*2.2. Methods of Reliability Verification*

Firstly, the capability of the sintered nano-Ag or nano-Ag-SiOx against the ECM was verified in this work. The ECM samples were formed by stencil-printed the proposed anti-ECM nano-Ag composite paste on an alumina ceramic substrate (30 × 30 mm2) to form pairs of electrodes with identical gaps of 0.5 mm for ensuring uniform electrical bias distributions (Figure 1c). The as-printed ECM samples were then heated to 280 ◦C at a ramp rate of 5 ◦C/min and sintered at 280 ◦C for 30 min. These processes are shown in Figure 1d. The leakage currents between the electrodes are monitored by a Pico ammeter (RIGOL DM3068). The time when the leakage current first reaches 1 mA was defined as the lifetime of the ECM. The environment temperature during the ECM test was 25 ± 2 ◦C and the environment humidity during the ECM test was 50%.

Then thermal shocking tests were carried out according to the standard of JESD22- A104C to verify the reliability of the nano-Ag composite paste. The temperature swung from −40 ◦C to +125 ◦C and the soaking time at the extreme temperatures was 10 min. Each period was about 25 min. The degradation of shear strength and thermal resistance of the ECM samples was recorded every 100 cycles until 1000 cycles. The shear strength was measured by a die-shearing tester (XYZTEC, CONDOR 150) and the thermal resistance

was recorded by a thermal resistance measurement system that used *V*ge as temperature sensitive parameter [17,18].

**Figure 1.** (**a**) Preparation process of nano silver paste; (**b**) preparation of proposed anti-ECM nano-Ag-SiOx composite paste; (**c**) preparation of ECM samples; (**d**) schematic of the sintering process and ECM test.

For the thermal resistance measurement, a 3.92 × 3.88 × 0.07 mm3 Insulated Gate-Bipolar-Transistor (IGBT) (INFINEON, IGC15T65QE) was sinter-bonded on a <sup>30</sup> × <sup>30</sup> × 1 mm3 silver-plated direct-bond-copper (DBC) substrate under the same process as that of the ECM samples.

For the die-shearing tests, a 3 × <sup>3</sup> × 0.25 mm<sup>3</sup> dummy silicon chip with bottom metallization layer of Al-Ti-Ni-Ag (1.2 <sup>μ</sup>m) was sinter-bonded on a 30 × <sup>30</sup> × 1 mm<sup>3</sup> silverplated DBC substrate using the same process as the ECM samples, under an additional pressure of 5 MPa.

In the end, a DSC power device was demonstrated to verify the anti-ECM of the proposed nano-Ag composite paste. The fabrication process of the DSC demonstration is shown in Figure 2. Firstly, the top side of a silicon carbide (SiC) dummy chip of <sup>4</sup> × <sup>4</sup> × 0.17 mm3 was metalized with Ti-Ni-Ag using magnetron sputtering technology. Then the proposed nano-Ag composite paste was printed onto two silver-plated DBC substrates of <sup>20</sup> × <sup>20</sup> × 1 mm3 and the top Ti-Ni-Ag metallization of the SiC chip separately. Then the chip, the silver-plated copper block buffer of 2 × <sup>2</sup> × 2.2 mm3, and the silver-plated power terminals were assembled with the DBC substrates. The as-assembled DBC substrates were sintered at 280 ◦C and 5 MPa as the process of the previous samples for the die-shearing tests.

During the ECM test for the demonstrated DSC power devices, a direct current (DC) power supplier (Topower TN-XX702) was connected with the power terminals of the device. A controllable heating plate was used to provide the ambient temperature required for the ECM test. The leakage currents in the device were monitored by the Pico ammeter (RIGOL DM3068).
