High-temperature reliability of low-temperature and pressureless micron Ag sintered joints for die attachment in high-power device (original) (raw)
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Thermal Cycling of Sintered Silver (Ag) Joint as Die-Attach Material
JOM, 2019
Sintered silver is a promising die-attach material capable of operating at temperatures of more than 200°C. However, while sintering reliably bonds Ag paste on Ag-plated substrate, reliability studies are needed to understand the behavior of this sintered bond on copper and direct bond copper (DBC) substrates. Here, we thermally cycled micron-Ag and nano-Ag joints created via sintering on Cu, DBC, and Ag-plated substrates between À 65°C and 150°C in order to understand their evolving microstructures and reliability. Short periods at high temperature did not oxidize the substrate, but the absence of copper oxides did not prevent adhesive failure of the nano-Ag joint at the Cu interface. We found that Ag filler size influences pore shapes, pore sizes, and shear strength; the micron-Ag joint produced a mixture of irregular and regular spherical pore shapes that reduced bond strength more than the predominantly spherical pores present in the nano-Ag joint. JOM
Thermal Ageing Studies of Sintered Micron-Silver (Ag) Joint as a Lead-Free Bonding Material
metals and materials international, 2020
The sintered silver (Ag) joint has proven to be a suitable die-attach material to be used under the operating conditions of wide bandgap semiconductors because of its high melting point and high thermal and electrical conductivities. However, to bond reliably, a sintered Ag joint needs a suitable metallized substrate (e.g. gold or silver) and the application of pressure during sintering. Hence, we investigated the evolving microstructure (i.e. the importance of pore shape factor) and shear strength of micron-Ag joints bonded without pressure on copper, Ag-plated substrate, and direct-bond copper (DBC) thermally aged at 300 °C for 1000 h. The DBC substrate maintained die-shear strength better because its coefficient of thermal expansion matched those of the sintered Ag and Si dies. Regardless of substrate, micron-Ag joints showed a decrease of large pores (> 0.16 µm 2) and an increase of spherical pore shapes during the aging period. These favourable changes maintained the mechanical integrity of the micron-Ag joints. This evolving microstructure of the sintered Ag joint provides guidelines for packaging engineers to consider as part of their selection of metallizations and substrates for power electronic packaging.
Journal of Materials Science, 2021
Ag sinter joining technology is emerging as a die attach material for next-generation power modules in high-temperature applications. Thermal shock test has revealed that the fracture characteristics and reliability of sintered Ag joint were influenced by thermo-mechanical stress. This was study conducted to understand the microstructure, vertical crack formation, and fracture behavior of sintered Ag joints which were designed with different metallization layers on a direct bonded aluminum (DBA) substrate, at different thermo-mechanical stresses during thermal shock tests. Two kinds of metallization layers were designed as Ti/Ag and Ni/Ti/ Ag layers. A finite element model (FEM) simulation confirmed that the Ni layer prohibited Al hillock-like deformation and generates different thermo-mechanical stresses during the thermal shock test from-50°C to 250°C. Depending on the degradation of the interfaces for both of the Ag-sintered joints, the sintered Ag grain necking thickness and microstructure characteristics including the Ag grain structures, which have a dominant influence on the bonding strength in terms of longterm reliability, are considerably different from the results by an electron back scatter diffraction (EBSD) analysis. This paper proposes a novel metallization technology that can induce joint fracture with complete recrystallization of sintered Ag joints by effectively suppressing interfacial degradation. The mechanism of this technology was systematically analyzed through experiments and FEM simulations.
Sintered Ag joint is a potential Pb-free die attach materials for power electronics because of its high operating temperature, high electrical and thermal conductivity as well as its thermo-mechanical reliability. While the long term reliability of the pressure assisted nano-Ag joint has been studied extensively , the performance of pressureless nano-Ag joint during long-term reliability remains a question. In this paper, we addressed the reliability gaps in this area by characterizing the evolution of porosity and microstructures of the pressureless sintered nano-Ag joints on Cu, direct bond copper (DBC) and Ag plated substrates during their high-temperature storage in air at 300 C. The die shear strength of the nano-Ag joint on DBC substrate maintained above 5 MPa for 1000 h of thermal aging while those formed on Cu substrate fell below 5 MPa after 50 h of thermal aging. The microstructure of the sintered nano-Ag joint showed Cu oxide accumulated at the sintered Ag and Cu interface, and upon reaching the critical thickness, they would weaken the sintered nano-Ag joints for the Cu substrate. This microstructure differed from those formed on the Ag plated substrate which showed void-less diffusion band to improve or maintain the die-shear strength. Nano-Ag joints on the DBC substrate also shared similar resilience because of their matching coefficient of thermal expansions with the Cu oxides. The initial increase of die shear strength could be attributed to the densification of the sintered silver joints in terms of the total volume of porosity, pore size and pore shape distribution. The relationship between the oxidation ki-netics of Cu, aging time and shear strength were also established to predict the reliability strength of sintered nano-Ag joints on DBC and Ag substrates. In the case of DBC, its rough surface provided additional anchoring to the sintered Ag joint, resulting in a higher die shear strength despite prolonged aging. These favourable results suggested that pressureless nano-Ag joint could perform as well as pressure assisted nano-Ag joint during the long-term reliability test.
