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In this article, an SiC die was directly attached on a bare DBA (Al/AlN/Al) substrate via micron-sized Ag sintering at 250 °C without pressure. The micron-sized structure of the Ag–Al joint revealed robust bonding (33.6 MPa), which was attributed to the excellent sinterability of the Ag paste. A high-temperature storage test was conducted for 1000 h at 250 °C, and the thermal shock test was conducted from −50 to 250 °C for 2000 cycles. The shear strength was >30 MPa even after 1000 h of high-temperature storage. Furthermore, the online thermal resistance (Rth) of the direct bonding of SiC–DBA was measured during the power cycling test employing an n-doped 4 H-SiC thermal engineering group (TEG) chip. The SiC–TEG chip exhibited ultra-high power density (1200 W/cm2) and ensured that the junction temperature reached 200 °C. The total Rth increased from 0.58 to 0.7 K/W after 10 000 power cycles at a swing temperature, ΔTj, of 175 °C, indicating that the increase in the Rth was 20.7% at such a large power density. The results revealed that the SiC–DBA power module structure exhibited very good bonding and thermal-resistance reliability, which can be employed as a new structure for next-generation SiC power modules. [ABSTRACT FROM AUTHOR]

Interface connection of sinter micron-sized Ag paste on Cu substrate in a low temperature pressure-less condition was improved by a novel 3D pyramid-shape Ag plating layer which possessed a larger surface area four times than that of a Ag flat plating. Interface connection ratio achieved to 65% corresponding to a satisfied interface bonding strength of 25.3 MPa at 180 °C pressure-less sintering. Finite element model for sinter Ag joining on the 3D pyramid-shape Ag plating was constructed and revealed that stress mainly concentrated at Ag plating protrusions but not at the interface bonding line like the Ag flat plating case. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Adoption of wide-bandgap semiconductors in power electronic systems enables these devices to operate at high switching speeds and at high temperatures (above 200 °C). Sinter Ag joining as an attractive die attach material received more and more attention. This study introduced a robust bonding of sinter Ag joining direct on an electroless Ni(P) surface-finished DBC (direct bonded copper) substrate with a low-cost micron-sized Ag flake particle. Die shear strength of sinter Ag joining increased from sintering temperature of 180 °C to 300 °C and achieved 43.6 MPa at 300 °C in air and without pressure during sintering. The shear strength decreased at the sintering temperature of 350 °C due to mass Ni oxide and dense voids generation. The bonding mechanism of sinter Ag joining direct on the Ni-plated structure was discussed via SEM, EDX, XRD and TEM analysis. There is not clear inter-diffusion between Ni and Ag element at the bonding interface. TEM observation reveals that many Ag nanoparticles with the size < 10 nm were self-generated and touched on the thin Ni oxide layer to form a robust interface bonding. In addition, the high-temperature aging test also was investigated for the sinter Ag joining at the aging temperature 250 °C for 1000 h. The die shear strength keeps 30 MPa after 500 h aging and down to 17.9 MPa after 1000 h, may mainly induce by the interface connection ratio decrease with the Ag grain coarsening during high-temperature aging. This work provided experimental and theoretical studies for high-strength connections between sinter Ag joining and Ni-finished substrate, which impacts the application of sinter Ag joining for die attach modules in high temperature. [ABSTRACT FROM AUTHOR]

Sintered silver (Ag) die-attach has attracted much attention in assembling power modules for high power density and high temperature operation. We have investigated a method to improve the thermal stability of a sintered Ag die-attach structure at high temperature storage (HTS). One of the issues of die-attach with sintered Ag is the growth of a microporous structure after HTS, since the microporous growth of the sintered Ag layer may have severe influence on bonding strength and reliability. It was verified that the addition of transition metal compound particles such as WO3, Nb2O5, MoO3, and RuO2 to sintered Ag paste successfully prevents microporous structure growth beyond 200 °C. From the results of cross-sectional scanning electron microscope observation, the inhibiting effect of microporous growth has a correlation with the difference between the total-circumference-length measured before and after HTS, among which WO3 and Nb2O5 additives were more effective. We propose this amazing die-attach material to power modules for high temperature operation providing excellent thermal stability. [ABSTRACT FROM AUTHOR]

Abstract Sinter joining with Ag particle paste has emerged as a promising lead-free die-attachment solution for power devices owing to its low-temperature and pressureless processing characteristics. Using this process, the achieved performance is superior to that of traditional solders. In this work, a robust, direct Cu bonding was realized using a low-cost, hybrid Ag paste. The bonding strength of the bare Cu joint structure achieves approximately 30 MPa when bonded at 250 °C without assisted pressure in air. To understand this excellent result, the bonding microstructure was investigated via SEM, EDS, XPS and TEM. This research revealed that the direct Cu bonding is closely related with the Cu 2 O nanoparticles formed on the bare Cu surface under the effect of solvent and self-generate Ag nanoparticles during the sintering. These two nanoparticles can form an intact bond, which allows a preferable bonding strength at pressureless, low-temperature and atmospheric sintering condition. Also, a possible mechanism has been proposed to interpret the formation of this successful direct Cu bonding. Highlights • A hybrid Ag paste was introduced for die-attachment on bare Cu substrates. • The sintering process can be finished under a mild condition. • The direct Cu bonding pattern was specifically studied via SEM and TEM. • A possible interpretation was introduced to explain the direct Cu bonding. [ABSTRACT FROM AUTHOR]

