Objective There is an increasing demand for die attach materials with the rapid development of SiC devices, which can be bonded at low-temperature and function at high temperature. Nano-Ag sintering has been extensively investigated for application in high-temperature power electronics. However, the electrochemical-migration of Ag ions is the main drawback. Pd is famous for its chemical stability, and various studies have focused on the influence of Pd content on the effectiveness and its mechanism. Recently, researchers have been trying to mix Pd and Ag nanoparticles (NPs) to improve the resistance to electrochemical-migration of the sintered layer. However, Pd has a melting point higher than that of Ag, whereas the alloying process needs high temperature (similar to 850 degrees C) to form Ag-Pd alloy. Pulsed laser deposition (PLD) is a physical method feasible for fabricationg Ag-Pd nanoalloy without using organic additives such as polyvinylpyrrolidone, which is required in the chemical method. In this work, Ag-10%Pd nanoalloy was fabricated by the PLD method, which can be used to connect SiC and Ag-coated direct bonding copper (DBC) substrates. The sintered layer enhances resistance to electrochemical-migration with low-temperature bonding characteristics. The microstructure of the bonding, shear properties, and its electrochemical-migration resistance are studied. Methods Ag-10% Pd NPs were fabricated using PLD with a pressure of 750 Pa of Ar atmosphere. The Ag-Pd target was fabricated by powder sintering with weight ratio of 90:10. A picosecond laser with a pulse width of 10 ps was employed to ablate the target. Ag-Pd NPs were deposited on the back side of SiC chip (G.P. Tech, Ti/Ni/Ag metallization), then the SiC chip was removed from the substrate and placed on the Ag-coated DBC (HuaSemi Electronics, Ni/Au metallization). The interconnecting process is performed at a temperature range of 200 t 350 t assisted with a pressure of 5 MPa for 30 min in air. The shear test is conducted using Dage 4000. The electrochemical-migration test is conducted using a water drop test. Results and Discussions The microstructure of as-deposited Ag-Pd film comprises various NPs with diameters less than 1 mu m (Fig. 3). Element results indicate that these deposited NPs are in alloy state with a uniform composition distribution. The sintered joint comprises SiC chip, bondline and Ag-coated substrates (Fig. 4). The bondline thickness is about 27 mu m, which is only 31.6% of the as-deposited state. Thus, the Ag-Pd film had excellent deformability. The bondline exhibited Ag-9.57% Pd alloy microstructure without obvious element segregation. The sintered joint achieved a shear strength of 21.89 MPa at the sintering temperature of 250 t, which is higher than the US military standard MIL-STD-883K (7.8 MPa). Therefore, Ag-Pd nanoalloy film can be used as die attach material for low-temperature bonding. The sintering temperature provides the driving force for sintering process, as a denser bondline is achieved when the temperature is increased to 300 t (Fig. 6). Fracture surface reveals that the failure mainly occurred at the bondline, indicating that high bonding quality interface is realized ( Fig. 7). Compared with pure Ag, Ag-Pd nanoalloy exhibited a more than quadruple resistance to electrochemical-migration during the water drop test (Fig. 8). For pure Ag electrode, the current reached 1 mA with only 81.4 s, while the Ag-Pd electrode required 349.7 s for the short-circuit process. The dissolution of Ag ion was blocked by PdO formation on the anode, which played a paramount role in extending the short-circuit time, whereas the migration product was cloud-like instead of dendritic growth. This work proposed a method for fabricating Ag-Pd nanoalloy films as die attach material without the high alloying temperature. It should be noted that, Pd has a higher melting point (1554 degrees C) than Ag (961.7 degrees C), and Ag-Pd nanoalloy sintering requires higher sintering temperature than pure Ag NPs. Moreover, adding Pd is costly. Consequently, the sintering temperature, demand of electrochemical migration resistance and its cost should be balanced when applying Ag-Pd nanoalloy in electronic packaging. Conclusions Ag-10% Pd nanoalloy was successfully fabricated as die attach material using PLD. The sintered joint achieved a shear strength of 21.89 MPa at the sintering temperature of 250 degrees C, which was higher than the US military standard MIL-STD-883K (7.8 MPa). Compared with pure Ag, Ag-Pd nanoalloy exhibited a more than quadruple electrochemical-migration resistance. The dissolution of Ag ion was blocked by PdO formation on the anode with obviously extended short-circuit time, whereas the migration product was cloud-like. Compared with conventional direct sintering of Ag and Pb nanoparticles, pulsed laser deposited Ag-Pd nanoalloy sintering avoids high-temperature alloying process (850 degrees C), which is promising for Ag-Pd low-temperature bonding and is expected to provide a solution for the high-reliability power electronic packaging.
