## 3 Results
Figure 6: Cross-sectional SEM images of the interfaces between the bonding layer and (**a**) chip and (**b**) substrate, as-bonded. Figure 7: Cross-sectional SEM images of a TLPS joint aged at (**a**) 150, (**b**) 175, and (**c**) 200 \({}^{\circ}\)C for 1000 h.  
Figure 8: XRD patterns of the TLPS bonding layer as-bonded and aged at 150, 175, and 200 \({}^{\circ}\)C for 1000 h. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Aging Condition** & **Area** & **Cu** & **Sn** & **Ni** & **Au** & **Phase** \\ \hline \multirow{2}{*}{150 \({}^{\circ}\)C 1000 h} & 1 & 74.2 & 24.7 & – & 1.0 & Cu3 Sn \\  & 2 & 46.9 & 42.0 & 4.1 & 7.1 & (Cu,Ni)6 Sn5 + Au \\ \hline \multirow{2}{*}{175 \({}^{\circ}\)C 1000 h} & 3 & 72.8 & 26.0 & – & 1.2 & Cu3 Sn \\  & 4 & 47.8 & 36.3 & 4.4 & 11.5 & (Cu,Ni)6 Sn5 + Au \\ \hline \multirow{2}{*}{200 \({}^{\circ}\)C 1000 h} & 5 & 71.7 & 26.6 & – & 1.7 & Cu3 Sn \\  & 6 & 47.6 & 38.4 & 5.8 & 8.1 & (Cu,Ni)6 Sn5 + Au \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition (at.%) in the marked areas in Figure 9 measured by EDX spectroscopy. Figure 9: Cross-sectional SEM images of the interfaces between the bonding layer and chip aged at **(a)** 150, **(b)** 175, and **(c)** 200 \({}^{\circ}\)C for 1000 h.  
Figure 11 illustrates the relationship between the shear strengths of the TLPS joints and aging conditions. The shear strength of the as-bonded joints is approximately 22.1 MPa. During the aging process, the shear strengths aged at 150 and 175 \({}^{\circ}\)C increase from 22.1 to 24.4 and 27.4 MPa in the first 500 h, respectively; afterward, they decrease to 22.8 and 24.0 MPa, respectively, after 1000 h. In contrast, that aged at 200 \({}^{\circ}\)C remains constant in the first 500 h and slightly decreases afterward to 19.0 MPa after 1000 h. The lowest shear strength of all the aged specimens is approximately 19 MPa, which is quite high when compared to the requirement of MIL-STD-883, Method 2019.7 (\(>\)0.5 MPa, 7 \(\times\) 7 mm in die-size). This indicates that the joints could be applied to high-temperature applications with an excellent shear strength stability. The representative fracture surfaces of the as-bonded and aged specimens are shown in Figure 12. The fractography of the as-bonded specimen shows a brittle fracture surface on the Cu\({}_{6}\)Sn\({}_{5}\), as shown in Figure 12a,b. The fractography of the aged specimens also shows a brittle surface. However, a fracture clearly appears at the interface between Cu and Cu\({}_{3}\)Sn, as shown in Figure 12c-h. The morphology of the fractured surface appears similar to a \"boiled egg\" and \"peeled eggshell\", which represents the Cu particle and Cu\({}_{3}\)Sn layer covering them. Such unique fracture surfaces are observed only on the aged specimens. Based on the results, the change in the fracture behavior could have occurred by the microstructural variation through thermal aging. Figure 11: Shear strengths of TLPS joints aged at 150, 175, and 200 \({}^{\circ}\)C for 1000 h.  
Figure 10: Cross-sectional SEM images of the interfaces between the bonding layer and substrate aged at (**a**) 150, (**b**) 175, and (**c**) 200 \({}^{\circ}\)C for 1000 h.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Aging Condition** & **Area** & **Cu** & **Sn** & **Ni** & **Au** & **Phase** \\ \hline
150 \({}^{\circ}\)C 1000 h & 1 & 75.7 & 24.3 & – & – & Cu\({}_{5}\)Sn \\  & 2 & 73.1 & 26.9 & – & – & Cu\({}_{5}\)Sn \\ \hline
175 \({}^{\circ}\)C 1000 h & 3 & 73.6 & 26.4 & – & – & Cu\({}_