This study reports a novel high-temperature and high-current wire interconnection by printing Ag sinter paste for WBG semiconductor power devices. The Ag sinter paste wire interconnection (APW) was successfully fabricated connecting two Cu electrodes coated by Ag. Both the high-temperature reliability and the high-current reliability of the APW were evaluated by aging test at 250 °C for 1000 h and by electro-migration (EM) test at 2.2 × 104 A/cm2 for 2000 h. In the aging, a resistivity of about 4.1 × 10−6 Ω cm was achieved, which further decreased to 3.4 × 10−6 Ω cm after aging for 1000 h. This reduction can be attributed to further sintering of the sintered Ag network. The initial shear strength was 17 MPa, which, while high enough, was maintained even after an aging test of 1000 h. In the EM test, the resistance of APW was not changed significantly up to 1000 h and then only slightly increased at 2000 h. This only slight change in resistance demonstrates greater stability than that of conventional wire interconnection material such as Al and Cu under high temperature and high current density. The influence of EM on the interface between APW and a Cu substrate was observed by scanning electron microscopy with EDX. The change of resistance of the APW was induced by the formation of Cu oxide at a cathode side, which leads to a crack extension along the interface between APW and a Cu substrate. Thus, the results obtained in the present work show the APW as a promising new alternative to wire bonding technology for WBG power electronic devices. [ABSTRACT FROM AUTHOR]

Low-stress design of bonding technology with a sandwich structure of sintered Ag and tungsten (W) thin film was developed for SiC power die-attached modules. The die-attached bonding layer was designed as sintered Ag/W/sintered Ag structure. Experiment results show that the initial bonding strength was larger than 65 MPa for this die-attached structure and larger than 35 MPa with a thermal shock test from − 50 to 250 °C for 1000 cycles. These results are largely better than that almost all sintering Ag technology reported in previous studies. Furthermore, the sandwich structure also compared with the sintered Ag structure which just using sintered Ag paste as bonding layer. The thickness of Ag paste is set as 100, 200 and 500 µm in the sintered Ag structure. The results show that the initial bonding strength of sintered Ag structure was about 60-70% of the value of W sandwich structure and about one-third of that after 1000 cycles. X-ray and SEM observation revealed that sandwich structure significantly decreased the size of crack extension in the sintered Ag layer during the thermal shock test. Finite element analysis reveal that the shear stress at the pore location of sandwich structure decreased to almost half values of the sintered Ag structure with the thickness of sintered Ag of 500 µm, and decreased almost 20% compared with the thickness of sintered Ag of 100 µm. The bonding technology with the W sandwich structure should be an attractive for low stress design in SiC power die-attached modules, which significantly increased its function for long-term high temperature applications. [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]

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 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]

In this work, we applied a Ag flake formed paste for sinter-joining on bare Si, SiC, and GaN surfaces. The sintered joints possess a high shear strength of over 40 MPa under a pressureless sintering condition at 250°C. The high bonding strength of the joints is achieved by a tight adhesion between Ag and bare surfaces, which attributed to an excellent sinter-joining ability of the Ag flake. The flakes, acquired by mechanical milling, can be rapidly sintered into a homogenous porous structure at a pleasureless and low-temperature sintering condition due to its dislocation-rich nano grain structure. During sintering, a drastic morphology reconstruction of Ag flake, from a flattened flake to a drop-like particle, can introduce robust interfacial connection structures, which is important for the robust bonding. This Ag flake paste can be regarded as a promising sinter-joining material for the connection of bare surfaces in high-temperature applications. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper proposes a method to prepare aluminum nitride (AlN) ceramic–metal joints by nano-grained Ag flake particle sintering under low temperate pressure-free conditions. A robust shear strength larger than 25 MPa was achieved at 280 °C sintering for 3 mm × 3 mm joints. Moreover, large chip (5 mm × 5 mm, 10 mm × 10 mm) bonding confirmed that the technology was not influenced by the size and can be applied to large-scale ceramic–metal joining. Further investigation confirmed that nano-grained Ag micro flake particles were easily necked together, and the AlN/Ag interface could be tightly bound at a low temperature of 180°C. This study provides a new method for low-temperature pressure-less conditions for bonding ceramic-metal structure materials on a large-scale. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In this study, the thermal shock reliability of sintered Cu joints on SiC power device application was investigated. Firstly, sintered Cu joints were used to bond chips and substrate consisting of various materials to evaluate their bondability. Secondly, SiC dummy chips were bonded to DBC substrate and thermal shock test from −40 °C to 250 °C were performed both in the ambient atmosphere and in vacuum. Finally, the SiC MOSFETs bonded by sintered Cu joints were evaluated by power cycle test from 25 °C to 200 °C and the thermal conductivity was evaluated by T3ster equipment. The results showed the sintered Cu exhibited extremely high reliability during the thermal shock aging test in ambient atmosphere although inferior reliability was observed in vacuum. This phenomenon was investigated and explained by field-emission scanning microscope, energy-dispersive X-ray spectroscopy, and X-ray diffractometer. • The thermal shock reliability of sintered Cu joints on SiC power device application was investigated. • Sintered Cu joints exhibited excellent reliability during thermal shock test. However, oxidation occurs. • The power cycle test on actual SiC-MOSFET/DBC indicates the promising application of sintered Cu joints. [ABSTRACT FROM AUTHOR]