Mechanical properties of nano-silver joints as die attach materials
This review traces the development of silver (Ag) as a die attach bonding material in the microelectronic packaging industry from its' early days as micron-scale silver flakes to the recent nanoscale Ag paste and other derivatives. Basic materials properties include the composition of Ag pastes, the methods of producing Ag nanoparticles, and product applications will be presented. Key processing conditions will be discussed to elucidate different factors which influence the mechanical properties of nano-Ag joints, principally the tensile and shear strength as well as thermal fatigue properties. Success in implementing nano-scale Ag pastes could only have been possible by deriving a fundamental understanding developed in the field of processing and using ceramic and metallic nano-powders.
Pressure-Less Silver Sintering Pastes for Low Porosity Joint and Large Area Die
2016
Under pressure-less sintering conditions silver sintering pastes for different sized die attachment to form robust silver joints have been developed. The sintering profiles of pastes developed in this work are compatible with conventional infrared reflow oven, resulting in a higher through-put as compared to that of box heating oven; the sintering work can be done under air and a highly reliable joint has been obtained as judged from the shear strength or thermal measurement results from aging tests. Ag joints generated on 3mm x 3mm Ag-die/Au-DBC combination displayed a “center-dense-edge-porous” structure. As the 250C aging continued, joints with ~10% total porosity have been observed; in the center area, porosity even reaching as low as 2 – 3%. The shear strength reached as high as ~70Mpa after aging, demonstrating a robust bonding. Two routes for Ag diffusion are revealed: (1) in the bulk phase Ag continuously densifies through grain boundary and lattice diffusion, resulting in ...
Ceramics International, 2018
Solid-state porous Ag was utilized to achieve interface bonding under low temperature and low-pressure for large area, high-reliability designs for high performance die-attach power modules in high temperature applications. In this process, solid-state porous Ag was first fabricated by sintering Ag particles onto both a Cu substrate and an Si die with different bonding areas. After polishing the surface of the solid-state porous Ag, the Ag-Ag interface was bonded together under low pressure (0.4 MPa) and low temperature (250°C and 300°C). The bonding strength was greater than 30 MPa for a 15 mm x 15 mm bonding area at a bonding temperature of 300°C. The electrical resistivity of the solid-state porous Ag was about 7 µΩ cm, or half that of Pb-free alloy solder materials. The bonding mechanism of the Ag-Ag interface was analyzed by a transmission electron microscope. It was observed that Ag hillocks and nanoparticles had been generated and bridged the Ag-Ag interface. This resulted in a large interface connection ratio and a high bonding strength. In addition, it was observed that the shear strength of the solid-state porous Ag with a thick bonding layer decreased thermomechanical stress in the die-attached power module and thus improved structural reliability. This bonding process featuring solid-state porous Ag presents an attractive technology for the fabrication of large area bonding and high-reliability die-attached module structures for high temperature applications.
Influence of Ag particle shape on mechanical and thermal properties of TIM joints
Microelectronics International
Purpose The purpose of this paper is to develop and test the thermal interface materials (TIM) for application in assembly of semiconductor chips to package. Good adhesion properties (>5 MPa shear strength) and low thermal interface resistance (better than for SAC solders) are the goal of this research. Design/methodology/approach Mechanical and thermal properties of TIM joints between gold plated contacts of chip and substrate were investigated. Sintering technique based on Ag pastes was applied for purpose of this study. Performance properties were assessed by shear force tests and thermal measurements. Scanning electron microscopy was used for microstructural observations of cross-section of formed joints. Findings It was concluded that the best properties are achieved for pastes containing spherical Ag particles of dozens of micrometer size with flake shaped Ag particles of few micrometers size. Sintering temperature at 230°C and application of 1 MPa force on the chip during ...
Journal of Alloys and Compounds, 2020
This study systematically investigates the effect of oxygen on the microstructural evolution as well as the macro/micromechanical characterization of Ag sinter paste during the aging process at 250 C for 1000 h. The sintered Ag joint is separated into two groups and, respectively, kept in air and vacuum (10 À4 Pa) atmospheres. The microstructure of sintered Ag paste become clearly coarsened after aging at 250 C in air for 100 h, while it remains almost the same as in the initial state in vacuum, even after aging for 1000 h. The different microstructural evolutions are investigated in detail by transmission electron microscopy, which reveals that coarsening are involved in the AgO reaction and form a large number of Ag nanoparticles around sintered Ag particles, as well as enhance coalescence. Furthermore, the macro/ micromechanical properties of sintered Ag paste, after aging in air and vacuum atmospheres, are evaluated by a die shear test and a nano-indenter system with a spherical diamond tip. Shear strength and fracture mode were systematically analyzed. The micromechanical properties of sintered Ag paste, including Young's modulus and hardness, depend on its microstructure and are largely influenced by its porosity, which decreases from initiation to 250 h and recovers after aging for 500 h in air. However, there is almost no change during the aging process in the vacuum atmosphere. The results of this study contribute to a better understanding of the microstructural evolution and the mechanical properties of sintered Ag joints, including the change in bonding quality and Young's modulus during aging for actual power modules, which are packaged by a resin.
Bonding technology based on solid porous Ag for large area chips
Scripta Materialia, 2018
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.