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Due to the mismatch between the thermal expansion coefficients of semiconductor laser chip and heat sink, giant thermal stresses will be generated after eutectic bonding. Immoderate thermal stress ...

Thispaper introduces a novel, inherently simple, and all-silicon wafer-level fabrication and hermetic packaging method developed for MEMS devices. The proposed method uses two separate SOI wafers to form highly-doped through-silicon vias (TSVs) and suspended MEMS structures, respectively. These SOI wafers are then bonded by Au-Si eutectic bonding at 400 °C, achieving hermetic sealing and signal transfer without requiring any complex via or trench refill process steps. The package vacuum is measured using encapsulated MEMS resonators to be as low as 15 mTorr with the help of successfully activated thin-film getters. The combined fabrication and packaging yield is around %89 and chips still maintain a package pressure below 100 mTorr after more than 6 years. The packages show an extremely high strong bonding strength (>40 MPa) and are proved to remain hermetic after temperature cycling (25 °C-85 °C) and harsh temperature shock (5 min@300 °C) tests. The all-silicon MEMS resonators fabricated and packaged using the proposed method project up to a $2.3\times $ enhancement in the bias instability and $\sim 4\times $ in the temperature sensitivity of frequency output compared to an identical MEMS resonator fabricated using the silicon-on-glass (SOG) technology.

Cu/Sn wafer-level bonding is an interesting solution for wafer-to-wafer stacking technologies, due to its compatibility with 3D interconnections as well as vacuum sealing applications. The Cu/Sn eutectic bonding process is favored by industry and researchers alike for its excellent process compatibility and low cost, and has been proven for high density interconnect packaging of MEMS devices. Because of the presence of heterogeneous materials (TSV -Cu, Si substrates, Cu bumps, and Sn bumps) with large differences in coefficients of thermal expansion, the Cu/Sn interface exists varying degrees of stress and strain during the bonding process which can have an impact on the reliability of the bonding results. By optimizing the layout of the Cu/Sn bumps and the parameters of the bonding process, the stress and strain distribution during the bonding process can be reduced and the reliability of the Cu/Sn bonding interface can be improved. Based on ANSYS finite element steady-state thermodynamic analysis the stress and strain distribution at the bonding interface can be effectively obtained for different Cu/Sn bump layout, reducing the strain on the bump surface during the bonding process and improving the process reliability by optimizing the design rules.

Vacuum packaging of Micro-electro-mechanical system (MEMS) devices is a hot topic for its advantages of improving performance and reducing power consumption. In this paper, the physics package of a chip scale atomic clock (CSAC), as a typical kind of MEMS device, is performed by vacuum packaging based on a systematic method proposed by us. The whole process, including low outgassing and thermal stable materials selection, prebaking for desorption, getter firing for absorption and solder reflow for vacuum sealing is introduced thoroughly. The thermogravimetric analysis or thermal gravimetric analysis (TGA) is used to analyze the thermal stability and desorption of materials. The leak rate of physics packages is measured to be less than 4 × 10−10 Pa·m3/s by helium leak detection. The residual gas pressure and composition in physics packages are analyzed after vacuum packaging. The results show a high vacuum ~0.1 Pa in the physics package. The frequency stability is improved from 4.68 × 10−11 to 1.07 × 10−11 @40,000 s. The presented method for high vacuum packaging is also applicable to other MEMS devices.

This paper presents the development of an 8-in six-degree-of-freedom (DOF) inertial sensor based on an advanced capacitive inertial sensor fabrication platform. The platform integrates three-axis gyroscopes and three-axis accelerometers on the same chip. The fabricated sensors are vacuum packaged at the wafer level with a polysilicon-based through-silicon interposer (TSI) using the aluminum–germanium eutectic bonding approach. Wafer-level measurement results indicate that a fabrication yield of greater than 92% and a vacuum level of $\sim$ 100 mtorr have been achieved. The fabricated inertial sensors and the customized application-specific integrated circuits are encapsulated in a 5 mm $\times$ 5 mm $\times$ 1.3 mm quad-flat no-leads package using the plastic molding technology. The system-level characterization of the developed six-DOF inertial sensors have been implemented. Several reliability tests conducted according to the relevant JEDEC standards prove that the packaged sensors are highly reliable and robust for a wide range of operating environments.

Advancements in miniaturisation and new capabilities of implantable devices impose a need for the development of compact, hermetic, and CMOS-compatible micro packaging methods. Gold-tin-based eutectic bonding presents the potential for achieving low-footprint seals with low permeability to moisture at process temperatures below 350\degree C. In this paper, a method for the deposition of gold-tin eutectic alloy frames via sequential electroplating from commercially available solutions, with no special fabrication process, is described in detail. Bond quality was characterised through shear force measurements, scanning electron microscopy, visual inspection, and immersion tests. Characterisation of seals geometry, solder thickness, and bonding process parameters were evaluated, along with toxicity assessment of bonding layers to the human fibroblast cells. With a successful bond yield of over 70\% and no cytotoxic effect, AuSn eutectic bonding appears as a suitable method for the protection of integrated circuitry in implantable applications., 16 pages, 12 figures, paper submitted to Journal of Micromechanics and Microengineering

Thermal management of microprocessors and power electronics face major challenges including but not limited to a need for reliable and mechanically robust micro-cooler and micro-heat spreader devices. While the thermofluidic performances of the embedded microchannel with 3D manifold cooling and heat spreaders have received much attention (and carefully studied) for the past two decades, the challenges associated with the mechanical design and reliability have not been the focus of the research in thermal management community. Gold-tin eutectic bonding is proposed to attach silicon-based evaporator and condenser substrates in vapor chambers, as well as silicon microchannels substrate to silicon 3D manifold in high performance micro-coolers. The bonding interface in a silicon embedded microchannel to a silicon 3D manifold requires careful design to withstand pressures ~ 500 KPa, for target heat flux of ~ 1 kW/cm2 using single-phase water as working Evaporator and condenser bonding interface in a silicon-based vapor chamber must withstand water burst pressure of ~ 0.8 MPa when it is subjected to elevated temperature of 175 °C. In this study, a set of silicon microchannels in two substrates are fabricated using conventional microfabrication process, which then bonded using gold and tin multilayer composite structure (150 nm Au/75 nm Sn/150 nm Au) at 350 °C. Subsequently, the tensile stress is "directly" measured using carefully designed "test" samples, by means of applying direct tensile force to the sample. We measured tensile stress for 11 samples with contact areas ranging from 50 µm x 100 µm to 150 µm x 150 µm, the resulting stresses ranges from 2.0 to 5.5 MPa, with an average value of 3.2 MPa. Findings of this study will be largely beneficial in design and fabrication of silicon-based vapor chambers and high performance microchannel with 3D manifold coolers.

