## 1 Introduction  
The demand for high heat endurance bonding solutions is rising steadily because of next generation power modules with wide bandgap semiconductor materials such as silicon carbide (SiC) [1,2]. There is a strong need for power modules to operate at high powers and frequencies with a high integration and miniaturization capability. Moreover, the SiC chips have lower power losses and higher switching speeds, even at elevated temperatures. These applications provide higher operation temperatures, which have recently exceeded 175 \({}^{\circ}\)C or reached 200 \({}^{\circ}\)C compared to the conventional 150 \({}^{\circ}\)C. The bonding layers are exposed to high temperature operation atmospheres. Therefore, high heat tolerance bonding technologies as alternatives to conventional Sn-based solders are in high demand. Sinter bonding using nano- or micro-particles is one of the most promising technologies and has been reported to provide outstanding thermal and electrical conductivity and reliable performance [3; 4; 5; 6]. However, the sinter bonding technology requires complicated industrial equipment and processing because of the necessity of the pressure-assisted bonding process, so it cannot be widely applicable to industrial use. Another high-heat-tolerance bonding solution is strongly required for these reasons.  
In recent years, transient liquid-phase (TLP) bonding has been developed as a promising packaging technique for next generation power modules [7; 8; 9; 10]. Low melting point materials (Sn, In) and high melting point materials (Cu, Ag, Ni) are provided as layers or mixed powders that isothermally solidify during processing, which have higher re-melting points than the employed bonding materials. The technique involving the mixed powders as the bonding material is called transient liquid-phase sintering [11; 12; 13]. In a Cu-Sn system, the bonding layer is composed of Cu\({}_{6}\)Sn\({}_{5}\) and Cu\({}_{3}\)Sn intermetallic compounds (IMCs), whose melting points are 415 \({}^{\circ}\)C and 640 \({}^{\circ}\)C, respectively. The mechanical properties of the Cu-Sn IMCs indicate higher strengths and creep resistances compared to conventional Sn-based solders [14]. These properties indicate that the Cu-Sn transient liquid-phase sintered (TLPS) joints could be appropriate as a temperature resistant method for power electronics applications; however, Cu-Sn IMCs have been generally used for the purposes of crack formation in conventional soldering because of their quite brittle and stiff properties [15; 16; 17]. These mechanical properties of the IMCs cause joint fracture during thermal stresses despite the higher melting point of the TLP joints [18; 19; 20]. The authors proposed a novel approach for reducing the stiffness of TLPS joints by controlling the microstructural morphology using a Cu-solder-resin composite as the bonding material [21; 22], where a skeleton shaped microstructure consisting of Cu particles connected with Cu-Sn IMC bridges partially filled with polyimide resin was observed. In addition, the Cu-Sn skeleton microstructure showed a superior thermal cyclic reliability to a conventional Sn-based solder owing to the stiffness reduction effects of the joints. However, the precise study of the microstructural evolution of the joints during high-temperature operation has not yet been investigated.  
In the present study, the evolution of the TLPS Cu-Sn skeleton microstructure during thermal aging was evaluated by shear strength tests, X-ray diffraction analysis, microstructure observation, and a fractographical approach. From these results, the microstructural and mechanical evolutions of the TLPS Cu-Sn skeleton joints are discussed.