## 2 Experimental Procedure  
The Cu-solder-resin composite investigated in this work is a paste that mainly contains Cu particles, Sn-3 Ag-0.5 Cu (SAC305) solder particles, and polyimide-type thermosetting resin, as shown in Figure 1. The melting point of the solder is 220 \({}^{\circ}\)C. The cure temperature of the polyimide resin is 230-240 \({}^{\circ}\)C. The sizes of the Cu particles and solder particles are approximately 10 and 3 \(\upmu\)m, respectively. The content of the copper in wt.% is approximately three times higher than that of the solder, which is designed to consume the solder through the formation of Cu-Sn IMCs. The content of the polyimide resin is approximately 20% in volume within the total of the Cu, solder, and polyimide resin. The polyimide resin is chosen by a curing temperature close to the melting point of the solder and by the soft mechanical properties, whose yield strength is approximately 1 MPa, which was expected to easily deform while embedded within the microstructure of the Cu and IMCs.  
Figure 2 shows a schematic of the die-attached specimen in this work. Properties were obtained from the joints between the Kovar (Ni-Co-Fe alloy) chips and the directly bonded copper substrates, composed of two 0.3-mm-thick copper electrodes bonded to a 0.6-mm-thick silicon nitride (Si\({}_{3}\)N\({}_{4}\)) substrate. The chips had dimensions of \(7\times 7\times 2\) mm and a Ni/Au surface metallization. The thickness of the Ni and Au metallization was 2.0 and 0.1 \(\upmu\)m, respectively. Before bonding, the chips were mounted onto the composite paste, which was printed on the substrate with a 100-\(\upmu\)m thickness in advance. After preheating at 100 \({}^{\circ}\)C for 60 min in air using a hotplate, the specimens were bonded at 250 \({}^{\circ}\)C for 1 min in nitrogen atmosphere using a reflow furnace (VSU28, Invacu, Ltd., Burgas, Bulgaria). No pressure was applied in this heating process. Afterward, the die-attached specimens were subjected to thermal aging testing at 150, 175, and 200 \({}^{\circ}\)C for 500 or 1000 h in air. Analyses of the cross-sections  
and fracture surfaces of the aged specimens were performed by field emission scanning electron microscopy (FE-SEM; SU8000, Hitachi High-Technologies Corporation, Tokyo, Japan) and energy dispersive X-ray spectrometry (EDX; EMAX Evolution, HORIBA, Ltd., Kyoto, Japan). The crystal structure of each phase in the TLPS layer was identified by X-ray diffraction (XRD; D8 DISCOVER, Bruker Corporation, Billerica, MA, USA) apparatus, as shown in Figure 3. The specimens for XRD were prepared by a tilted cross-sectioning method for resin-molded specimens. The die-attached specimens were subjected to shear tests with a cross head speed of 100 \(\upmu\)m/s at room temperature by a bond tester (CONDOR 150, XYZTEC). The average value of the two samples was regarded as the shear strength. After the shear test, the fracture surfaces were analyzed using FE-SEM and EDX.  
Figure 1: SEM image of Cu-solder-resin composite paste.  
Figure 3: XRD method using tilted cross-sectioning: (**a**) before cross-sectioning, (**b**) after cross-sectioning, (**c**) schematic of XRD arrangement, and (**d**) irradiated area of the transient liquid-phase sintered (TLPS) layer.  
Figure 2: Schematic of a die-attached specimen.