The high thermal conductivity, lightweight, and low cost of aluminum (Al) make it a promising substrate material for high power electronic and photonic packages and housings. It is particularly attractive for aerospace and space applications due to its lightweight. A main challenge for these applications is poor bondability. The native aluminum oxide (Al2O3) prevents aluminum from bonding by using popular die-attach materials such as solders. Zincating process is often needed to dissolve the Al2O3 layer and deposit a protection zinc layer which provides a basis for subsequent metallization or soldering process. However, the zincating and metallization processes could increase the processing cost and bring more reliability issues. In this research, a novel Ag foil bonding technique has been developed to bond Ag foils directly to Al substrates to produce Ag-cladded Al substrates. Two Ag-Al bonding processes are developed: solid-state and eutectic. Subsequently, Si chips are bonded to the Ag-cladded Al substrates using solid-state process at 300 °C without any additional die-attach material. For the Ag-Al bonding processes, no surface treatment is applied to Al substrates to remove the native Al2O3 layer. In the Ag-Al soli-state bonding process, Ag and Al atoms inter-diffused through the thin Al2O3 to react and form Ag2Al and Ag3Al compounds. In the Ag-Al eutectic bonding process, Ag2Al+(Al) eutectic structure forms at the Ag/Al interface without Ag3Al compound formation. The native Al2O3 layer, a potential fracture path, is broken into pieces during eutectic reaction and possibly dispersed into the eutectic structure. Shear test results of Si/Ag/Al joint samples far exceed the military criterion (MIL-STD-883H method 2019.8). The Si/Ag/Al structures break either along the Ag/Al interface or within the Si chip. With the advantages of high thermal conductivity, high reliability, lightweight, and process simplicity, the Ag-cladded Al structures should be highly valuable for applications in packages and housings where lightweight and high heat-conducting are necessary.

GaAs monolithic microwave integrated circuits (MMICs) with different back metallization systems (TiW/Au and Au/Ti/Au) exhibit different problems in the automatic Au-Sn eutectic bonding process, such as edge breakage or excessive voids. In this study, the formation mechanism of the edge breakage and excessive voids were investigated to prevent the damage of the MMICs in mass production scenarios. The microstructure and elemental distribution were studied using a scanning electron microscope and energy-dispersive spectroscopy. The void contents of the brazed region were measured with three-dimensional computed tomography. The top Au layer of the TiW/Au metallization partially dissolved in the melting An-Sn solder. Consequently, liquidus temperature of the solder increased, leading to isothermal solidification with the formation of &zeta, Au5Sn in the scrubbing process, which was the reason for the edge breakage. The terminal Au film of the Au/Ti/Au metallization completely dissolved in the melting An-Sn solder. The metallurgical combination was achieved by the formation of the TiAu4 intermetallic compound between the Au-Sn solder and the Ti layer. The wettability of Au-Sn solder on Ti layer should be improved to prevent the formation of the excessive voids.

Ultrathin buffer layers (UBLs) with varied thickness ranging from 10 to 100 nm and different materials were used in Cu/Sn eutectic bonding. A Cu/Sn film thinner than $2~\mu \text{m}$ could fully react and became stiff and rough Cu-Sn intermetallic compound layer, which leads to failure bonding. Four kinds of semiconductor compatible materials including Ti, Pd, Co, and Ni were inserted between Cu/Sn to delay interdiffusion prior to eutectic bonding. In addition to symmetric Cu/Sn bonding with UBL, asymmetric Cu/Sn-Cu bonding scheme with 50-nm Ni UBL was demonstrated. With good mechanical properties, bonding quality, and electrical characteristics, the application of submicrometer Cu/Sn wafer-level bonding by assistance of buffering layer gives a promising and flexible platform for future 3-D integration applications.

A high quality factor ( $Q$ ) and low-impedance aluminum nitride (AIN) on silicon piezoelectric resonator is reported in this paper. The piezoelectric resonator is fabricated based on the AIN-on-cavity-Silicon-on-Insulator (SOI) platform and vacuum encapsulated using the wafer-level chip-scale packaging by aluminum–germanium eutectic bonding approach. The fabricated resonator achieves a loaded $Q$ of 9520 and a motional impedance of 32.64 $\Omega$ at its series resonant frequency of 27.19 MHz. Thermal cycles (TC) and highly-accelerated stress test (HAST) are performed to evaluated the packaging reliability of the fabricated devices. No obvious impedance change caused by the TC and HAST is observed. The TC results indicate that the packaged devices have an equivalent life of more than 11.65 years based on the Coffin–Manson model. The piezoelectric resonator-based programmable oscillators demonstrate an overall frequency drift of $\pm$ 3 ppm over the temperature range of $-$ 20 $^{\circ }$ C to 70 $^{\circ }$ C thanks to the passive and active temperature compensations. With this stable frequency reference, the programmable oscillator produces a 75.01-MHz frequency output. The measured integrated phase jitter is 2.02 ps (rms) from 12 to 10 MHz.

