Active Metal Brazing Research Articles

Effect of Gold on the Microstructural Evolution and Integrity of a Sintered Silver Joint

There is a need for next-generation, high-performance power electronic packages and systems employing wide-bandgap devices to operate at high temperatures in automotive and electric grid applications. Sintered silver joints are currently being evaluated as an alternative to Pb-free solder joints. Of particular interest is the development of joints based on silver paste consisting of nano- or micron-scale particles that can be processed without application of external pressure. The microstructural evolution at the interface of a pressureless-sintered silver joint formed between a SiC die with Ti/Ni/Au metallization and an active metal brazed (AMB) substrate with Ag metallization at 250°C has been evaluated using scanning electron microscopy (SEM), x-ray microanalysis, and x-ray photoelectron spectroscopy (XPS). Results from focused ion beam (FIB) cross-sections show that, during sintering, pores in the sintered region near to the Au layer tend to be narrow and elongated with long axis oriented parallel to the interface. Further densification results in formation of many small, relatively equiaxed pores aligned parallel to the interface, creating a path for easy crack propagation. X-ray microanalysis results confirm interdiffusion between Au and Ag and that a region with poor mechanical strength is formed at the edge of this region of interdiffusion.

Accelerated Degradation of Ag-Cu-Ti Alloy in Molten Sodium Via Dissolution of Sliver

Ag-Cu-Ti alloys are widely used in active metal brazing (AMB) to bond between heterogeneous metal-ceramic joints. This is mainly because brazed joints with the alloys show acceptable bonding strength and good oxidation resistance at high temperatures of 400~600 oC through a simple but low cost bonding process. For example, a brazed joint between 304 stainless steel and α-Al2O3 with a commercial Ag-35.2Cu-5.4Ti (in wt.%) alloy showed excellent shear bonding strength of 100~200 MPa, and the oxidation kinetics of the alloy in air obeys the parabolic rate law with the rate constant of about 10-10~10-12 g2 cm−4 s−1 in the temperature range of 400~600oC. Previous studies showed that the alloy formed slowly growing protective Cu- and/or Ti- oxide layers. However, when the brazing alloys are applied to applications associated with molten sodium (Na) environments, such as cooling pipe joints in nuclear power plants and heterogeneous joints of high temperature sodium beta-alumina batteries, the joints show poor bonding performance resulting in failure in less than 100 hours at 350oC. In order to elucidate the phenomena different from a general expectation, there is a need to investigate the effect of molten sodium on the degradation kinetics of Ag-Cu-Ti alloys. In this study, disk shaped coupon specimens (10 mm in diameter, 5 mm in thickness) made of 430 stainless steel and α-Al2O3 were prepared and the heterogeneous specimens were brazed with a Ag-35.2Cu-5.4Ti alloy. The bonded assemblies were exposed to molten sodium at 350oC. Their degradation behaviors were characterized via (1) cross-sectional observation through optical microscopy (OM), secondary electron microscopy (SEM), transmission electron microscopy (TEM), and field-emission electron probe micro analyzer (FE-EPMA), and (2) kinetics analysis with Na penetration depth measurements at different exposure times. As the exposure proceeds, it was characterized that Ag in the Ag-Cu-Ti alloy continuously dissolves into the Na melt at the alloy-Na interface leaving the unreacted Cu and Ti to be a porous structure. The kinetics of the Na penetration depth into the alloy is governed by the linear rate law. It turns out that (1) no continuous and protective layer at the reaction front between the Na melt and alloy is formed and (2) Ag concentration in the melt in the vicinity of the reaction front is maintained below its maximum solubility in molten Na by sufficiently rapid homogenization of the Na melt (with a small amount of dissolved Ag) at this temperature. It was concluded that rapid degrdation of Ag-Cu-Ti in molten Na is not via corrosion but via dissolution of alloying element(s) in the melt although its resulting microstructure is hardly distinguishable from that formed by corrosion process. Since the reaction front is continuously refreshed, the velocity of the reaction front, i.e., the penetration depth of Na into the alloy as a function of time, is governed by the linear rate law which is analogous to gaseous corrosion forming porous or discontinuous corrosion products.

