# In-Situ Observation of Adhesion Behavior During Ultrasonic Al Ribbon Bonding
## 4 Discussion  
Large shaking of Al ribbon usually occurs at the initialization of Al ribbon bonding. This often fluctuated, affected by the initial fixing situation. The bonding conditions of \(F_{b}\) = 7 N and \(P_{u}\) = 3 or 4 W adopted in the present study provide some fluctuation to the initial large sliding; however, these can provide stable bonding in the early, middle, and later stages [1,5,22,23,24,25,26,27,28,29,36]. The friction slip was the dominant mechanism in the early stage. Plastic deformation became predominant in the middle stage; however, the friction slip occurred because the island streaks increased. The central belt zone grew; the interfacial reaction occurred in the later stage. Figure 11: Twist and peel test of ultrasonic bonded interface between Al ribbon and silica substrate (\(F_{b}\) = 7 N and \(P_{u}\) = 4 W): (**a**) Before twisting; (**b**) Immediately after twisting Al ribbon (start time of stage 1); (**c**) When twisting Al ribbon by 25\({}^{\circ}\) in clockwise direction; (**d**) Twisting by 40\({}^{\circ}\) when step 1 is complete; (**e**) At the beginning of step 2 in counterclockwise direction; (**f**) Twisting by \(\sim\)18\({}^{\circ}\) in step 2; (**g**) Immediately after fracture occurs due to peeling (step 3); (**h**) After fracture. Mark D and Area B\({}_{1}\) remain on silica side. Figure 12 illustrates the interfacial adhesion process in the ultrasonic bonding between the Al ribbon and transparent silica substrate. Immediately after large sliding, a few island streaks appeared as illustrated in Figure 12a. The number of island streaks increased and several islands combined with one another in Figure 12b. The central belt zone, B, was formed in the middle stage and widened to become B\({}_{1}\) in Figure 12c. Area B\({}_{2}\) was formed around B\({}_{1}\) in Figure 12c. Area C was the touch and detach area. The peripheral bond area was between Area A and Area C even in the final stage. Areas B\({}_{1}\) and B\({}_{2}\) must consist of thin layers formed by some material reaction between Area A and the silica substrate [14; 22; 39; 40; 41; 42]. Figure 13 illustrates the mechanisms of increasing adhesion area. Figure 13a expresses the cross section of the Al ribbon and silica substrate. The macro folding was primarily produced parallel to the direction of ultrasonic vibration. Even after the macro folding was complete, the apparent contact area had multiple small voids (or gaps) [16]. As illustrated in Figure 13b, blackish gray area were island streaks. Long voids existed between the island streaks. The voids were crushed by two mechanisms of microslipping (c) and microfolding (d) after the macro folding was complete. The voids between island streaks were required to shrink due to the microfold mechanism at the void tip. The island streaks combined with one another due to the microfold mechanism. This was the interfacial plastic deformation enhanced by ultrasonic power. Figure 12: Schematic illustrations of interfacial bonding process between Al ribbon and silica substrate, which are observed from silica side; (a) Early stage after large sliding; (b) Middle stage in which island streaks increase and central belt zone B forms; (c) Later or final stage in which B\({}_{2}\) appears around B\({}_{1}\). B\({}_{2}\) is behind B\({}_{1}\). Figure 13: Schematic illustrations of bonded interface between Al ribbon and silica substrate; (a) Cross section parallel to US direction; (b) Drawing of bonded interface and island streaks; (c) Microslipping produced in longitudinal direction of Al ribbon; (d) Microfolding at void tip produced in width-direction of Al ribbon. A mechanical alloying could be produced at the bonding interface by ultrasonic vibration [42; 43; 44]. It is necessary to microscopically analyze the interfacial reaction layers at the bonded interface [42; 43; 44; 45].