Aluminum-silicon alloys contain coarse, acicular Si plates that are detrimental to material properties. However, trace additions of Sr have been found to modify the eutectic Si to have a fine, fibrous structure, which improves elongation, strength, and thermal conductivity. In this study, in-situ neutron diffraction was conducted during the solidification of unmodified and Sr-modified binary Al-6 wt.% Si to obtain a novel view of the modification mechanism. Neutron diffraction intensity data was collected at temperatures ranging from 660 °C to 200 °C and was integrated to create fraction solid curves for the individual Al and Si phases in each alloy. Scanning electron microscopy revealed that the Sr-modified alloy microstructure contained many micro-sized Al-Si-Sr intermetallics scattered about the eutectic regions at the Al-Si interfaces, and a few Si flakes close to some β-(Al,Si,Fe) intermetallics, suggesting important interactions between both the Fe-bearing phases and Sr in the solidification of eutectic Si. Comparing the isolated phase evolution temperatures for each alloy revealed in-situ for the first time that Sr suppresses the nucleation temperatures of both the eutectic Al and eutectic Si phases by several degrees. Accordingly, the eutectic reaction was observed to evolve at a higher primary Al solid fraction with Sr modification.

Microstructure transition and selection of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics with minimal eutectic spacing of 50 nm are studied by laser zone melting under rapid solidification rate. When the eutectic spacings are in micronscale, the ‘Chinese script’ structure is the main microstructure. As increasing the solidification rate (≥ 50 µm/s), the eutectic colony structure is obviously formed. The irregular-regular structural transition and coexistence are produced around eutectic spacing of 100–200 nm. The mechanism of irregular/regular eutectic structure transition and coexistence is revealed by considering the relative kinetic undercooling of each phase. In addition, the strong formation tendency of amorphous/nano-eutectic structure is observed at the surface of as-solidified ceramic drops by rapid liquid-metal quenching.

Eutectic high-entropy alloys are potential replacements of structural alloys, due to their microstructure and properties. In this work, CoCrFeNiTax (x = 0.1, 0.2, 0.3, 0.395, 0.4 and 0.5, x value in molar ratio) eutectic high-entropy alloys were produced by arc melting technique. Two phases, FCC solid solution and Laves phase, are identified in the alloys. The alloys are transformed from hypoeutectic to hypereutectic solidification by increasing Ta content. CoCrFeNiTa0.395 eutectic high-entropy alloy exhibits an ultrahigh yield strength of 1.4 GPa, while CoCrFeNiTa0.3 hypoeutectic high-entropy alloy shows a compressive strength of 2.5 GPa and a considerable fracture strain of 44%. Such remarkable strengthening effects are shown to result from the ideal combination among the second-phase, solid solution, and boundary strengthening.

Ni-Sn anomalous eutectic is obtained by single track laser cladding with the scanning velocity from 1 mm/s to 10 mm/s using the Ni-32.5 wt.%Sn eutectic powders. The microstructure of the cladding layer and the grain orientations of anomalous eutectic were investigated. It is found that the microstructure is transformed from primary α-Ni dendrites and the interdendritic (α-Ni + Ni3Sn) eutectic at the bottom of the cladding layer to α-Ni and β-Ni3Sn anomalous eutectic at the top of the cladding layer, whether for single layer or multilayer laser cladding. The EBSD maps and pole figures indicate that the spatially structure of α-Ni phase is discontinuous and the Ni3Sn phase is continuous in anomalous eutectic. The transformation from epitaxial growth columnar at bottom of cladding layer to free nucleation equiaxed at the top occurs, i.e., the columnar to equiaxed transition (CET) at the top of cladding layer during laser cladding processing leads to the generation of anomalous eutectic.

Dissimilar transient liquid phase joints were fabricated between Ni-based GTD-111 and Co-based FSX-414 superalloys using a 50 μm thick amorphous Ni Si B interlayer. The athermally solidified zone in the middle of joint contained the ternary eutectic of γ/Ni3B/Ni6Si2B and few amounts of Ni3Si. The amount of eutectic phase was decreased by increasing the bonding time. The complete isothermal solidification was obtained at the joining time of 100 min. The completion of isothermal solidification was controlled by several factors including the diffusion of Si and B from the bonding zone into the base metals, the diffusion of Ti, Al, Co and Cr from the base metals into the bonding zone and proportional re-distribution of them throughout the joint. The width of bonded zone (about 80 μm) was greater than the thickness of interlayer due to the coupled diffusion and dissolution phenomena. After the homogenization heat treatment, an increase of 34% in shear strength of the joint was observed due to the uniform distribution of strengthening precipitates across the bonding area.

A series of CoCrFeMnNiPdx (x = 0.2–2 at.%) eutectic high entropy alloys (EHEAs) were prepared and analyzed to show the alloying effect of Pd on the microstructure and phase constituents. It was found that the microstructure as the increase of x changes from the divorced eutectic to the coarse granular divorced eutectic around which are the fine lamellar dendrite eutectic and then to the seaweed dendrite eutectic. Hitherto, such seaweed eutectic dendrite was rarely reported in either high entropy alloys or conventional alloys. Specifically, a multimodal grain size distribution including several hundred microns for the eutectic-dendrite, several microns for the coarse granular eutectic, several hundred nanometers for the fine lamellar eutectics and the lath-shaped microstructures in the intermetallic phases, and even several nanometers for the nano-twins was observed. Since the CoCrFeNiMnPdx EHEAs obey all the design rules of HEAs with a single solid-solution phase but exhibits a two-phased microstructure, mere consideration of average physical properties of constituent elements was concluded to be not sufficient to the design of HEAs. The present work is not only enrichment of current EHEA systems but also of importance to the design of HEAs.

In this paper, we report that a selective laser melted (SLM) AlSi10Mg alloy has unique hierarchical microstructures. There are micro-sized Al grains and transgranular ultrafine cellular structures around primary Al, and the cell boundary is made up of alternate eutectic Si and eutectic Al phases. The SLM AlSi10Mg alloy has both high tensile strength and high strain-hardening capability, which are superior to Al-Si alloys fabricated by conventional powder metallurgy and cast methods. Post-tensile high-resolution microscopy analyses provide further evidence supporting Orowan looping as a strengthening mechanism of the SLM Al-Si alloy.

Glass-forming ability of Ge0.08Se0.92 (eutectic composition) and Ge0.20Se0.80 (Phillips-Thorpe threshold where the mean coordination number equals to 2.4) are compared employing very slow melts cooling. Using X-ray diffraction the presence of crystalline phase (Ge4Se9) was found in slowly cooled melt of Ge0.20Se0.80. It is suggested that in Se-GeSe2 system the maximum of glass-forming ability is shifted to the vicinity of the eutectic composition.

