## 3 Results  
Figure 4 presents XRD results of the Cu-solder-resin composite paste and a bonding layer in the as-bonded state. Sn, Ag\({}_{3}\)Sn, and Cu peaks are detected in the composite paste, while Cu\({}_{6}\)Sn\({}_{5}\), Cu\({}_{3}\)Sn, Ag\({}_{3}\)Sn, and Cu peaks are detected in the as-bonded state. In the as-bonded state, the Sn peaks, which are detected in the composite paste, fully disappear, while the Cu\({}_{6}\)Sn\({}_{5}\) and Cu\({}_{3}\)Sn peaks appear. The disappearance of Sn and appearance of Cu-Sn IMCs could have occurred through isothermal solidification reactions between the Cu particles and solder particles during bonding. A cross-sectional SEM image of the as-bonded TLPS joint is shown in Figure 5, which indicates that the dark gray Cu particles are connected with the light gray bridges of the Cu-Sn IMCs within a black polyimide resin matrix. This shows the unique microstructure of the TLPS joint, which is a Cu-IMC skeleton shaped microstructure partially filled with polyimide resin. Figure 6a shows the magnified structures of the chip side interface. (Cu,Ni)\({}_{6}\)Sn\({}_{5}\) is observed on the Ni/Au metalized chip surface, and no intermediate layers are detected between them. An intermediate layer composed of Cu\({}_{3}\)Sn IMCs is observed between the Cu particles and (Cu,Ni)\({}_{6}\)Sn\({}_{5}\) IMCs. Figure 6b shows the substrate side interface, where Cu\({}_{6}\)Sn\({}_{5}\) IMCs are observed on the Cu electrode of the substrate with an intermediate layer of Cu\({}_{3}\)Sn IMCs. These phases were confirmed by EDX results, as shown in Table 1. In addition, it should be noted that submicron voids are observed in the Cu\({}_{3}\)Sn layer, as shown in Figure 6. The submicron voids are observed in the Cu\({}_{3}\)Sn layer on the Cu particles but not on the Cu substrate. Figure 4: XRD patterns of the Cu-solder-resin composite paste and the as-bonded TLPS bonding layer. Figure 5: Cross-sectional SEM image of an as-bonded TLPS joint. Cross-sectional SEM images of the TLPS joints aged at 150, 175, and 200 \({}^{\circ}\)C for 1000 h, which are shown in Figure 7, show little change compared to the as-bonded state, as shown in Figure 5. The XRD results of the TLPS bonding layer aged at 150, 175, and 200 \({}^{\circ}\)C for 1000 h, as shown in Figure 8, show that Cu, Ag\({}_{3}\)Sn, and Cu\({}_{3}\)Sn peaks--and no Cu\({}_{6}\)Sn\({}_{5}\) peaks--are detected in the aged state. This indicates that the Cu\({}_{6}\)Sn\({}_{5}\), which are detected only in the as-bonded state, fully transformed into the Cu\({}_{3}\)Sn phase during the thermal aging for 1000 h. Focusing on the interfacial microstructure on the chip side (Figure 9 and Table 2) surface and substrate side (Figure 10 and Table 3) surface, both the Cu\({}_{3}\)Sn phase and residual (Cu,Ni)\({}_{6}\)Sn\({}_{5}\) phase are observed at the chip side interface, whereas only the Cu\({}_{3}\)Sn phase is observed at the substrate side interface. Although a small amount of the (Cu,Ni)\({}_{6}\)Sn\({}_{5}\) phase could remain because of the Ni dissolution from the chip metallization, it can be concluded from these XRD and EDX results that the highest amount of the Cu\({}_{6}\)Sn\({}_{5}\) phase, as seen in the as-bonded state, transforms into Cu\({}_{3}\)Sn through the thermal aging. In addition, it should be noted that the number of submicron voids at the interface between the Cu\({}_{3}\)Sn and Cu particles apparently increases through the thermal aging, as can be seen from a comparison between Figure 6 (as-bonded) and Figures 9 and 10 (aged). \begin{table}
\begin{tabular}{c c c c c c} \hline
**Area** & **Cu** & **Sn** & **Ni** & **Au** & **Phase** \\ \hline
1 & 73.8 & 25.0 & – & 1.2 & Cu\({}_{3}\)Sn \\
2 & 44.2 & 44.6 & 5.3 & 6.0 & (Cu,Ni)\({}_{6}\)Sn\({}_{5}\) + Au \\
3 & 75.6 & 24.4 & – & – & Cu\({}_{3}\)Sn \\
4 & 56.0 & 44.0 & – & – & Cu\({}_{6}\)Sn\({}_{5}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition (at.%) in the marked areas in Figure 6 measured by EDX spectroscopy.