# Thermal Fatigue Properties of Ultrasonically Bonded Copper Joints
## 3 Results and Discussion
### Discussion on the Suppression of Thermal Fatigue Fracture in C1940 Joints  
In this section, we discuss the reason why thermal fatigue fracture was suppressed in C1940 joints and not in C1020 joints. Figure 9 shows the average grain size in the vicinity of the bonding interface of both the C1020 and C1940 joints, where the blue and red bars respectively correspond to the C1020 and C1940 joints. The average grain size was estimated as the average value of grain sizes within a region of height 10 \(\upmu\)m and the width of the bonded interface. The left axis is the distance from the bonding interface to the center of the region. The direction from the bonding interface toward the Cu terminal is the positive direction.  
As shown in Figure 9a, the grain size in C1940 joints was smaller than that of the C1020 joints at both the substrate and terminal sides before the thermal cycling test. Grain refinement on the substrate side for C1940 joints was attributed to the use of a substrate already having a small crystal grain size. In contrast, as C1020 was used in the terminal of both C1020 and C1940 joints, it was supposed that the difference in the grain size of the terminal originated from the ultrasonic bonding process. Although the mechanism of the bonding process in ultrasonic bonding is not completely understood [15], it is roughly understood to consist of two processes. (1) The surface layer such as an oxide film is broken by the sliding friction between the bonding materials. (2) The exposed clean surfaces adhere to each other owing to plastic deformation [15; 16]. As the C1940 substrate is harder than the C1020 substrate, the time required for these two processes after the onset of ultrasonic bonding increases. It has been reported that the grain size of copper subjected to sliding friction becomes finer [17]. Grain sizes in the terminal of C1940 joints were thought to be finer owing to an increase in the time for which they were subjected to sliding friction. This behavior also explains why the peel strength and bonding area of C1940 joints before the thermal cycle test were lower than those of the C1020 joints. Because the sliding friction time was longer on the C1940 joint, less time was spent on bonding than on the C1020  
Figure 9: Average grain size of C1020 joints and C1940 joints (**a**) before and (**b**) after the thermal cycling test.  
joint, so the peel strength and the bonding area were considered to be reduced. Figure 9b shows the average grain size of C1020 and C1940 joints after the thermal cycling test. The grain size of C1940 joints was finer than that of C1020 joints on both the substrate and terminal sides even after the thermal cycling test. Thus, during the thermal cycling test, in the C1940 joint, where the vicinity of the bonding interface was constituted by finer grains, it was found that crack propagation was suppressed because of many grain boundaries and the thermal fatigue resistance was better than the C1020 joint.  
Therefore, to effectively suppress thermal fatigue fracture in ultrasonically bonded copper joints, it is recommended to use a hard substrate with fine grains that can form fine grains around the bonding interface.