An Al2O3/Y3Al5O12 eutectic ceramic was prepared with a c-axis sapphire seed by an optical floating zone furnace. The crystallographic relationships in the initial and the steady growth sections of the as-grown eutectic ceramic were investigated by electron backscattering diffraction and transmission electron microscopy. The corresponding results were: {101}Y3Al5O12 || {112¯0}Al2O3 and {21¯1}Y3Al5O12 || {0001}Al2O3, respectively. The steady orientation of the latter one shows smaller planar disregistry. Interfacial strain played the decisive role in affecting the solidification behavior of the Al2O3/Y3Al5O12 eutectic ceramic. The stability of interfaces with minimum interfacial strain and better ionic charge balance in irregular microstructure prevail upon the constraint of the seed. These results might cast light for the interfacial design of the Al2O3/Y3Al5O12 binary eutectic ceramic. This article is protected by copyright. All rights reserved.

Peculiarities of the structure of a refractory eutectic alloy of the Nb – Si system, formed by the method of directed crystallization with liquid-metal coolant, have been studied. Characteristic zones of microstructure of the ingot obtained upon directed crystallization are considered, the alloy composition is analyzed, and volume fractions of phases in the Nb – Si composite are determined.

The emergence of eutectic high-entropy alloys containing Laves phase provides a new and exciting research direction towards developing advanced structural alloys, in light of the inherent advantages of Laves phase at elevated temperatures. In this work, CoCrFeNiZrx alloys were prepared with varying zirconium contents by vacuum arc-melting method. Typical eutectic microstructure was identified in the as-cast alloy having x = 0.5. The alloys consist of face-centered cubic solid solution and C15 Laves phase in the form of lamellae. Crystallographic orientation relationship between these two phases was determined. With the increase of the volume fraction of hard C15 Laves phase, the resulting alloys showed an increase in strength, but became more brittle at room temperature; the fracture process changed from ductile inter-lamellar fracture to brittle trans‑lamellar fracture. With the increase of test temperature, however, the fracture mode converted to be ductile fracture. As such, the eutectic microstructure can accommodate considerable plastic strain and has potential for engineering applications involving elevated temperatures. Keywords: High-entropy alloy, Laves phase, Eutectic microstructure, Mechanical properties, Elevated temperature

Effects of Re on the formation of surface eutectics have been investigated by using Ni-base single crystal superalloys with different Re additions. It was found that Re promotes the segregation of Al and Ta to the eutectic melt, leading to an increase of the surface and internal eutectics. In addition, the addition of Re also increased the freezing range, the local solidification time, and the permeability of the dendritic network within the mushy zone. These factors ultimately promoted the outflow of the interdendritic residual liquid with the action of solidification shrinkage, and led to the formation of more surface eutectics. In contrast, the addition of Re had no obvious influence on the surface eutectic microstructures.

The demand of small-feature-size, high-performance, and dense I/O density applications promotes the development of fine-pitch vertical interconnects for 3-D integration where microbumps are fabricated with Cu through-silicon via and under-bump metallization. Small dimension Cu/Sn bonding has to be developed to address the needs of increasing I/O density and shrinking pitch and size for future applications. For fine-pitch microbumps, it is important to select right UBM and solder materials to obtain lower UBM consumption, which means lower intermetallic compound (IMC) thickness. To find the best binary system material for fine-pitch microbumps with a different annealing temperature and time, we investigate the interfacial reaction and intermetallic compound morphologies of Co UBM with Sn, SnCu, and SAC solders. A thin, uniform, and single-phase IMC between solder and UBM facilitates finer pitch and more reliable microbumps development; the higher activation energies imply longer solder lifetime. Co, as an ultrathin buffer layer (UBL), is also used in Cu/Sn bonding. A comparison between Cu–Sn bonding with and without UBL is conducted. From this study, Co as UBL and UBM is explored and could be applied in semiconductor applications.

The fragility of Au80Sn20 eutectic solder is the major bottleneck of its application, which origins from the coarse primary ζ′-phase and the intrinsic brittleness of irregularly eutectic structure. To tackle this problem, various solidification processes have been adopted in this work, and the influence of those processes on phase composition and eutectic microstructure is studied. The results show that simply increasing the cooling rate does not prevent the formation of detrimental ζ′-Au5Sn phases, although the resulting eutectic morphology changes from dendritic to isometric. Applying magnetic field into rapid solidification can effectively remove the primary ζ′-Au5Sn phases, which is attributed to the fission of primary crystal nucleus during the fast eutectic nucleation course. The alloys through magnetic-field-induced rapid cooling exhibit a superior room-temperature compressive property, and the fracture stress and strain reach up to 1456.7 MPa and 64% respectively. Our finding on the effect of cooling rate and magnetic field on solidification not only sheds light on future research direction but also provides practical guidance to the engineers in solder industry.

A series of Al-40Zn-5Si alloys with different additions of Eu were prepared by permanent mold casting. The nucleation phenomenon incorporating the modification of silicon phase was investigated by using the synchrotron radiation imaging technology, scanning electron microscopy, electron probe microanalysis and high resolution high-angle annular dark-field scanning transmission electron microscopy. It was found that the primary phase was α-Al dendrites and the pre-eutectic Si particles were precipitated continuously at the front of the solid/liquid interface in Al-40Zn-5Si alloys. The nodular pre-eutectic Si particles were observed when the Eu addition was 0.3%. Meanwhile, a flake-to-fibrous transition of eutectic Si was also observed. The needle-like eutectic Si crystals were seen to shoot out from the vicinity of primary α-Al dendrites into melt with high velocities leading the irregular eutectic interface in the unmodified Al-40Zn-5Si alloy. However, the eutectic grains with smooth interfaces nucleated mainly near primary or secondary α-Al branches in 0.3% Eu-modified Al-40Zn-5Si alloy. The eutectic grains began to nucleate on the nodular pre-eutectic Si particles when the α-Al surface reached so close to them. The formation of nodular pre-eutectic Si particles was attributed to the adsorption of Eu atoms along the growth direction of Si and at the intersection of Si twins. This is fully consistent with the well-known poisoning of twin plane re-entrant edge (TPRE) and the impurity induced twinning (IIT) modification mechanisms proposed for interpreting the modification of eutectic Si.

