## 4 Results and Discussion
### Reliability of TLPS Joints during Thermal Cycling  
Figure 7 shows the relationship between the number of temperature cycles and the ratio of unbonded areas of SiC-DBC joints of TLPS and SAC305 obtained from the SAT images. The SAT images of the joints after thermal cycles can be found in our previous work [33]. The size of the detected unbonded regions is larger than 0.1 mm \(\times\) 0.1 mm, which is the resolution of the SAT equipment used in this study. The ratio of the unbonded areas of the SAC305 joints increases to 6%, while that of the TLPS shows a minor increase after 1200 thermal cycles. Figure 8 shows the cross-sectional SEM images  
Figure 5: Temperature dependence of the homogenized (**a**) Young’s modulus and (**b**) yield strength calculated by the 15-μm-sized models of Cu-IMC-resin and Cu-SAC-resin, where the resin region is PI, epoxy, or air. The values are normalized by that of the Cu-IMC-resin (PI) at 22 \({}^{\circ}\)C.  
Figure 6: Deformation properties calculated by the 50-μm-sized Cu-IMC-resin microstructure model based on 3D image reconstruction at 22 \({}^{\circ}\)C.  
of the TLPS and SAC305 joints after thermal cycles, which displays crack-like connected cavities in the SAC305 joints and no apparent deterioration in the TLPS joints. These observations confirm that the TLPS joints are more reliable than the SAC305 solder joints, i.e., the former exhibits fewer crucial delamination or cracks, which are less likely to affect the heat dissipation. Figure 9 shows the FE simulation results of the thermal cycling tests for the SiC-DBC joint bonded by the TLPS layer, where the homogenized properties based on 3D image reconstruction are used. The joints exhibit strain peaks at the edges and corners of the bonding layer, where cracks were expected. The homogenized TLPS joint shows the maximum strain \(\sim\) 0.4%, which was generated during the thermal cycle with minimum temperature. In contrast, quite low Mises stress was generated in the TLPS layer as compared to that in the DBC substrate. In addition, no stress concentration on the DBC substrate was identified. The relationships between the number of thermal cycles and the calculated Mises strain and stress are shown in Figure 10a,b, respectively. Remarkably, the Mises strain of the homogenized TLPS joint did not increase through the cycles, while that of the SAC305 joint increased in every cycle. Further, the normalized Mises stress of the homogenized TLPS joint (0.08, \(-\)92%) is low compared to that of IMC (1.00, reference) and SAC305 (0.11, \(-\)89%). These results suggest that the TLPS facilitates the reduction of the both Mises strain and stress macroscopically through thermal cycling. Figure 8: Cross-sectional SEM image of (**a**) the TLPS joint after 1,200 thermal cycles and (**b**) the SAC305 joint after 600 thermal cycles in the temperature range of \(-\)55 to 175 \({}^{\circ}\)C.  
Figure 7: Unbonded area of TLPS and SAC305 solder joints through 1200 thermal cycles in the temperature range of \(-\)55 to 175 \({}^{\circ}\)C.  
In an earlier study [34], we demonstrated the excellent thermal reliability of the TLPS joints after thermal aging at 200 \({}^{\circ}\)C for 1000 h through temperature storage tests, where IMC of the microstructure transformed from Cu\({}_{6}\)Sn\({}_{5}\) to Cu\({}_{3}\)Sn along with the formation of submicron voids at the interface between Cu and IMCs during the thermal aging. The IMC was treated as the Cu\({}_{6}\)Sn\({}_{5}\) in the simulations. Although, Cu\({}_{3}\)Sn generally exhibits superior properties: higher strength, higher elongation, and lower elastic modulus compared to Cu\({}_{6}\)Sn\({}_{5}\)[18, 42], the transformation of the IMCs and formation of the submicron voids needs to be explained, which will be discussed in future work. The results of FE analysis in this study suggested the stable mechanical properties of the IMC-based composites at high temperature, which is a significant advantage of these composites over the solder-based ones.