최신의 고능률 브레이징 기술개발 동향

Recent developing tendency for technologies of high efficient brazing are studied by searching of NDSL, Science Direct, KIPRIS, PCT and so on. Active metal brazing, arc brazing, fluxless brazing, brazing with low melting point, reactive air brazing, laser brazing, laser droplet brazing are investigated. By optimal selecting of the above mentioned technologies, it needs to investigate an economical metallurgical design and the advanced brazing methods. To improve the embrittlement of intermetallic compound at brazing interface, it must be studied the inexpensive variant metals including nonmetals and the heat sources(MIG, TIG, Laser) by hybrid techniques. Key words: Brazing, fluxless brazing, Laser brazing, Active metal, Laser droplet, Intermetallic compound

Active metal brazing of silicon nitride ceramics using a Cu-based alloy and refractory metal interlayers

Silicon nitride/silicon nitride joints with refractory metal (W and Mo) interlayers were vacuum brazed using an active braze, Cu-ABA (Cu–3Si–2Al–2.25Ti, wt%), and two interlayer arrangements in a double-lap offset configuration: Si3N4/Cu-ABA/W/Cu-ABA/Mo/Cu-ABA/Si3N4 and Si3N4/Cu-ABA /Si3N4. Titanium segregated at the Si3N4/Cu-ABA and Mo/Cu-ABA interfaces, but not at the W/Cu-ABA interface. The room-temperature compression-shear strength values of Si3N4/Cu-ABA/Si3N4 and Si3N4/Cu-ABA/W/Cu-ABA/Mo/Cu-ABA/Si3N4 joints were 118±24MPa and 22±5MPa, respectively. Elevated-temperature compression tests showed that Si3N4/Cu-ABA/Si3N4 joints had strength of 31±6MPa at 1023K and 17±3MPa at 1073K. Likewise, Si3N4/Cu-ABA/W/Cu-ABA/Mo/Cu-ABA/Si3N4 joints had strength of 19±4MPa at 1023K and 13±3MPa at 1073K. Knoop microhardness profiles revealed hardness gradients across the joints. The effect of joint microstructure and test configuration on the mechanical behavior is discussed.

(Invited) Characteristics of a Wire-Bonding-Less SiC Power Module Operating in a Wide Temperature Range

Introduction Using high-speed switching, wide bandgap semiconductor devices can reduce on-resistance in the on-state to realize low power converter losses. In addition, SiC power devices can be operated in high temperatures; thus, their cooling systems can be reduced in size or removed all together, thereby increasing the power density of the power converter. In this study, we designed and fabricated a SiC power module with a built-in snubber circuit. This power module is operable in high temperatures over 200 degrees C. Design of Power Module for High-Temperature Operation The manufactured power module, its components, and its circuit diagram are shown in Fig. 1. The module size is 30 × 44 × 29 mm3. Active metal brazing copper (AMC) substrates sandwich the four SiC-metal–oxide–semiconductor field-effect transistors (MOSFETs) in this structure. Two AMC substrates connect via the metal block. The C–R snubber circuit is built-in on the surface of AMC1 (Fig. 1(b)). The SiC-MOSFET and AMC1 are joined by a flip-chip-bonding technique using an Al bump. The others are joined by Au–Ge solder. Fig. 2 shows the 50-kHz switching waveforms. Fig. 3 shows the circuit configuration for the Fig. 2 waveforms. This power module can realize high-speed switching in a fall time tf @ 25 ns. Surge voltage and ringing are suppressed by the built-in snubber circuit. Acknowledgment This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), "Next-generation power electronics/Consistent R&D of next-generation SiC power electronics", and "the Novel Semiconductor Power Electronics Project Realizing Low Carbon Emission Society" (funding agency: NEDO). Figure 1

