# In-Situ Observation of Adhesion Behavior During Ultrasonic Al Ribbon Bonding
## 1 Introduction  
Solid state ultrasonic bonding renders it possible to bond similar and dissimilar materials directly and within a short duration [1; 2; 3; 4; 5; 6]. Al ribbon bonding is applied to power electronics packaging as well as to Al wire bonding [1; 6; 7; 8]. Various studies of solid state ultrasonic microjoining have been reported [1; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. The dominant mechanism consists of friction slip, plastic deformation, frictional heating and interfacial reaction (interdiffusion) [9; 10; 11; 12; 13; 14; 22]. These mechanism elements influence each other; additionally, their contributions are changed by the bonding conditions (bonding force, \(F_{b}\), and ultrasonic power, \(P_{u}\)) [1; 2; 13; 16; 17; 18; 19; 20; 21; 22; 14; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81 of the direct observation have been reported for Al wire bonding [1; 29] and Al ribbon bonding [1]; however, these are insufficient for understanding the ultrasonic bonding process.  
If the bonding condition is appropriate, the general bonding process is as follows. Immediately after ultrasonic vibration is introduced in a direction parallel to the bonding interface, the friction slip occurs initially. The surface oxide film is scrubbed by the ultrasonic vibration and locally broken [16; 22]. Then, the clean metallic surface is locally created, that is, local adhesion is produced. The local adhesion indicates that the friction coefficient at the bonding interface increases, that is, an increase in the shear force [13; 21; 23; 24; 25; 26]. Shear stress is a component of equivalent stress which is the driving force behind plastic deformation [34; 35; 36; 39]. The rate sensitive bonding materials are easily deformed by the enhanced equivalent stress, causing the softening effect. The uniaxial yield stress normal to the bonding interface apparently decreases [16; 31]. In ball (or wedge wire) bonding, the side surface can be folded to the substrate due to the plastic deformation facilitated by the softening effect. The apparent bonded (or contact) area grows to the peripheral area. This is the fold mechanism which is redefined as the macro fold mechanism in the present paper. The sequence mechanism from friction slip to fold was defined as \"slip and fold mechanism\" in a previous paper [1]. Some microslips occur at the peripheral bond area after the macro fold is complete [11; 12].  
As the shape of the ribbon is roughly flat, compared with that of Au ball and Al wire, the ribbon bonding exhibits a different mode from ball (or wire) bonding. The macro fold mechanism works easily and the apparent contact width rapidly increases in Al ribbon bonding. Then, numerous strips (island streaks) are formed in a direction parallel to the ultrasonic vibration; additionally, a belt-shaped bond-area (belt zone) is formed at the central bonded area normal to the direction of ultrasonic vibration. However, the adhesion behavior cannot be clearly explained using only slip and the macro fold mechanism. Therefore, in the present study, the in-situ observation of the interfacial adhering process during the ultrasonic bonding between Al ribbon and transparent silica (crystal glass) substrate is performed using a high-speed video camera with frame rates of \(10^{4}\) fps or \(10^{3}\) fps. The bonding interface can be observed directly, using a transparent silica substrate. The adhering process is discussed, comparing the results observed at \(10^{4}\) fps with those of \(10^{3}\) fps. The purpose of the present study is to understand the interfacial adhesion behavior during ultrasonic Al ribbon bonding. Furthermore, the complex adhesion behavior is discussed through the theoretical idea of the common bonding mechanism, consisting of friction slip, plastic deformation, and interfacial material reaction.