In order to confirm the eutectic composition, eutectic temperature and phases of Al-rich Al-RE (La, Ce, Pr, Nd) binary systems, the eutectic Al-RE alloys with the controversial compositions were investigated. The scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), differential scanning calorimetric (DSC) and cooling curve tests analysis indicated that a similar eutectic composition (∼12 wt.% RE) on the Al-rich region was observed in the Al-La, Al-Ce, Al-Pr and Al-Nd alloys, and the eutectic structure of the alloys was consisted of α-Al and α-Al11RE3 phases by combining with X-ray diffraction (XRD) tests. In addition, the eutectic temperatures determined by DSC and cooling curve tests were very close and around 645 °C.

Two kinds of Sn-3.5Ag eutectic solder ribbons of 0.13 mm thickness were prepared by a casting–rolling process and a rapid solidification process. The microstructure, phase constitution, melting characteristics, wetting behavior and soldering strength were compared using optical microscopy, scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy, differential scanning calorimetry and a MTS ceramic testing system. The results show that the microstructure of rapidly solidified solder is finer and more uniform, and the eutectic structure has a higher solid solubility and more homogeneous distribution of Ag in a Sn matrix. The solidus and liquidus temperature decreased, resulting in a 3.3% reduction of pasty range. In addition, the wettability and shear strength of the solder joints increased by 13.2% and 7.9%, respectively.

Fine-grained aluminum alloys are very heat sensitive and overheating can reduce the strength of the material. Filler metals of Zn-5Al and Zn-5Al-3Cu alloys were used to join fine-grained 7034 aluminum alloy pieces by ultrasonic-assisted soldering at 420 °C. We observed excellent dissolution of the filler metal to the base metal. For the Zn-5Al solder, the Al content of the bond layer increased from 25 to 54 at.% when the ultrasound time increased from 5 to 60 s, resulting in a decrease of the volume fraction of the eutectic phase from 60% to 15%. The maximum tensile strength of the joints reached 208 ± 8 MPa and all fractures occurred in the brittle eutectic phase. For the Zn-5Al-3Cu solder, η-Zn and α-Al phases were identified in the bond layer. The maximum tensile strength of the joints was 249 ± 12 MPa, which was ~80% of the base metal after the same soldering thermal cycle. In this case, fracture occurred in the η-Zn phase. The higher strength is attributed to the absence of the eutectic phase. When the ultrasonic treatment time was increased to 60 s, the volume fraction of α-Al in the bond layer increased to 93%. The strength of the joints decreased as the brittle ZnAl eutectoid phase crystals appeared inside the α-Al phase in the bond layer. Keywords: Fine-grained Al alloy, Ultrasonic-assisted soldering, Zn-based solder, Eutectic phases, Mechanical properties

We report a novel microstructure evolution and corresponding mechanical behavior of bimodal eutectic (Ti 63.5 Fe 26.5 Co 10 ) 82 Nb 12.2 Al 5.8 alloys processed by semi-solid sintering (SSS) of the as-milled alloy powders with various glass contents resulted from various milling times. Results show that the as-milled alloy powders have more homogeneous element distribution and higher content of glassy phase with increased milling time. Correspondingly, although the SSSed bulk alloys possess the same constituted phases of the bcc β-Ti, bcc B2 Ti(Fe, Co) and fcc Ti 2 (Co, Fe), their eutectic structures containing bcc β-Ti and bcc B2 Ti(Fe, Co) evolve from irregular eutectic, to partial coarse eutectic, to fine cellular eutectic matrix, and finally to typical nano and ultrafine lamellar eutectic matrix. Interestingly, it is the first time to report that the lamellar eutectic matrix has an bimodal structure consisting of interleaving nano- and ultrafine-grained B2 Ti(Fe, Co) and bcc β-Ti lamellae. Corresponding to the evolution of eutectic structure, the SSSed bulk alloys exhibit a gradual increase in yield strength and plastic strain. Especially, the SSSed bimodal eutectic alloy has ultra-high yield strength of 2050 MPa and large plasticity of 19.7%, superior to those of equivalent counterparts. Theoretically, strengthening mechanism of the SSSed bimodal eutectic alloy can be mainly rationalized as ordering strengthening of B2 superstructured Ti(Fe, Co) and coherency strengthening between bcc β-Ti and bcc B2 Ti(Fe, Co) lamellae inside the lamellar eutectic matrix.

The results of investigation of the structure of a heat-resistant eutectic niobium–silicon alloy, which was prepared by directional solidification using a liquid-metal coolant, are reported. The typical areas of the macrostructure of an ingot undergone directional solidification are considered, and the composition and the volume fractions of the Nb–Si composite have been determined.

This paper presents micro-Pirani vacuum gauges using low-resistivity monocrystal silicon for the heaters and the heat sinks. We designed a new Pirani gauge with an embedded Wheatstone bridge made by thick-silicon resistors that allows an improvement in resolution and sensitivity. With the optimized Wheatstone bridge structure, the temperature coefficient of the gauges pressure measure decreases from 2.5%/°C to 0.74%/°C, and the resolution was optimized from 0.071 to 0.008 Torr at 1 Torr. In addition, the gauges were packaged into non-hermetic cavities by wafer-level AlGe eutectic bonding to undergo the wafer-dicing process, with sensitivity improved from 1.65 to 17.1 V/Torr at 1 Torr.

A CoCrFeNiMnPd eutectic high-entropy alloy was designed and prepared. A unique eutectic-dendritic solidification pattern was observed in the as-cast alloy. Heretofore, this kind of pattern was rarely reported no matter in binary or multi-component alloys. Through a series of experimental characterization and theoretical analysis, it could be concluded that the sluggish diffusion effect of high-entropy alloys makes the interface deviate considerably from the local non-equilibrium condition and the alternate arrangement of CoCrFeNiPd-rich FCC and Tetragonal Mn7Pd9 lamellae results in weak interface energy anisotropy, thus forming the seaweed eutectic-dendrite. Application of the designing rules of high-entropy alloys to the current work indicates that mere consideration of the weighted average physical properties of constituent elements is not sufficient to the design of high-entropy alloys with a single solid-solution phase.

