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We investigate the role of an Ag interlayer to suppress Kirkendall void formation in Ti/Ni surface finishes on the die-attached joints during harsh thermal aging tests at 250 °C. GaN dies were attached to the Ti/Ag/Ni sputtered direct bonded aluminum (DBA) and Ti/Ni sputtered DBA substrate using a high-temperature soldering, respectively. Without the Ag interlayer, void size increased to 0.69 µm after 1000 h of aging, but with the Ag interlayer, it reduced to 0.0794 µm. The Ag interlayer retards the development of cracks and void coalescing by controlling the diffusion rate between Ni/Sn at the bonding interface during thermal aging. • Analyzing how a silver interlayer affects Ni-finished GaN power semiconductors in in-wheel motor integration. • Quantifying the link between mechanical capabilities and microstructure in crystallographic characteristics. • Exploring how the Ag interlayer controls Ni-Sn IMC system diffusion rates. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) power modules with Ag sinter-bonding die attach were designed on the basis of thermal stress analysis for reliable high-temperature operations. Both the finite-element analysis (FEA) simulations and preliminary experiments confirmed that inserting the direct-bonded-copper (DBC) substrates can effectively reduce the maximum thermal stress in the module. A prototype SiC power module using sintered Ag die attach with a DBC substrate was designed and fabricated. The modules exhibited excellent durability in power cycling between 65 °C and 250 °C up to 20 000 cycles. FEA calculations of cumulative thermal strain and stress distributions adequately predicted the initial cracking position in the specimens after prolonged power cycles, observed by scanning electron microscopy. [ABSTRACT FROM AUTHOR]

Micron Ag paste had a more affordable price, feasible large-scale synthesis, and longer storage life compared to nano Ag paste, thus it attracts much industrial interest for die attachment of high-power devices. However, the previous studies of high-temperature reliability were mainly focused on nano Ag joints, the research about reliability of micron Ag joints, especially low-temperature and pressureless, was very limited. Therefore, we evaluated high-temperature stability of low-temperature and pressureless micron Ag joint, involving in the changes of mechanical behaviors, evolution of microstructure and interfacial reliability. The average joint strength of micron Ag joints was independent of aging time and kept approximately 35 MPa after aging for 1000 h. The fracture of the micron joint was dominated by the ductile deformation of Ag grains during the fracture process. On the other hand, the microstructure of porous structure evolved greatly during aging process. Ag grains were oriented randomly before and after aging process, but the Ag grains increased slightly from 827.2 nm initially to 1178.4 nm after 1000 h aging. Meanwhile, the pores size in porous structure increased, the number decreased significantly, and the porosity also decreased slightly. Moreover, the barrier layers at interfaces of micron Ag joint remained stable and reliable during aging at 250 °C. The results would promote the large-scale application of the commercially available micron Ag paste in high-power devices. [ABSTRACT FROM AUTHOR]

A bonding technology is introduced by using surface polished porous Ag in die-attachment structure. Bonding strength did not change much as the chip size varied from 3 × 3 mm 2 to 15 × 15 mm 2 . This confirms that the technology was not influenced by the chip size, and thus can be used in large area bonding. Bonding mechanism based on stress-induced migration was discussed with the three dimensional finite element analyses. Transmission electron microscopy (TEM) observation further confirmed that single crystal hillocks and Ag particles formed at the bonding interface, bridging the interface together. [ABSTRACT FROM AUTHOR]

Ag sinter joining technology has been used in the advanced power applications to replace conventional soldering technology due to its high temperature stability, along with its excellent electrical and thermal conductivity. In this paper, we report the high-temperature reliability (250 °C for 1000 h) for die-attachment structures using Ag sintering technology on Cu substrates with different top metallization layers (Au and Ag), formed via different deposition processes (electroplating and sputtering). The bonding strength over 40 MPa and high-temperature reliability of sintered Ag on the sputtered Ag surface was the best among the systems studied here. Bonding quality and the bonding fracture behavior of sintered Ag on the different metallization substrates were characterized. Ag-Au solid solution was formed due to metallizaion Au atoms diffused into sintered Ag layer, leading to decreased shear strength under high temperature process. The influence of grain structure on the bonding quality at the interface between sintered Ag and the metallization Ag layers were discussed. It revealed that the grain size and orientation of the top metallization Ag layer influenced the bonding quality. The sintered Ag layer formed by Ag hybrid particles may have a selective orientation of metallization layer on the surface (111) of the Ag crystal. These results will be helpful to understand both technological perspectives for design and the applications of sintered Ag from the viewpoint of high-temperature reliability, as well as the fundamental understanding of its bonding quality mechanism with top metallization layers. [ABSTRACT FROM AUTHOR]

