# In-Situ Observation of Adhesion Behavior During Ultrasonic Al Ribbon Bonding
## 2 Experimental Procedures  
Annealed pure aluminum ribbon made by Tanaka Denshi Kogyo, Japan was used in the present study. The purity was 99.99 mass\(\%\). The width and thickness were 1.0 mm and 0.20 mm, respectively. The surface roughness of the Al ribbon was measured using a stylus profilometer made by Kosaka Laboratory Limited (Ltd.), Japan. Because the surface roughness of the silica substrate was significantly lower than that of Al ribbon surface, the surface roughness of the silica substrate was ignored when local contact behavior was examined.  
Figure 1 is a schematic illustration of the ultrasonic bonding apparatus and measuring system. The bonding tests were performed at room temperature (about 298 K). The ultrasonic frequency used was 60 kHz. The bonding force, \(F_{b}\), was 7 N and the ultrasonic power, \(P_{u}\), was 3 W or 4 W. A flat bonding tool was used. The chuck width was 250 \(\upmu\)m and the maximum surface roughness of the flat chuck-face was about 0.5 \(\upmu\)m. A very simple bonding sequence was adopted. Initially, the bonding force was applied to the ribbon. The ribbon was kept pressed on the substrate for 400 ms to damp the vibration generated by the impact of the bonding force. Then, the ultrasonic vibration was applied for 400 ms, parallel to the longitudinal direction of the Al ribbon. Finally, the bonding force was unloaded at 400 ms after stopping the ultrasonic vibration. The vibration of the tool top was measured by a laser Doppler vibrometer produced by PI-Polytec Ltd., Japan. The sampling rate was 2.56 MHz. The bonding interface image was obtained from the back of the silica substrate, using the high-speed video camera made by Nac Image Technology Ltd., Japan. The frame rate, \(F_{rate}\), was set at \(10^{3}\) or  
\(10^{4}\) fps. The shutter speeds in \(F_{nte}=10^{3}\) and \(10^{4}\) fps were 1 ms and 5 us, respectively. The bonding interface was illuminated with a light emitting diode (LED) ring lamps. Illumination using a spot laser pointer with a wavelength of 650 nm was added to the LED lamps, allowing for the adhesion behavior to be observed clearly at a high speed of \(10^{4}\) fps. The correlation coefficients, \(r\), of pixel gray scale intensities, \(z\) (\(i,j\)), were calculated for examination of the image-matching between video captures and still images, where (\(i,j\)) was the pixel position of digital images [3; 24].  
If simple peel tests were carried out [14], the fracture occurred on the Al ribbon (or the plug fracture). In the present study, a twist and peel test was performed manually allowing the fracture to occur on the bonded interface. As illustrated in Figure 2, the end of the Al ribbon was twisted by \(40^{\circ}\) in step 1, applying a poor peeling tensile force. In step 2, it was twisted by \(20^{\circ}\) in the reverse direction and a poor tensile force was applied manually. As a next step, the tensile force was increased and the Al ribbon was peeled along the surface of the silica substrate in step 3. The bonded interface during the twist and peel test was recorded through the high-speed video camera at \(F_{nte}=10^{3}\) fps. The aim of the twist and peel test was to understand the origin of the interfacial fracture as well as to confirm the positional relation of the island streak and central belt zone in the direction normal to the bonding interface.