Bonded Ceramic-Metal Layers for Fabrication of Thermal Conduction Plates

The work described in this article is part of an effort to build reliable and efficient liquid cooling modules for high-power laser diodes. The cooling system is designed to mount at least 12 laser diodes to a common microheat exchanger, thus requiring a large-size thermal conduction plate. Fabrication of the thermal conduction plate involved void-free bonding of copper layers on both sides of an aluminum nitride (AlN) plate. In the current study, ceramic-metal bonding methods using moly-manganese metallization and active metal brazing were investigated. Bonded AlN/copper plates were characterized and evaluated by optical microscopy, scanning electron microscopy, and energy dispersive spectrometry. For detecting voids, cracks, and delamination, some of the plates were analyzed by scanning acoustic microscopy (C-SAM). Results indicated that >99% void-free bonded AlN/Cu plates can be fabricated by using properly selected metallization conditions and brazing temperature profiles. The active metal brazing approach was found to be a cost-effective method of fabricating reliable, void-free thermal conduction plates.

TiOx Based Thermoelectric Modules – Manufacturing, Properties and Operational Behavior

The class of reduced (non-stoichiometric) titanium dioxide is a promising material for thermoelectric modules with regard to its availability, economy, and operation and alteration behavior. The thermoelectric efficiency of TiOx is minor compared to established thermoelectric materials. However TiOx is very attractive for economic reasons and there are still expectations for efficiency rise by modification of the material structure. This paper gives the latest results of manufacturing technologies and test results of TiOx modules close to real conditions of service. A manufacturing process for unileg n-type modules based on hot pressed and shaped n-type TiOxlegs was established including joining of modules with reliable electrical connections and supporting substrates. Modules with copper metallized AlN, Al2O3 and Si3N4 substrates were built to evaluate the influence of ceramic substrate material on module behavior. In contrast to the widely used direct copper bonding process in combination with Al2O3substrates in this study the active metal brazing technology for the realization of planar and structured metal to ceramic compounds was applied.A lab scale testing device was used for acceptance trials of themodules with a maximum hot side temperature of 600°C while the cold side was kept at 100°C. This initial high temperature test revealed similar power output and internal resistance of all fabricated modules. To investigate the reliability and durabilityof the modules thermal cycleswere carried out on the basis of a reduced version of a thermoelectric generator unit, whichwas equipped with 2 x 3 modules arranged along a planar gas heat exchanger. The test rigwas designed to simulate the thermal stresses during power generation in the exhaust line of a passenger car. The characterization of the module resistance and microstructural investigations of cross-sectionsafter 1000 thermal cycles showed differences in the behavior and failure modes of the modules which can be correlated to the ceramic substrates used.

Effects of Extreme Temperature Swings ( <inline-formula> <tex-math notation="TeX">$-\hbox{55}\ ^{\circ}\hbox{C}$</tex-math></inline-formula> to 250 <inline-formula> <tex-math notation="TeX">$^{\circ}\hbox{C}$</tex-math></inline-formula>) on Silicon Nitride Active Metal Brazing Substrates

Reliability of power electronic substrates has been one of the main issues of high-temperature packaging technologies. Widely used direct bonded copper (DBC) substrates, if subjected to wide temperature swings, suffer from copper layers peeling off from a ceramic because of the large thermal stresses resulting from the difference in coefficient of thermal expansion (CTE) between the copper and the ceramic (e.g., $\hbox{Al}_{2}\hbox{O}_{3}$ or AlN). Recently, silicon nitride active metal brazing ( $\hbox{Si}_{3}\hbox{N}_{4}$ -AMB) substrate has been developed with the aim to utilize power electronic substrates for extreme environmental conditions. $\hbox{Si}_{3}\hbox{N}_{4}$ -AMB has superior reliability because of the high mechanical strength of $\hbox{Si}_{3}\hbox{N}_{4}$ . In this paper, the effects of extreme temperature swings ( $-\hbox{55} ^{\circ}\hbox{C}$ to 250 $^{\circ}\hbox{C}$ ) on $\hbox{Si}_{3}\hbox{N}_{4}$ -AMB substrates were investigated to evaluate their applicability to harsh environment. Their high resistance to the peeling off of copper layers was confirmed. However, the surface-roughening phenomenon was observed on the surface of copper layers after the temperature cycling test. The mechanism of surface roughening was analyzed. It was found that the out-of-plane displacement of single grains, which is called the orange-peel phenomenon, occurred on the surface of the copper layers.

