This study was undertaken to investigate the effect of bonding pressure profiles and ultrasonic vibration triggering timing on the deformation of gold balls and bond intermetallic formation process in Au-Al thermosonic wire bonding. The following results were obtained. Under the condition of ultrasonic vibration triggered after bonding pressure, which makes contact surface of a gold ball flat, small intermetallics are formed dispersively at the flat contact interface of a gold ball and an aluminum pad. They grow in the following two processes. One is the process forming slender islet intermetallics at the middle portion of the bond by micro frictional slip. The other is the process forming intermetallics shaped in arcs at the periphery of the bond by plastic deformation. A dynamic load leads to larger deformation and intermetallics area than a static load during bonding pressure. Under the condition of ultrasonic vibration triggered before bonding pressure, intermetallics are initially formed at the small contact area of spherical surface. They grow rapidly into islet shapes by micro frictional slip and are increased in numbers orthogonally to the direction of ultrasonic vibration. Then, arc-shaped intermetallics are also formed rapidly at the periphery of the bond by plastic deformation. The ultrasonic vibration triggered before pressure causes larger deformation and bond area in a short time than the ultrasonic vibration triggered after pressure. It is effective for reducing bonding time.
This work was undertaken to develop reliable ball bonding technique for copper wire. In this investigation, the effect of the shielding gas on ball formation, ball quality and the bondability of the copper ball was examined. The shielding gases of only high purity argon and high purity argon containing hydrogen produced spherical copper balls without cellular structure surfaces or necking at the ball-wire interface. Auger and SEM analyses revealed that the copper ball formed in the high purity argon gas containing 10 vol. % hydrogen had a very smooth surface without oxidation. Therefore, this gas was selected for copper ball formation. The copper balls which were thermosonically bonded to aluminum electrode pads failed at a shear strength level comparable to that of gold. It was found that good stitch bonding of copper wire to unplated copper lead frame fingers could be achieved by preventing them from oxidizing. Simple thermal aging tests at 300°C verified that the copper/aluminum intermetallic layer grew considerably slow without degradation of mechanical characteristics.
The effect of metal salt generation bonding method on the bond strength of the solid-state bonded interface of tin has been investigated by SEM observation of the interfacial microstructures and fractured surfaces. Metal salt generation was carried out by boiling a tin surface in organic-acid. Solid-state bonding was carried out in a vacuum chamber at bonding temperature T of 393~483 K and bonding pressure P of 7 MPa (bonding time: t = 1800 s). The bond strength increased with an increase in the bonding temperature, independently of the metal salt generation method. Because of the metal salt generation, bonded joints were obtained at a bonding temperature that was 40 K or more than the typical temperature required, and the bond strength was comparable to that of the base metal. When metal salt generation is not applied, the oxide film is destroyed by the improved plastic deformability of the tin. Thus the tensile strength of the joint is increased. On the other hand, when metal salt generation is applied, a high-tensile-strength joint is obtained at a low bonding temperature because metallic tin are exposed at the bond interface as a result of the decomposition of metal salt in the bond interface at a low temperature.