# Thermal Fatigue Properties of Ultrasonically Bonded Copper Joints
## 3 Results and Discussion
### Crack Propagation Behavior during Thermal Fatigue Fracture  
Figure 5 shows the cross-sectional OM and SEM images of the C1020 joints obtained perpendicular to the ultrasonic vibration before and after the thermal cycling test. In Figure 5a, the black line at the joint interface corresponds to the unbonded region, which became enlarged after the thermal cycling test. As shown in Figure 5b, narrow cracks were present near the bonded region, which propagated from the thick unbonded region after the thermal cycling test, although the width of the unbonded region was nearly the same as that before the thermal cycling test. This result revealed that the initial  
Figure 3: Stereomicroscope image of fracture surface of the C1020 joint.  
unbonded region was enlarged by the thermal stress produced during the thermal cycling test, and the cracks were initiated and propagated to the bonded regions. It should be noted that the narrow cracks propagated diagonally near the bonded region. To investigate crack propagation during the thermal fatigue test, EBSD was performed for the C1020 joints.  
Figure 6 shows SEM and EBSD images of the cross-section of the C1020 joints. Figure 6a,b respectively show the SEM image and IQ + IPF map before the thermal cycling test. The black region at the edge of the bonded region corresponds to the unbonded region. Crystal grains were refined in the bonded region approximately 20 \(\upmu\)m above and below the bonding interface. The grain size at the bonded region was reduced to 2.5 \(\upmu\)m (bonded region at the left) or 12.1 \(\upmu\)m (bonded region at the center) from 46.2 \(\upmu\)m on the C1020 substrate or 16.5 \(\upmu\)m at the C1020 terminal. These results  
Figure 4: Measurement results of the peel strength and bonding area of the C1020 joints before and after the thermal cycling test. (**a**) Results of peel strength and bonding area. (**b**) Representative stereomicroscope image of the fractured surface. (**c**) SEM image of the boundary of a stripe. (**d**) Representative backscattered electron (BSE) image of the fractured surface after thermal cycling tests.  
Figure 5: Cross-sectional images of the heel region in C1020 joints before (**a**,**c**) and after (**b**,**d**) thermal cycling test. (**a**,**b**) optical microscope (OM) images of the joint. (**c**,**d**) SEM images of the bonded region.  
revealed that ultrasonic bonding induced grain refinement at the interface rather than in the base material. Figure 6c,d show the SEM image and IQ + IPF map after the thermal cycling test. EBSD analysis was performed on not the completely fractured region but the same region during fracturing to evaluate the crack propagation behavior. The cracks were present at the edge of the bonded region. We confirmed that these cracks, initiated from the unbonded region as shown in Figure 5, propagated around the fine grains of the bonded region. On the other hand, cracks propagating in the refined region on the right side corresponded to the initially bonded region. It has been reported that the grain boundary can be an obstacle to crack propagation [13; 14]. Therefore, we inferred that the cracks did not propagate into the refined region that had many obstacles and that cracks propagated along a route with few grain boundaries around the fine grain region. When the refined region is narrow, there are few obstacles, and these cannot inhibit crack propagation. This finding suggests that increasing the grain boundary could be effective in suppressing crack propagation caused by thermal fatigue.