Thermal cycling behavior of alumina-graphite brazed joints in electron tube applications

Alumina was joined with graphite by active metal brazing technique at 895, 900, 905, and 910 °C for 10 min in vacuum of 0.67 mPa using Ti-Cu-Ag (68.8Ag-26.7Cu-4.5Ti; mass fraction, %) as filler material. The brazed samples were thermal cycled between 30 and 600 °C and characterized. X-ray diffraction results show strong reaction between titanium and carbon as well as titanium and alumina. Scanning electron microscopy and helium leak tests show that the initial and thermal cycled brazed samples are devoid of cracks or any other defects and hermeticity in nature. Brazing strength of the joints is found to be satisfactory.

Co-fired AlN–TiN assembly as a new substrate technology for high-temperature power electronics packaging

New wide-band gap semiconductors (SC) for power electronics such as SiC, GaN and diamond will allow higher power densities, leading to higher operating temperatures. However, the surrounding materials will also undergo an increase in temperature, meaning that a parallel effort is needed in SC packaging technologies research. One of the essential components, the substrate, is used to insulate electrically the SC from the rest of the system, drain the generated heat and provide a path to connect the SC to the rest of the system. Direct bonded copper (DBC) and active metal-brazed (AMB) substrates have limited temperature and cycling operation, owing to the large differences in the thermal expansion coefficients between the ceramics and the metals. In this work we propose a new and original substrate technology based on two co-sintered ceramics: an insulating ceramic (AlN) and a conductive one (TIN). The microstructure, the chemical compatibility and the electrical properties indicate that the proposed substrate could operate at a temperature above 200 1C the current substrate technologies, which makes it particularly attractive for high-temperature power electronics applications.

Microstructure and Interfacial Reactions During Active Metal Brazing of Stainless Steel to Titanium

Abstract Microstructural evolution and interfacial reactions during active metal vacuum brazing of Ti (grade-2) and stainless steel (SS 304L) using a Ag-based alloy containing Cu, Ti, and Al was investigated. A Ni-depleted solid solution layer and a discontinuous layer of (Ni,Fe)2TiAl intermetallic compound formed on the SS surface and adjacent to the SS-braze alloy interface, respectively. Three parallel contiguous layers of intermetallic compounds, CuTi, AgTi, and (Ag,Cu)Ti2, formed at the Ti-braze alloy interface. The diffusion path for the reaction at this interface was established. Transmission electron microscopy revealed formation of nanocrystals of Ag-Cu alloy of size ranging between 20 and 30 nm in the unreacted braze alloy layer. The interdiffusion zone of β-Ti(Ag,Cu) solid solution, formed on the Ti side of the joint, showed eutectoid decomposition to lamellar colonies of α-Ti and internally twinned (Cu,Ag)Ti2 intermetallic phase, with an orientation relationship between the two. Bend tests indicated that the failure in the joints occurred by formation and propagation of the crack mostly along the Ti-braze alloy interface, through the (Ag,Cu)Ti2 phase layer.

