Search

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 long-term 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.

The application of Co-W-P plating technology in high-temperature package structure is advantageous from a point of structural reliability because Co-W-P metallization is known to deliver strong bonding to both high-temperature-compatible Ag-sintered joining and high-temperature-compatible encapsulation resins. However, Co-W-P, unlike a noble metal, has a potential risk of surface oxidation in the module fabrication process. This surface oxidation can result in a decrease in resin adhesion. In this paper, the effects of W content (7 wt%, 11 wt%, 21 wt%) in Co-W-P metallization on both the oxidation resistance and the resin adhesion were studied. The resin adhesion on the annealed Co-W-P metallization with a high W content (21 wt%) was found to be sufficiently strong even after 250 °C anneal for 1 h. This resin adhesion strength was not present in other Co-W-P metallization tests. SEM–EDS analysis revealed that the oxidization of the Co-W-P-metallized surface during the anneal process proceeded more slowly in the case of the Co-W-P metallization with a doping 21 wt% W. XPS analysis revealed that Co(OH)2, necessary for a chemical reaction with the resin, exists mainly on the Co-W-P-metallized surface in the case of doping 21 wt% W, even after 250 °C anneal. XRD analysis revealed its structure to be a characteristic Co-W solid solution, unlike the structures found in other Co-W-P metallization. The findings in this study are significant for the promotion of Co-W-P metallization in the module fabrication process, as well as to the fundamental understanding of oxidation resistance and adhesion behavior on Co-W-P metallization.

Ag sinter joint is gaining greater attention for use with wide-bandgap die attach modules in high-temperature applications. In this study, we have investigated both the necking growth of Ag grain and the mechanical properties of sintered Ag with different Ag particle shapes in a low-temperature, pressure-less sintering process in air and N2. The necking growth ratio of Ag grains plays an important role in the mechanical properties of sintered Ag. Large necking growth corresponds to stronger tensile fracture strength of sintered Ag paste and stronger die shear strength. Self-generated Ag nanoparticles (AgNPs) with of a size less than 10 nm were observed under an air atmosphere, an atmosphere that produces larger necking growth than does that of N2. In addition, self-generated AgNPs were observed in sintered Ag flake particles specimens. Ag flake particles also lead to larger necking growth than that of sintered Ag spherical particle specimens. Self-generated AgNPs accumulated together due to their high surface energy, thus improving the necking growth of Ag grain and leading to a stronger fracture strength. In addition, it was revealed that both the oxygen element and the grain residual strain of initial Ag particles play important roles in the AgNPs self-generation during the sintering process. This study enhances our understanding of the sintering mechanism of Ag particles and the relationship between grain necking growth and its mechanical properties.

In typical CuNWs-based conductors, CuNWs are deposited on or semi-embedded in the surface of substrates, which cannot avoid the oxidation. Here, we report a strategy to fabricate highly conductive and stretchable conductor with enhanced reliability and robustness through fully embedding CuNWs into the surface layer of poly(dimethylsiloxane) (PDMS) and followed with a high-intensity pulsed light technique. The light energy absorbed by the film not only removes the oxides on the surface of nanowires but also enhances the inter-nanowire connection to achieve high conductivity. The fully embedded CuNWs/PDMS structure showed excellent oxidation resistance in high-humidity and high-temperature environments (85 °C, 85% RH), and maintained a low sheet resistance when subjected to 30% of strain for 1000 cycles. Stretchable dipole antenna was fabricated using the fully embedded CuNWs/PDMS conductors, which retained its sensitivity to specific radio frequencies even after 500 cycles of the stretching/releasing process. Furthermore, a stretchable conductive heater was demonstrated, verifying the applicability of our fully embedded conductor for wearable electronics. We believe this work might open up new opportunities for the wide range of practical applications of CuNW-based conductors.