We report on a novel approach to design typical ultrafine lamellar eutectic structure in bimodal alloys fabricated by semi-solid sintering (SSS) of a eutectic mixture. In our work, ultrafine lamellar eutectic structure was implemented by controlling the phase composition of eutectic reaction, and consequently by regulating the structure of eutectic reaction-induced liquid phase through varying component number. Microstructure analysis indicate that although all SSSed alloys have the same three phase constitutions of bcc β-Ti, bcc Ti(Fe, Co), and fcc Ti 2 (Co, Fe), the morphology and distribution of the eutectic structure transforms from limited length and minor quantity, to partial fine alternating bcc β-Ti and bcc Ti(Fe, Co) lamellae, and further to typical complete ultrafine alternating continuous lamellae in the SSSed ternary Ti-Fe-Co, quaternary Ti-Fe-Co-Nb, and quinary Ti-Fe-Co-Nb-Al alloys. Interestingly, the SSSed Ti-Fe-Co-Nb-Al alloy presents a novel bimodal microstructure of coarse fcc Ti 2 (Co, Fe) surrounded by an ultrafine lamellar eutectic matrix containing ultrafine bcc β-Ti and bcc Ti(Fe, Co) lamellae. This bimodal microstructure exhibits ultra-high yield strength of 2050 MPa with plasticity in compression of 19.7%, which exceed published values of equivalent materials. Our results provide a novel pathway for fabricating new-structure metallic alloys for high-performance structural applications.

The present investigation is based on the results of a directionally solidified (DS) Sn–9 wt%Zn–2 wt%Cu alloy, including primary/secondary/tertiary dendrite arm spacings of the Sn-rich matrix, the morphologies of the eutectic mixture and the corresponding interphase spacing, the nature and proportion of the Cu–Zn intermetallic compound (IMC). The main purpose is to establish interrelations of these microstructure features with experimental solidification thermal parameters, such as cooling rates and growth rates (v), macrosegregation and hardness. Such interrelations are interesting for both industry and academy since they represent a tool permitting the preprogramming of final properties based on the design of the microstructure. In the case of Sn–Zn–Cu alloys, hardly anything is known about the combined effects of the length scale of the microstructure and fraction and distribution of the primary IMC on hardness. The alloy microstructure is composed of a β-Sn dendritic region, surrounded by a eutectic mixture of α-Zn and β-Sn phases and the γ-Cu5Zn8 IMC. The eutectic interphase spacing varies in the range 1.2–3.6 μm, with the α-Zn phase having a globular morphology for v > 0.5 mm/s and a needle-like morphology for v

The mechanism of refinement of eutectic Si with Sb additions in an Al–10 wt.% Si alloy is studied. The increased holding time designates a transition of the eutectic nucleation temperature from a decreased value to an increased one, accompanied by a morphological transformation of eutectic Si from lamellar to a shorter, rod-like shape. By ab initio molecular dynamics simulation, the chemical short-range order of −0.002 demonstrates weak affinity in the Al–Sb pair, which does not favor the existence of AlSb clusters in the melts. Sb–Sb segregation and the Sb–Si repulsive force are revealed, which promote the formation of Al-enriched precursors and the accumulation of Si atoms adjacent to precursors, inducing precursor nucleation. Continuous attachment of precursors and individual Si atoms during Si growth results in the high number-density nano-particles and suppresses the formation of twin grains within eutectic Si. Keywords: Al–Si alloys, High number-density nano-particle, Precursor-assisted nucleation, Atomic structure, Eutectic Si refinement

Ni-30wt%Sn alloy powder beds were remelted using a laser beam to examine regular eutectic and anomalous eutectic growth behavior. The remelted microstructure mainly consisted of primary α-Ni dendrites and refined regular lamellar eutectic in the interdendrite space. At the top of the molten pool, the competition between primary α-Ni dendrites and the regular lamellar eutectic can be explained by the maximum interface temperature criterion. Anomalous eutectic was observed at the bottom of the molten pool. The remelting of primary α-Ni dendritic arms formed in the first laser remelting scan had an important effect on the formation of the anomalous eutectic in the second laser remelting scan. This effect led to the formation of globular α-Ni particles with similar Euler's angles in the anomalous eutectic.

Flip-chip ultraviolet light-emitting diode (FC UV-LED) fabricated by direct AuSn eutectic package is of high interest in Research and Development due to its excellent thermal performance and good reliability. However, the voids in eutectic bonding layer due to the lack of AuSn filled have a big influence on the thermal management and optical performance of FC UV-LEDs, and it is believed that the eutectic voids can affect the thermal-conduction resistance (the following unified called thermal resistance) and the junction temperature of FC UV-LEDs. In this paper, modeling and thermal simulation using finite element analysis is developed by considering the geometrical model of eutectic FC UV-LEDs with 3%, 10%, 20%, and 30% bonding voidage. Meanwhile, to validate the simulation, the thermal parameters of FC UV-LEDs are determined and measured using thermal transient tester, and it is found that UV-LED with 3% voidage shows lowest thermal resistance and junction temperature compared with the other samples in both simulation and experiment. Moreover, the optical performance of UV-LEDs is evaluated via the photoelectric analysis system, and the results confirm that the lowest thermal resistance leads to the lowest junction temperature but the highest light output power.

This paper reports a deep-UV LED (DUV-LED) package based on silicon MEMS process technology (Si-PKG). The package consists of a cavity formed by silicon crystalline anisotropic etching, through-silicon vias (TSVs) filled with electroplated Cu, bonding metals made of electroplated Ni/AuSn and a quartz lid for hermetic sealing. A DUV-LED die is directly mounted in the Si-PKG by AuSn eutectic bonding without a submount. It has merits in terms of size, heat dissipation, light utilization efficiency, productivity and cost over conventional AlN ceramic packages. Uniform light emission at an optical output of 30 mW and effective reflection on Si (111) slopes in the Si-PKG were observed.

