## 3 Results and Discussion
Figure 3c also shows the change in thermal resistance after the temperature cycling. The thermal resistance grows steadily as the number of cycles increases. The thermal resistance of the nano-Ag paste improves by \(\sim\)9% after 1000 cycles, while the thermal resistance of the nano-Ag-SiO\({}_{\rm x}\) paste sintered with 5 MPa pressure increases by \(\sim\)12%. It was consistent with the variation of the die shear strength. Fortunately, the increase in thermal resistance of the nano-Ag-SiO\({}_{\rm x}\) pastes is still less than the common industrial failure criterion in power electronics, which is usually required as less than 20% of the thermal resistance after 1000 cycles of the temperature cycling. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Temperature (K)**} & \multirow{2}{*}{**Electrode Spacing (mm)**} & \multirow{2}{*}{**Voltage (V)**} & \multicolumn{2}{c}{**Lifetime (min)**} \\ \cline{3-4}  & & & **Sintered Nano-Ag** & **Sintered Nano-Ag-SiO\({}_{\rm x}\)** \\ \hline
673 & 0.5 & 200 & 332 & 512 \\
673 & 0.5 & 260 & 294 & 477 \\
673 & 0.5 & 300 & 236 & 412 \\
773 & 0.5 & 200 & 134 & 341 \\
773 & 0.5 & 260 & 81 & 244 \\
773 & 0.5 & 300 & 74 & 208 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: ECM testing conditions and lifetime comparisons. Figure 4 shows the microstructures and porosity of sintered Ag and sintered nano-Ag-SiO\({}_{\rm x}\) before and after the thermal shocking aging. The porosity of the sintered nano-Ag-SiO\({}_{\rm x}\) before and after the thermal shocking aging is always lower than that of the sintered nano-Ag. It was believed that the nucleation of metallic Ag occurs preferentially adjacent to the SiO\({}_{\rm x}\) nanoparticles as seeds; this is called heterogeneous nucleation [25; 26]. Therefore, for example, the porosity of the as-sintered nano-Ag-SiO\({}_{\rm x}\), i.e., 21.8%, can be much lower than that of the as-sintered nano-Ag, i.e., 34.5%. Furthermore, the grains of the sintered nano-Ag-SiO\({}_{\rm x}\) grew more than that of the sintered nano-Ag under a similar sintering profile. The larger grain size indicated a smaller grain boundary density as well. Considering that the grain boundaries are close to the free surface as fast diffusion paths [27], the more the grain boundary, the more the Ag hillocks could be formed by atomic diffusion or stress migration through the grain boundaries. The fewer grain boundaries of the sintered nano-Ag-SiO\({}_{\rm x}\) may be also beneficial to retard the silver ECM to some extent as a result. Figure 3: (**a**) Microstructures of as-sintered nano-Ag-SiO\({}_{\rm x}\); (**b**) the corresponding magnified image; (**c**) the thermal impedance of sintered nano-Ag-SiO\({}_{\rm x}\) variation with the number of thermal shocking cycles. For the ECM verification for device packaging applications at 400 \({}^{\circ}\)C under the applied voltage of 400 V, the ECM lifetime of the DSC device using the sintered nano-Ag-SiO\({}_{\rm x}\) and the sintered nano-Ag as die attachment is 240 h and only 96 h, respectively. The ECM of silver was suppressed successfully. As seen from Figure 5, although silver dendrites appeared after 240 h ECM testing of DSC devices using sintered nano-Ag-SiO\({}_{\rm x}\), significant improvement in ECM lifetime was achieved by nano-Ag-SiO\({}_{\rm x}\) paste. Herein, the nano-Ag-SiO\({}_{\rm x}\) paste could be a promising option to be used as a bonding material against specific metal corrosion. Figure 4: (**a**–**d**) Microstructures and (**e**–**h**) porosity comparisons of sintered nano-Ag and sintered nano-Ag-SiO\({}_{\rm x}\) before and after the thermal shocking test.