Solder joint reliability in power modules is one of the most important issues for hybrid, electric, and fuel cell vehicles; these modules must have highly reliable solder joints, i.e., they must be highly thermostable at temperatures over 175 °C in the future. The soldering surfaces in power modules are often finished with electroless Ni–P plating. Thus, for Cu/Ni–P plating/Sn–0.7Cu joints, it is necessary to suppress Ni diffusion into the solder. Ni diffusion can be suppressed in the presence of a continuous Cu6Sn5 intermetallic compound (IMC) layer at a Ni–P plating/solder interface. To form this IMC, we investigated the composite Sn–0.7Cu solder added with Cu balls. It was confirmed that the addition of 2.5 wt% Cu balls formed a continuous (Cu, Ni)6Sn5 IMC layer between the solder and the Ni–P plating. It is concluded that the IMC layer works well as a Ni diffusion barrier in multiple reflow tests, of which the peak temperature was 330 °C, and in a high-temperature storage test at 200 °C for 1000 h.

A new solvent composed of 4-(tert-butyl) cyclohexyl acetate as a dilute agent and HPMDA as a thickener was developed to make micron-sized Ag paste and to bond power semiconductors. The new Ag paste achieved sentinel printing to form sharp Ag patterns with uniform thickness and defined boundaries, which is a key to printing fine pitches in the electronics industry. The electrical resistivity of Ag patterns of 3 μΩ cm was obtained at 280 °C for 30 min. Two Ag-plating copper substrates were successfully bonded with the new Ag paste and the shear strength gradually increased with the bonding temperature. A high strength of 80 MPa was achieved at 280 °C under a small sintering pressure of 0.4 MPa. This value is far higher than the value obtained by using an expensive Ag nanoparticle paste. The detailed mechanism is discussed in this work by comparison with common ethylene glycol solvent.

Triangular and hexagonal-shaped single-crystal Au nanosheets were prepared by the reduction of HAuCl4 with the polyol method, after addition of a polyvinylpyrolidone (PVP) capping agent and salts. Both capping agent and salts played a crucial role during nanosheet growth. The high PVP concentration was found to result in a mixed Au morphology of sheets and spherical particles, which could be tailored with the addition of salts to provide solely nanosheets. The main product is always nanosheets after the salts have been added. The selective adsorption of PVP and salt ions onto certain crystallographic facets is thought to be important in the proposed growth mechanism. This synthetic route is simple and fast, and can be extended to other morphology metal particles with careful salt and capping agent selection.

Boron nitride (BN) nanotubes, nanohorns and nanocoils were synthesized by annealing Fe4N/B, FeB and Fe/B powders at 1000 °C for 1–24 h in nitrogen gas atmosphere, and large amounts of BN nanotubes were obtained by annealing Fe4N/B. The growth mechanism and atomic structures were investigated on cup-stacked BN nanotubes synthesized from Fe4N/B by X-ray diffraction, high-resolution electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. As-produced BN soot was purified by removing non-BN nanomaterials such as metal catalyst particles and unreacted boron, and high purity BN nanotubes were obtained.

The transformation kinetics of the β-phase from an as-solidified structure composed of α and e in the Fe–Si system was investigated by using rapidly, unidirectionally or conventionally solidified FeSi2 alloys containing a small amount of Cu (0.1–1 at%). The addition of Cu decreased the size of primary e and slightly changed the solidified eutectic morphology. The solubility of Cu in the α-Fe2Si5 phase was estimated to be less than 0.2 at%. A needle-like Cu enriched phase was newly formed in the conventionally solidified alloys containing more than 0.2 at % Cu. Microdifferential thermal analysis (DTA) clearly showed that the addition of Cu drastically accelerated β-phase formation. X-ray diffraction analysis and microstructural observation of the isothermally heat-treated specimens showed that Cu addition was effective in increasing the rate of eutectoid decomposition (α → β + Si) and the initial stage of the peritectoid reaction (α + e → β). For complete β formation, heat treatment for a long time was still required because it took a long time for the coarse e-phase in the slowly solidified alloy to be eliminated by peritectoid reaction. The effect of Cu depended on the annealing temperature. The decomposition rate of α in the Cu-added cast specimen was about 15 times higher at 1073 K than that of the binary cast specimen and exceeded more than 30 times at 873 K.

MnO containing soda-lime silicate glasses were vitrified and nitrogen was introduced into them. The prepared glasses were subject to density measurement and alkaline durability tests. MnO addition caused poorer alkaline durability in 2N NaOH solution at 82°C. Needle-like precipitations were observed on the MnO containing glass surface after 96 h durability tests. Glass components were found to dissolve selectively in the alkaline solution in the order SiO2 > CaO > MnO. The density of the obtained glasses increased linearly with increasing amount of nitrogen content in the glasses. A considerable improvement in alkaline durability of the glasses by nitrogen addition was observed, and its effect was larger for the MnO-free glasses.

