# Evaluation of Ultrasonic Bonding Strength with Optoacoustic Methods
## 4 Results and Discussions  
### Formation of Bonds under the Ultrasonic Bonding  
Figure 7 shows the relation between the vibration time and the bonding strength. The vertical axis is the vibration time. The horizontal axis is the bonding strength. With vibration times shorter than 200 ms, a joint strong enough to measure the bond strength was not obtained. This time period can be considered as an incubation time to form the micro-bonds though the removal of oxide film. The bonding strength roughly increased with the vibration time of 200 ms, while it had a large variance especially in the shorter vibration time. In addition, the fracture mode changed with the vibration time. The specimen bonded with the shorter vibration time broke at the bonded interface between the two sheets as shown in Figure 8a. When the vibration time increased to over 400 ms, the fracture occurred around the bonded part as shown in Figure 8b. We call this fracture mode the base material fracture. During the vibration, the upper bonding sheet in contact with the ultrasonic horn is compressed by the normal force. This causes thinning of the bonding part. The base metal feature is due to a decrease of the fracture strength of the specimen by the thinning.  
Figure 8: Fracture pattern after the tensile shear test. (**a**) Interface fracture, the vibration time 300 ms; (**b**) Base material fracture, the vibration time 400 ms.  
Figure 7: Vibration time vs. bonding strength.  
The changes in bonding strength are associated with the development of the bonded area. In order to estimate the bonded area, the fracture surface that exhibited the interface fracture was observed. Figure 9 shows an example of an SEM image of the interface fractured surface. This specimen showed a bonding strength of 532 N at the vibration time of 200 ms. In the fracture surface, two characteristic patterns are observed in white on the fractured surface: the scratch pattern shown in Figure 9b,c, and the dimple pattern in Figure 9d,e. In the initial stage of ultrasonic bonding, the ultrasonic vibration causes friction between metal sheets forming the scratch pattern. Thus, the scratch pattern observed in the SEM image is not considered to be the \"un-bonded region\". The intermetallic bond is achieved thorugh removal of oxide films and interfacial adhesion on the scratched surface, resulting in formation of micro-bonds. In addition, these phenomena mainly occur at the stress concentration part caused by the horn and anvil tips, or uneven contact part due to rolling. Thus, the formation of micro-bonds occurs dispersedly at the bonding interface. Since the micro-bond is torn off in the tensile shear test, the resultant dimple pattern is observed in the fractured surface. The variation in the bonding strength observed in Figure 7 is considered to be due to the transient formation of micro-bonds. Figure 10 shows the relation between the bonding strength and the total area of the white region in the SEM image computed with the image binarization. The total area includes both the scratch pattern and the dimple pattern. The bonding strength increases with the calculated bonded area, indicating the development of a bonded region. However, it was difficult to identify the accurate bonded area by the image analysis, because the size of the micro-bonds, about 10 to 100 \(\upmu\)m, was so small that it was difficult to distinguish between the scratch pattern and the dimple pattern. Actual area of the bonded region is surmised to be smaller than the values shown in Figure 10.  
Figure 9: Fracture surface observation at bonded area by SEM. (**a**) Bonded interface; (**b**) Scratch part; (**c**) Magnified scratch part; (**d**) Dimple pattern; (**e**) Magnified dimple pattern.