Microstructure and Strength of γ-TiAl Alloy/Inconel 718 Brazed Joints

Intermetallics and superalloys brazing development is a current topic owing the extending use of these alloys in industrial applications. In this work a γ-TiAl alloy was joined to Inconel 718 by active metal brazing, using Incusil-ABA as filler. Joining was performed at 730 °C, 830 °C and 930 °C, with a 10 min dwelling time. The interfaces were characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). For all processing conditions, the reaction between the base materials and the braze alloy produced multilayered interfaces. For all processing temperatures tested (Ag), (Cu), AlNi2Ti and AlCu2Ti were identified at the interface. Raising the brazing temperature increased the thickness of the interface and coarsened its microstructure. The increase of the extension of the interface was essentially due to the growth of the reaction layers formed near each base material, which were found to be mainly composed of intermetallic compounds. The mechanical behavior of the joints, at room temperature, was assessed by microhardness and shear tests. For all processing conditions the hardness decreases from periphery towards the Ag-rich centre of the joints. Brazing at 730 °C for 10 min produced the joints with the highest average shear strength (228±83 MPa). SEM and EDS analysis of the fracture surfaces revealed that fracture of joints always occurred across the interface, preferentially through the hard layer, essentially composed of AlNi2Ti, resulting from the reaction between Inconel 718 and the braze alloy.

Microstructural evolution during reactive brazing of alumina to Inconel 600 using Ag-based alloy

A metal–ceramic bonding process was developed to produce vacuum tight alumina–Inconel 600 joints using an Ag-based active metal brazing alloy that can withstand continuous operating temperature up to 560°C. The microstructure and microchemistry of the braze zone was examined using extensive microanalysis of the constituent phases and a mechanism for the interfacial reactions responsible for the bonding is proposed. Prolonged heat treatment at 400 and 560°C under simulated in-service conditions revealed that the microstructure of braze zone of the joints was stable and maintained leak-tightness and strength. The bond strength of the interface was high enough to cause failure in the alumina side of the joints. Failure of the joints was caused by initiation of crack on the surface of alumina as a result of high tensile residual stress adjacent to the metal–ceramic interface.

Active Brazing Joins Titanium-Graphite Beam Target for Subatomic Particle Detection

Abstract Active metal brazing played a key role in the development and fabrication of a modified beam target for particle-physics experiments conducted by Fermilab. The new target consists of a titanium tube filled with stacked graphite fins that were bonded in place by means of active metal brazing. This article describes the vacuum process and the methods used to evaluate the brazed titanium-graphite joints.

Microstructure and mechanical properties of joints in sintered SiC fiber-bonded ceramics brazed with Ag–Cu–Ti alloy

Active metal brazing of a new high thermal conductivity sintered SiC-polycrystalline fiber-bonded ceramic (SA-Tyrannohex®) has been carried out using a Ti-containing Ag–Cu active braze alloy (Cusil-ABA®). The brazed composite joints were characterized using scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM–EDS). The results show that this material can be successfully joined using judiciously selected off-the shelf active braze alloys to yield metallurgically sound joints possessing high integrity. Uniform and continuous joints were obtained irrespective of differences in the fiber orientation in the substrate material. Detailed interfacial microanalysis showed that the titanium reacts with C and Si to form TiC layer and a Ti–Si compound, respectively. Furthermore, the evaluation of shear strength of the joints was also conducted at ambient and elevated temperatures in air using the single-lap offset (SLO) shear test. The perpendicular-type SA-Tyrannohex joints exhibited apparent shear strengths of about 42MPa and 25MPa at 650°C and 750°C, respectively. The fracture at the higher temperature occurred at the interface between the reaction-formed TiC layer and braze. This might be caused by generation of stress intensity when a shear stress was applied, according to μ-FEA simulation results.

ZrO2-Ti합금의 활성금속 브레이징

In this study, active metal brazing methods for <TEX>$ZrO_2$</TEX> and Ti alloy were discussed. To get a successful metal-ceramic bonding, various factors (melting temperature, corrosion, sag resistance, thermal expansion coefficient etc. of base materilas and filler metal) should be considered. Moreover, in order to clarify bonding between the metal and ceramic, the mechanism of the interfacial structure of the joints should be identified. The driving force for the formation of metal and ceramic interfaces is the reduction of the free energy which occurs when their contact becomes complete. Interfacial bonding depends on the material combinations and the bonding processes. This study describes the bonding between ceramic and metal in an active metal brazing.

