# Evaluation of Ultrasonic Bonding Strength with Optoacoustic Methods
## 1 Introduction  
Ultrasonic bonding is one of the solid-state bonding techniques that uses high frequency vibration to bond metal sheets. This technique is utilized in the automotive industry for joining parts such as wire bonding on a semiconductor substrate, wiring components with electrode terminals, and thin film electrodes of lithium ion batteries. The importance of the ultrasonic bonding is increasing with electrification of automobiles and the expansion of demand of electric vehicles. In addition, as the ultrasonic bonding has advantages such as short-time and low energy compared with other solid-state bonding techniques [1], the ultrasonic bonding is expected to be a promising tool in dissimilar bonding for automotive body panels. Since the industrial importance and the demand of the bonding quality increase, inspection methods for assuring the reliability of the bonding are required.  
Destructive methods including a tensile testing, tensile shear testing, etc. are utilized to evaluate the bonding strength. The bonding reliability is evaluated statistically by a sampling inspection. On the other hand, nondestructive methods including ultrasonic testing or radiographic testing are conducted for evaluation of the state of the bonding. Some researchers evaluated the deterioration inside materials with ultrasonic waves [2; 3; 4], electrical methods [5], thermography [6; 7], and radiography [8]. The ultrasonic testing detects the reflected waves coming from the discontinuous interfaces due to cracks and un-bonded regions. In addition, some researches [9; 10; 11] tried to detect the residual stress around the bonded area by nonlinearity of the sound wave. These methods used the acoustoelasticeffect, in which the sound velocity depends on the elasticity of materials. The bonding strength is often estimated with the bonded area. The bonded area was visualized by ultrasonic C-scan testing [12; 13; 14; 15] and radiography [16; 17]. In these studies, it was indicated that the relation between the bonded area and the bonding strength are linearly correlated. However, C-scan testing and radiography are not suitable for in-situ measurement of the bonding strength, because it needs measurement equipment outside of the production line and is time-consuming. There have been other ways to evaluate the bonding area with ultrasonic techniques including the A-scan method [18; 19], resonant frequency measurement [20], cantilever test [21], electrical method [22], and thermographic test [23]. Application of an optical technique is an effective methodology for non-contact measurement. However, there are few studies that relate the optical measurement to evaluation of the bonding strength. The bonded area detected by the optical coherence tomography [24] had a correlation with the bonding strength. Furthermore, nondestructive evaluations are of technical interest, as well as fundamental interest, in the measurement of interfacial strength between dissimilar materials such as metal/polymer dissimilar joints [25] and fiber reinforced composites [26; 27; 28]. The interfacial strength is extremely important in controlling the mechanical property of joints.  
In our study, we propose a new method to evaluate the bonding strength that is non-destructive and quick by analyzing the elastic behavior of the bonded materials. We expect that the elastic behavior will change depending on the bonding strength. By analyzing the elastic behavior of bonded materials, it is expected that the bonding strength can be evaluated. We focus on the optoacoustic methods to analyze the elastic behavior in a non-destructive way. The ultrasonic bonding uses acoustic vibration, so we considered that it will be possible to measure the elastic behavior in situ. In addition, the vibration behavior can be measured without contact using the optics with the acoustics.