In this paper, silicon substrate wafer level fabrication process of liquid cooling microchannel is introduced. Fig. 1 shows the microchannels fabricated on the silicon wafer surface. This microchannel component is to be integrated into a heat sink package served as an advance liquid cooling solution for high power server electronics cooling. Sambhaji [1] suggested liquid microchannel cooling method as more effective, reliable and lesser mechanical vibration to draw intense heat flux from high power electronic devices; compared with conventional air-flow convection cooling. Nicholas [2] illustrated micro structure jet impingement onto mirco-porous structure to eliminate flow instabilities, meeting high heat dissipation from concentrator photovaltics devices. Hence, novelty of multi-layers microchannel fabrication becomes the key to meet these requirements. This paper demonstrated using silicon wafer level process to build the main components of microchannels, micro-fins and jet impingement-nozzles. Fig. 2 shows these three main key components fabrication on silicon substrate. These structures were fabricated through silicon deep reactive ion etching process. Key process parameters were developed and optimized to meet structural dimensions control limits, uniformity and surface roughness conditions. Additional dielectric layers were added before the lithography patterning for silicon-etch to ensure smoother etched surface achieved after long process time of plasma etching process. Optimized proportion ratio of dielectric layers and photoresist layers thickness were evaluated to meet the plasma etching selectivity requirement between the silicon, dielectric and photoresist materials. The remainder oxide layer on the silicon surface is critical acting as an adhesive layer for subsequent metallization process. Liquid chemical etching process was used to remove the photoresist layer after silicon plasma etching. The key challenge within the fabrication process flow is to perform double sided silicon micro-structures fabrication. Silicon substrate as a carrier wafer to temporarily bonded on the microchannel surface in order to perform silicon etching process on the others side of wafer. Bonding materials and conditions were evaluated in order to ensure no delamination and breaking of silicon substrate during the silicon plasma etching on “structured” substrate. Optimized process flow was developed as well to ensure there is no penetration of chemicals and gas onto the bonding surface which could deteriorate the materials properties, consequently failed to debond or broken during the debonding process after the silicon plasma etching process completed. Metalization process was introduced to deposit eutectic bonding layers on the microchannel surfaces. Process flow sequences and conditions were established to ensure good bonding quality between the components layers without liquid leakage. Experimental flow test was carried out. This shows good pressure drop trend with increasing flow rate without leaking and blockage issue during the test. Experimental test show good repeatability results and illustrated good reliability results of the silicon microchannel samples.

In this study, a chip level Cu pillar to In/Sn pad thermocompression bonding (TCB) structure was investigated. After reaching the target bonding force and the melting point of the pad, the In/Sn pad was in its liquid phase for a short time and then quickly diffused into the Cu pillar, which demonstrates that Cu-solder bonding with low bonding temperature (150 °C) and short bonding time (1 min) can be realized in atmospheric environment. These bonding results with good electrical performance and bonding quality show the potential of this chip level stacking process.

Power LED is expected to be used in indoor and outdoor lighting, which will replace incandescent lamp, fluorescent lamp and high pressure sodium lamp. However, for high-power LED, the heat flux produced by high current injection is high. The heat generated by the LED chip cannot be dissipated outside from the chip in time, which leads to the increase of junction temperature and the decrease of reliability and lifetime of the power LED. With the increasing demand for high-power LED in general lighting applications, more chip bonding materials or processes are being developed to improve the heat dissipation capacity of LED. At present, Au-Sn eutectic alloy is widely used in packaging of high power LED devices because of its higher melting point, good wettability and high thermal conductivity. Eutectic bonding technology is expected to become the most promising high power LED die-bonding process. However, the bonding technology is mainly mastered by large companies, and the related research results are confidential. The die-bonding technology of high power LED packaging has become one of the technical bottlenecks hindering the development of China's LED industry. The interfacial microstructure between chip and heat sink was investigated by scanning electron microscopy (SEM). This study provides some basic data for the LED sealing industry, and also provides theoretical guidance for the development of key technologies in the LED industry.

Developments of advanced Vertical Interconnection Technologies have been indispensable parts of achieving MEMS three-dimensional (3D) integration. This paper presents a TSV-free Vertical Interconnection Technology using Au-Si eutectic bonding applied on MEMS wafer-level packaging (WLP), which simplifies processes and promotes all-Si fabrication abilities in MEMS 3D WLP. Specifically, by forming Au-Si ohmic contacts in Au-Si bonding, it is cost-efficacious to employ low-resistivity Si columns (rather than conventional metal TSV structures) in Cap wafers as vertically electrical pathways while accomplishing sealing functions. Based on testing results of signal interfacing and hermetic packaging for MEMS devices, the proposed technology possesses extensive application prospects.

Au-Ge-based solder alloys are promising alternatives to lead containing solders due to the fact that they offer a combination of interesting properties such as good thermal and electrical conductivity and high corrosion resistance in addition to a relatively low melting temperature (361 °C for eutectic Au-28Ge at.%). By adding a third element to the eutectic Au-28Ge alloy not only the Au content could be reduced but also the melting temperatures could be further decreased. In this study, in addition to the eutectic Au-28Ge (at.%) two ternary alloys were chosen from the Au-Ge-Sb and Au-Ge-Sn system, respectively. The soldering behavior of these alloys in combination with the frequently used metals Cu, Ni, and Ti was investigated. The interface reactions and microstructures of the joints were characterized in detail by SEM and EDX analysis. For the determination of the mechanical properties, shear tests were conducted. Mean shear strength values up to 104 MPa could be achieved.