Active Metal Brazing of Machinable Aluminum Nitride-Based Ceramic to Stainless Steel

Shapal™-M machinable AlN-based ceramic and AISI 304 stainless steel were joined by active metal brazing, at 750, 800, and 850 °C, with a dwell stage of 10 min at the processing temperature, using a 59Ag-27.25Cu-12.5In-1.25Ti (wt.%) filler foil. The influences of temperature on the microstructural features of brazed interfaces and on the shear strength of joints were assessed. The interfacial microstructures were analyzed by scanning electron microscopy (SEM), and the composition of the phases detected at the interfaces was evaluated by energy dispersive X-ray spectroscopy (EDS). The fracture surfaces of joints were analyzed by SEM, EDS, and GIXRD (Grazing Incidence X-Ray Diffraction). Reaction between the liquid braze and both base materials led to the formation of a Ti-rich layer, adjacent to each base material. Between the Ti-rich layers, the interfaces consist of a (Ag) solid-solution matrix, where coarse (Cu) particles and either Cu-In or Cu-In-Ti and Cu-Ti intermetallics phases are dispersed. The stronger joints, with shear strength of 220 ± 32 MPa, were produced after brazing at 800 °C. Fracture of joints occurred preferentially not only through the ceramic sample but also across the adjoining TiN layer, independent of the brazing temperature.

Thermal Stress Reliability Study on Substrates for High Temperature, Silicon Carbide Power Electronic Modules

The goal of this study is to investigate packaging techniques for extreme environment (i.e., −50 °C to 250 °C) power electronics. Areas of interest include substrate materials and baseplate/heat spreader materials. Multiple substrate technologies are evaluated, including direct bond copper (DBC), direct bond aluminum (DBA), and active metal braze (AMB) substrates. When possible, varying thicknesses of ceramic and metal layers are included to determine if that parameter affects reliability. Within each material and thickness sample lot, different layout geometries and overall sizes are tested, again to determine if that specific parameter has an impact on reliability. In a power electronics application, these substrates are required to be attached to a baseplate/heat spreader for mechanical support and to improve thermal performance. Therefore, a smaller sample size of substrate materials was also tested for reliability when soldered to a metal matrix composite (MMC) baseplate. To evaluate the reliability of said materials, several inspection methods are utilized, comprising of scanning acoustic microscopy (SAM), optical microscopy, florescent penetrant inspection, cross section analysis, and optical profilometery. This paper will present the results of thermal stress studies on each of the materials under varying conditions. The conditions include: thermal dwell conditions of 250 °C in excess of 1000 hours and thermal cycling from −50 °C to 250 °C in excess of 1000 cycles. Current results will be presented and discussed.

Characterization of alumina–alumina/graphite/monel superalloy brazed joints

Active metal brazing of alumina with alumina, graphite and monel 404 superalloy was performed at 910 °C for 5 min using TICUSIL (68.8Ag–26.7Cu–4.5Ti in wt.%) as the braze alloy in vacuum of 5 × 10 −6 mbar. The brazed joints were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, nanohardness and Young's modulus measurements by depth sensitive indentation technique. X-ray diffractrometry showed that the Ti-based compounds were formed at the substrate–filler alloy interfaces of the brazed joints as reaction products. The cross sectional microstructures of the brazed joints were observed by scanning electron microscopy. The energy dispersive X-ray analysis determined the elemental compositions at the selective points of the joint cross section, which supported the X-ray diffraction results. The nanohardness and Young's modulus of the substrate–filler alloy interface showed no abrupt change, which suggests reliable joint performance in service.

Microstructure and Mechanical Properties of Joints in Sintered SiC Fiber-Bonded Ceramics

Active metal brazing of a new high thermal conductivity SiC-polycrystalline fiber-bonded ceramic (SA-Tyrannohex™) has been conducted using a Ti-containing Ag-Cu active braze alloy (Ticusil®). The brazed joints were characterized using SEM-EDS and Knoop hardness scans across the interfaces. The effects of fiber orientation in the composite on the microstructure, elemental composition, and microhardness are presented. Results show that this material can be successfully joined using judiciously selected off-the shelf active braze alloys to yield metallurgically sound joints possessing high integrity.