measurement was designed. The resistance variations of the bonds obtained with the three kinds of pads were measured. It was found that with the increase of the bonding parameters, the bond resistance firstly decreased and then increased rapidly, and the maximum ratio was above +10%, furthermore, the curves possessed periodical features. The real contact area of the bond interface and the cross-sectional deformation of the bond wire were the causations respectively for the fluctuation and wholly increase of the resistance. Compared with pull force method, this factor was complementary with the pull force and had real physical meaning to evaluate the bond properties. The fluctuation of the bond resistance also reflected the periodic features of the ultrasonic bonding process.By mechanically peeling and chemically etching the bond wire away, the footprints on the bond pad were observed under SEM. It was found that directional joint marks were evident on the thin Au layer Au/Ni/Cu pad with a relatively hard surface. The longitudinal (along ultrasonic vibration directions) and the above lateral (perpendicular to the ultrasonic vibration) joint marks possessed stereo and periodic features. They were ascribed to the ultrasonic vibration and lateral plastic flow of the bond wire, respectively. However, the directional features were unobvious for the other two pads with relatively softer surfaces.By using the high temperature aging method, the evolution processes of the bond interfaces in the Au/Al and Al/Au systems were compared, and certificated that the actual joining area started from the bond periphery, especially at the bond heel and toe.To analyze the interfacial characteristics of the bond cross-section, the TEM samples were prepared by FIB equipments. In nano-scale, the interfacial layered structures were observed on TEM bright field images (BFI) and dark field images (DFI); the interdiffusion distance between elemental Al and Au was measured and the intermediate phase of Au8Al3 among the reactant layer was identified by convergent beam electron diffraction (CBED). In atomic scale, the high resolution electron microscope (HREM) images showed that the intermediate phase was with a diameter of a few nanometers and distributed discontinuously. At the same time, the diffusion of elemental Al into Au layer has evident features: multistep waveform W type diffusion along the direction perpendicular to the bond interface and interference patterns with dark and bright bars F type diffusion along the direction parallel to the bond interface. Large amounts of crystal twins were observed in the bond wire near the interface and among the reactants. Increasing the ultrasonic power enhanced the diffusion process, and also made the interface of the reactants and the Au layer overburdened.Based on the experimental results, the models of the ultrasonic wedge bonding process and the interfacial bonding characteristics were illustrated. The effects of the heat, friction, plastic deformation and ultrasonic (phonon and vibration) on the formation of the metallurgical bonds were analyzed one by one. Combining the theoretical analysis and experimental results, the secondary effects of the ultrasonic vibration, such as removing the oxides and contaminants away, producing friction heat, enhancing plasticity of the bond wire, producing rapid shear strain/stress flow in the bond wire and interface, stirring the plastic and viscous bonding couples at local interface and so on, provided not only the fast diffusion path, but also the activation energy for the elemental interdiffusion at bond interface at relatively lower temperature.