# Thermal Fatigue Properties of Ultrasonically Bonded Copper Joints
## 3 Results and Discussion
### Suppression of Thermal Fatigue Fracture in C1940 joints  
To suppress the thermal fatigue fracture of the ultrasonically bonded copper joints, C1940 was prepared as the bonding substrate, as C1940 has finer grains than C1020. In order to clarify the influence of grain size on the thermal fatigue properties of the ultrasonically bonded joint, the same bonding conditions and the same number of thermal cycles were applied to the C1020 and C1940 joints. Thermal cycling tests were also performed for the C1940 joints. Figure 7 shows the peel strength and bonding area of the C1940 joints before and after the thermal cycling test; the values of the peel strength and bonding area of the C1020 joint are also presented for comparison. The blue and red bars  
Figure 6: Electron backscatter diffraction (EBSD) results of the cross-section of C1020 joints perpendicular to the ultrasonic vibration. **(a,c)** SEM images and **(b,d)** image quality (IQ) + inverse pole figure (IPF) maps of the joints before and after thermal cycling tests, respectively.  
correspond to the C1020 and C1940 joints, respectively. Figure 7a shows the peel strengths of the C1020 and C1940 joints. The peel strength decreased by 41% after the thermal cycling test for the C1020 joints. On the other hand, that of the C1940 joints decreased by 25% after the thermal cycling test. Figure 7b shows the bonding area of the C1020 joint and C1940 joints. The bonding area decreased by 70% and 7% after the thermal cycling test for the C1020 and C1940 joints, respectively. These results revealed the low decrease rate of the bonding area after the thermal cycling test led to the suppression of the decrease rate of the peel strength for the C1940 joints. Thus, it was found that thermal fatigue fracture could be suppressed by using C1940 as the substrate material.  
Figure 8 shows cross-sectional SEM and EBSD images of the C1940 joints. Figure 8a,b show the SEM image and IQ + IPF map, respectively, before the thermal cycling test. As with the C1020 joint, the crystal grains were refined in the bonded region. However, in stark contrast with the EBSD image of the C1020 joint, the grain size of the substrate was finer. Nonetheless, the microstructures of the two joints were not considerably different. Figure 8c,d show the SEM image and IQ + IPF map, respectively, after the thermal cycling test. Figure 8 shows that the cracks progressed very little, thus, the difference in the composition of the alloy can be excluded as the main factor influencing the suppression of thermal fatigue failure. If the difference in the alloy was the main factor in suppressing thermal fatigue, the fracture should have progressed on the C1020 terminal side. In the next section, we focus on particle size, which is the reason why C1940 was prepared as the substrate, and discuss each factor that suppressed thermal fatigue fracture in the C1940 joint.  
Figure 8: EBSD results of the cross-section of C1940 joints perpendicular to the ultrasonic vibration. (**a**,**c**) SEM images and (**b**,**d**) IQ + IPF maps of the joints before and after thermal cycling tests, respectively.  
Figure 7: Results of (**a**) peel strength and (**b**) bonding area of the C1020 joints and the C1940 joints before and after the thermal cycling test.