The thermal stability of silver (Ag) paste sintering on coated copper (Cu) and aluminum (Al) substrates has been investigated. Instead of conventional zincating or nickel plating, magnetron sputtering was used to achieve coating with titanium (Ti) and Ag. Silicon (Si) chips were bonded to coated Cu and Al substrates using a mixture of submicron Ag flakes and particles under 250°C and 0.4 MPa for 30 min. The joints were then subject to aging testing at 250°C for duration of 200 h, 500 h, and 1000 h. Two types of joints exhibited satisfactory initial shear strength above 45 MPa. However, the shear strength of the joints on Al substrate decreased to 28 MPa after 1000 h of aging, while no shear strength decline was detected for the joints on Cu substrate. Fracture surface analysis revealed that the vulnerable points of the two types of joints were (1) the Ag layer and (2) the interface between the Ti layer and Cu substrate. Based on the results of scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and simulations, cracks in the Ag layer were identified as the cause of the shear strength degradation in the joints on Al substrate. The interface evolution of the joints on Cu substrate was ascribed to Cu migration and discontinuity points that initialized in the Ti layer. This study reveals that Al exhibited superior thermal stability with sintered Ag paste. [ABSTRACT FROM AUTHOR]

The application of microporous sintered silver (Ag) as a bonding material to replace conventional die-bonding materials in power electronic devices has attracted considerable interest. Characterization of the mechanical properties of microporous Ag will enable its use in applications such as lead-free solder electronics and provide a fundamental understanding of its design principles. However, the material typically suffers from thermal and mechanical stress during its production fabrication, and service. In this work, we have studied the effect of microporous Ag specimen size on fracture toughness from the microscale to the macroscale. A focused ion beam was used to fabricate 20-, 10- and 5-μm-wide microscale specimens, which were of the same order of magnitude as the pore networks in the microporous Ag. Micro-cantilever bending tests revealed that fracture toughness decreased as the specimen size decreased. Conventional middle-cracked tensile tests were performed to determine the fracture toughness of the macroscale specimens. The microscale and macroscale fracture toughness results showed a clear size effect, which is discussed in terms of both the deformation behavior of crack tip and the influence of pore networks within Ag with different specimen sizes. Finite element model simulations showed that stress at the crack tip increased as the specimen size increased, which led to larger plastic deformation and more energy being consumed when the specimen fractured. [ABSTRACT FROM AUTHOR]

The mechanical properties of sintered Ag paste with microporous structure have been investigated by tensile and shear tests, focusing on the temperature-dependent plastic deformation at various temperatures from 25°C to 300°C, corresponding to the target operating temperature range of emerging wide-bandgap semiconductor devices. Specimens were prepared by sintering hybrid Ag paste consisting of microflake and submicron spherical Ag particles, simulating a typical bonding process for power semiconductor die attach. Mechanical tests revealed that the unique microstructure caused a brittle-to-ductile transition at temperature of around 160°C, remarkably lower than that of bulk Ag. The obtained Young's modulus and shear modulus values indicate obvious softening with increasing temperature, together with a remarkable decrease in Poisson's ratio. These plastic behaviors at elevated temperature can be explained based on Coble creep in the microporous network structure. Fracture surfaces after tensile and shear tests indicated unique features on scanning electron microscopy, reflecting the variation in the ductile behavior with the test temperature. Furthermore, these temperature-dependent mechanical parameters were employed in three-dimensional finite-element analysis of the thermomechanical stress distribution in wide-bandgap semiconductor module structures including Ag paste die attach of different sizes. Detailed thermal stress analysis enabled precise evaluation of the packaging design for wide-bandgap semiconductor modules for use in high-temperature applications. [ABSTRACT FROM AUTHOR]

We studied the thermal fatigue behavior of submicron silicon carbide particle (SiC)-doped silver (Ag) microflake sinter joints for die attachment in next-generation power devices. Si dummy chips and direct bonded copper substrates with various metallization schemes were bonded using SiC-doped Ag microflakes under mild conditions (250°C, 30 min, 0.4 MPa). The SiC was distributed homogeneously in the porous Ag network and inhibited morphological evolution during thermal cycling tests. The shear strength of as-sintered pure Ag and SiC-added joints was ∼50 MPa and 35 MPa, respectively. Thermal cycling tests from −40°C to 250°C were conducted for up to 1000 cycles (hours) to characterize the thermostability of the bonded joints. After 1000 cycles, joints with and without SiC experienced bonding degradation, with shear strength of ∼25 MPa and 20 MPa, respectively. Thus, after 1000 cycles, the shear strength of pure Ag and SiC-doped joints decreased by 58% and 42%, respectively, compared with their maximum value. Coarsening of porous Ag occurred in pure Ag joints. SiC addition inhibited morphological evolution of SiC-doped joints during thermal cycling. However, vertical cracks generated by thermal stress were observed in joints both with and without SiC, which may limit long-term reliability. [ABSTRACT FROM AUTHOR]