The interface structure between silicon and silver after the eutectic reaction at 950°C has been examined primarily by using high-resolution transmission electron microscopy. Silver almost became a single crystal and a unique interface structure was found, where a special orientation relationship across the interface was recognized. The dominant relationship was obtained as {111}Ag ∥{111Si and [110]Ag ∥ [110]Si. The lattice arrangement model of the interface showed that the interface expressed by this relation has a good lattice match across the interface.

Mechanical properties and microstructures of machinable silicon carbide, fabricated by pressureless sintering of silicon carbide fine powder with the aid of polysilastyrene, have been examined. Drastic changes in microstrucyure and in mechanical properties between specimens sintered at below 1773 K and at above 1873 K were observed. By sintering at above 1883 K the macinable silicon carbide had a good strength of more than 200 Mpa with high reliability, which was maintained beyond 1773 K. Polysilastyrene was converted in β-phase silicon carbide and ribbon carbon in the pores. The (001) plane of carbon is parallel to the (111) planes of β-phase silicon carbide.

A study of wetting of silicon single crystals and electrode metals was undertaken to investigate the wetting mechanisms and to clarify the interfaces between the silicon wafer and silver and tin. The experiments were performed using a high-temperature surface tension measuring equipment in vacuum and 5% H2/Ar atmospheres (105 Pa). The contact angles were measured by taking photographs through a telescope during the wetting experiment. Silicon was wetted by silver with a contact angle of 42°, whereas it was not wetted by tin in vacuum and in the 5% H2/Ar atmosphere. However, heat treatment of silicon in vacuum or 5% H2/Ar atmosphere prior to the wetting experiment reduced the contact angles compared to the cases with no heat treatment. The bonding interfaces between silicon and silver are also discussed.

Joined interfaces of HIPed additive-free silicon nitride ceramics/aluminium braze bonded at a low temperature of 1073 K for 18 ks or at a high temperature of 1473 K for 1.8 ks in vacuum of 1.3 mPa and of β silicon nitride powders/aluminium powders bonded at the low temperature for 1.8 ks or 18 ks in the same vacuum are identified by analytical transmission electron microscopy and X-ray diffraction method. Mullite, some small crystals and β′-sialon are detected at the interface of the ceramics/aluminium braze bonded at the low temperature and 15R AIN-polytype sialon, β′-sialon, aluminium nitride, mullite and silica-alumina noncrystalline are detected at that bonded at the high temperature. At the interface of the two kinds of powders, aluminium nitride and silicon are also detected besides β′-sialon and silica-alumina noncrystalline even though the bonding was conducted at the low temperature. The interfacial reactions of the joints are influenced not only by bonding temperature but also by the oxide formed at the interface before bonded.

A new joining process for ceramics to ceramics and ceramics to metals, “SQ brazing”, has been developed. This process utilizes squeeze casting; a brazing material is squeezed into the interface channel to be brazed and is solidified under a high pressure. This new process has several advantages, low cost, mass producibility, high interface strength and high reliability, no severe reaction, etc. Alumina to alumina and silicon nitride to silicon nitride brazing with pure aluminium are shown as examples. Alumina containing silica as a sintering additive brazed by a conventional method severely reacted with aluminium braze so that the joint strength was low. After SQ brazing, reaction was moderate and the strength almost reached that of the parent alumina. Silicon nitride could be brazed by SQ brazing. Although the simple SQ brazing could not make a strong interface, pre-oxidization treatment of silicon nitride increased the joint strength beyond 400 M Pa.

From the results of the bending strength and Weibull modulus of the joints of silicon nitride ceramics brazed using aluminium-silicon and aluminium-magnesium alloy filler metals at a temperature of 1073 K for 0.9 ksec in a vacuum of 1.3 × 10−3 Pa, silicon, especially, present in a small amount in the filler metals, was found to be effective in improving the bond strength, while magnesium in the filler metals was harmful to the joining. This can result in the formation of a thick stable alumina layer on the surfaces of the filler metals containing magnesium during brazing which prevented contact of the filler metals with the silicon nitride ceramics.