**3. Results and Discussion**

The ECM experimental lifespan findings of the suggested nano-Ag composite paste and the control samples of conventional nano-Ag paste are listed in Table 1. According to Table 1, the nano-Ag-0.1%SiOx paste has a substantially longer failure lifetime than the nano-Ag paste under identical conditions. The 0.1wt% SiOx doping in the original nano-Ag paste serves a critical function in preventing the silver from migrating electrochemically. Furthermore, the lifetime reduces greatly when the temperature and the electric field are increased.


**Table 1.** ECM testing conditions and lifetime comparisons.

It was believed that the ECM behavior of the sintered nano-Ag at high temperatures initiates from the oxidation of the sintered anode and the decomposition of Ag2O [6]. When the temperature is constant, the higher the electrode voltage applied, the faster the Ag<sup>+</sup> formed after anodic oxidation. These Ag ions would reduce at the negative electrode as metallic Ag under the action of the electric field, resulting in the accelerated growth of conductive silver dendrites from the cathode and extended to the anode. As a result, the leakage current can be detected between the anode and the cathode. When the voltage remains constant, the higher the ambient temperature, the faster the ions travel. Then the produced Ag<sup>+</sup> speeds up the creation of silver dendrites under the specific electric field. Therefore, the ECM lifetime of the sintered nano-Ag-SiOx in the temperature range of 573–673 K is 1.5 to 3 times that of the sintered nano-Ag. It was likely that the SiOx covered on the silver, as shown in Figure 3a,b, should be susceptible to oxidation. The oxidation electrode potential of SiOx oxidation to SiO2, i.e., ~0.06 *V*SCE, was reported as more than one time lower than that of Ag oxidation to Ag2O, i.e., 0.13 *V*SCE [7,19]. As a result, silver oxidation was reduced and the ECM was delayed.

Figure 3c shows the change in shear strength following temperature cycling. The shear strength falls as the number of cycles increases. It should be noted that the shear strength of the Ag-SiOx paste drops to less than 30 MPa after 600 cycles, but the shear strength of the nano-Ag control samples remains ~30 MPa after 1000 cycles. It was likely that the atomic diffusion rate of Ag into Ag, i.e., ~1.455 × <sup>10</sup>−<sup>23</sup> <sup>m</sup>2/s [20], is much higher than that of Ag into SiOx, i.e., ~8.76 × <sup>10</sup>−<sup>29</sup> <sup>m</sup>2/s [21], resulting in an insufficient driving force for the sinter-bonding of Ag-SiOx heterogeneous particles under the same pressure-free and sintering temperature conditions [22]. The Young's modulus of the SiOx nanoparticles, on the other hand, is much higher than that of the Ag nanoparticles, and the thermal expansion coefficient is much lower than that of the sintered nano-Ag, so the interfacial bonds of the sintered Ag-SiOx were subjected to thermo-mechanical stress fatigue during temperature cycling, eventually leading to its shear strength dropping faster in the case of pressureless sintering [23].

It was known the driving force for the sintering can be increased greatly if assisted pressure was applied [24]. An extra 5 MPa pressure was applied to promote the sintering of the Ag-SiOx paste for better long-term reliability. Fortunately, the shear strength of the pressure-assisted sintered Ag-SiOx can be kept at ~35 MPa even after 1000 cycles. It was even higher than the shear strength of the nano-Ag control samples after 1000 cycles. It was believed that the pressure can ensure the better thermo-mechanical reliability of the nano-Ag-SiOx paste.

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 ~9% after 1000 cycles, while the thermal resistance of the nano-Ag-SiOx paste sintered with 5 MPa pressure increases by ~12%. It was consistent with the variation of the die shear strength. Fortunately, the increase in thermal resistance of the nano-Ag-SiOx 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.

Figure 4 shows the microstructures and porosity of sintered Ag and sintered nano-Ag-SiOx before and after the thermal shocking aging. The porosity of the sintered nano-Ag-SiOx 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 SiOx nanoparticles as seeds; this is called heterogeneous nucleation [25,26]. Therefore, for example, the porosity of the as-sintered nano-Ag-SiOx, 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-SiOx 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-SiOx may be also beneficial to retard the silver ECM to some extent as a result.

**Figure 3.** (**a**) Microstructures of as-sintered nano-Ag-SiOx; (**b**) the corresponding magnified image; (**c**) the thermal impedance of sintered nano-Ag-SiOx variation with the number of thermal shocking cycles.

**Figure 4.** (**a**–**d**) Microstructures and (**e**–**h**) porosity comparisons of sintered nano-Ag and sintered nano-Ag-SiOx before and after the thermal shocking test.

For the ECM verification for device packaging applications at 400 ◦C under the applied voltage of 400 V, the ECM lifetime of the DSC device using the sintered nano-Ag-SiOx 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-SiOx, significant improvement in ECM lifetime was achieved by nano-Ag-SiOx paste. Herein, the nano-Ag-SiOx paste could be a promising option to be used as a bonding material against specific metal corrosion.

**Figure 5.** (**a**) Schematic diagram of DSC device, (**b**) metallographic micrograph of the DSC device after ECM failure, and (**c**) the corresponding magnified image.

#### **4. Conclusions**

Silver ECM at high temperatures could be significantly retarded by the addition of SiOx nanoparticles in the nano-Ag paste. The thermo-mechanical reliability of the proposed nano-Ag-SiOx paste has been verified and proved to be improved by sintering under 5 MPa assisted pressures. It seems suitable as a new promising bonding material to prevent silver ECM failure because its die shear strength and thermal resistance degradation still did not reach the common failure criterion even after 1000 cycles of the thermal shock test. Finally, the ECM verification of the DSC power device further confirms the improvement of the sintered nano-Ag-SiOx as die attachment against the sintered nano-Ag ECM failure at high temperatures. It is interesting to pay more attention to reducing the ECM failure of sintered nano-Ag in economical and promising ways in the future.

**Author Contributions:** Conceptualization, Y.-H.M. and G.-Q.L.; methodology, Y.-H.M. and B.Z.; validation, Z.D.; formal analysis, Z.D., Z.W. and Y.-H.M.; investigation, Z.D. and Y.-H.M.; resources, Y.-H.M. and Z.W.; data curation, B.Z. and Y.-H.M.; writing—original draft preparation, Z.D. and Y.-H.M.; writing—review and editing, Y.-H.M. and B.Z.; visualization, Z.D. and Y.-H.M.; supervision, Y.-H.M.; project administration, Y.-H.M.; funding acquisition, Y.-H.M. and Z.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China, grant numbers 52177189, 51922075, and 52107203, and by Tianjin Municipal Science and Technology Bureau, grant number 21JCJQJC00150. The APC was funded by the National Natural Science Foundation of China.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** In this section, you can acknowledge the equipment support given by Xin Li and her students from Tianjin University.

**Conflicts of Interest:** The authors declare no conflict of interest.