Driven by high-density and high-reliability demands on three-dimensional (3D) interconnections, electrical and mechanical properties of interconnection structures have been paid more attention. Aiming at the novel enable-3D interconnections realized by Au-Si wafer bonding, this study demonstrates the design, fabrication and test results of testing structures for dimension-define parameters of Au-Si bonds. In order to measure closely-realistic ohmic contact resistances of Au-Si bonds after wafer bonding processes implemented under certain bonding pressures and temperatures, the convex Si structures formed by the local oxidation of silicon (LOCOS) process have been designed. Therefrom the specific contact resistance (SCR) values of Au-Si bonds have been measured as 3.9-8.1×10-10 Ωm2 for different contact radii (3-20 μm), which indicates the low-resistance potentiality for Au-Si bonds applied on 3D interconnections. On the other hand, two findings on bonding strengths are summarized from finite element simulations and tensile experiments: 1) Au-Si bonding strengths of circular bonding patterns (35-75 MPa) are superior to the ones of square bonding patterns (18-30 MPa) due to the stress concentration phenomenon on four corners of square bonding patterns; 2) There is an inverse relationship between bonding strengths and bonding areas owing to uneven stress distributions in bonding structures. Overall, this study provides a basic optimization design strategy at electrical and mechanical levels for 3D interconnections actualized by Au-Si bonding. Incidentally, these introduced analysis methods are also feasible for homologous Si-based eutectic bonding techniques to extend the forms of 3D interconnections.

(Co 76 Sn 24 ) 100- x Nb x ( x = 0, 0.5, 0.8, 1.0) eutectic alloy melts were solidified at small undercooling for investigating the effect of Nb addition on microstructural development. With increasing Nb content, the solidification interface transits from eutectic seaweed ( x = 0) to eutectic dendrite ( x = 0.5) as a response to the change in the anisotropy of interfacial energy. Coupled eutectic growth can no longer be maintained within the main stems at x = 0.8 because the enrichment of Nb in the liquid ahead of the interface causes a significant difference in growth velocity between the α-Co and β-Co 3 Sn 2 phases and, as such, Co 3 Sn 2 doublons form. For x = 1.0, the difference in growth kinetics between the two eutectic phases is so large that divorced eutectic growth takes place on a large scale. Letting the undercooling prior to solidification increase, the eutectic interface morphology of (Co 76 Sn 24 ) 99.5 Nb 0.5 eutectic alloys returns from dendritic pattern back to factual seaweed pattern at more than 38 K undercooling and then transits to compact seaweed pattern at more than 181 K undercooling. The eutectic growth velocity slightly increases at low and intermediate undercooling but obviously decreases at large undercooling due with Nb addition.

Low-melting-point alloys have an extensive applications in the fields of materials processing, phase change energy storage, electronic and electrical automatic control, continuous casting simulation, welding, etc. Specifically, the eutectic compositions make up a large number of low-melting-point alloys that are exploited because of their desirable features like single melting peaks, excellent operational reliability, and casting fluidity. However, the fundamental physicochemical properties from the current available literature on low-melting-point multi-component eutectic alloys (LMP-MCEAs) are rather rare and lowly accurate, including the exact melting temperatures and compositions, constituent phases, microstructures and morphologies, melting enthalpies, specific heats, densities, and so on. This lack of information seriously limits the development and application of low-melting-point multi-component eutectic alloys. In this paper, the low-melting-point multi-component eutectic alloys composed of Bi, Cd, Sn, Pb, and In elements synthesized by high vacuum induction melting and fundamental data were investigated by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and density analysis instrument. Most of the LMP-MCEAs with complex eutectic morphology structures and XRD diffraction patterns could be explained with the fact that they were three-phase eutectic alloys with mixed growth way. Generally, LMP-MCEAs present an extremely low melting point between 48.3 and 124 °C and high density between 8 and 10 g/cm3.

New compositions in the melt-grown eutectic ceramics field are investigated for thermomechanical applications. This paper is focused on the Al2O3–Sm2O3–(ZrO2) system. The studied compositions give rise to interconnected microstructures without anisotropy along the growth direction. At variance with the binary eutectic Al2O3–SmAlO3, the homogeneity of the microstructure of the Al2O3–SmAlO3–ZrO2 ternary eutectic is less sensitive to the growth rate. Interfaces between the alumina and perovskite phases are investigated by high-resolution transmission electron microscopy (TEM). They are semi-coherent. In stepped interfaces, the facets are parallel to dense planes of each phase. The steps have a dislocation character and may accommodate both misfits. The ternary eutectic displays a very good creep behaviour with strain rates very close to those obtained on other previously studied eutectics in the Al2O3–RE2O3(RE = Y, Gd, Er)–ZrO2 systems. The deformation micromechanisms are analysed by TEM in the three eutectic phases. After creep, dislocations are present in every phase. The activation of unusual slip systems (pyramidal slip in the alumina phase) shows that high local stresses can be reached. The presence of dislocation networks with low energy configurations is consistent with predominance of dislocation climb processes controlled by bulk diffusion.

The use of Ni-30Cr components in nuclear power reactors requires a compatible filler metal to ensure reliable welded joints associated with reactor vessels, steam generators, and pressurizers. The high chromium content provides resistance to stress corrosion cracking (SCC), and niobium (Nb) additions provide ductility dip cracking (DDC) resistance. Previous work showed that solidification cracking susceptibility increases with 2–4 wt% Nb and decreases with 6–8 wt% Nb. This decrease is due to a eutectic backfilling phenomenon which “fills” freshly formed cracks when sufficient low-melting eutectic liquid is present upon solidification. A reduction in cracking susceptibility was also seen when molybdenum (Mo) was added in conjunction with high Nb levels, but the mechanism behind this effect is unknown. The objective is to address the potential effect Mo additions have on eutectic grain boundary wetting to better understand how crack healing may change with composition variance. Since Mo additions have no apparent effect on the fraction eutectic upon solidification, it is postulated that Mo may increase the “wettability” of the eutectic liquid along grain boundaries, allowing crack backfilling to occur more efficiently. Potentially, Mo additions may support more complete crack healing, thereby a further cracking susceptibility reduction, when the fraction eutectic is sufficiently high. A preliminary, qualitative approach was taken to induce local remelting and wetting of eutectic in weld metal samples with and without Mo additions. The results suggest that Mo changes the eutectic remelting and/or wetting characteristics to some degree. Fundamentally understanding molybdenum’s role in eutectic grain boundary wetting could facilitate the development of filler metals that rely on eutectic healing via backfilling to provide solidification cracking resistance.