Transient Liquid Phase Bonding Research Articles

Development of a boron-free filler metal for brazing nickel-based superalloys utilizing the Cu–Mn–Ni system

A new material for boron-free brazing of nickel-based superalloys is described that utilizes a transient liquid-phase bonding (TLPB) mechanism. Commercially available AMS 4764 powder (52.8Cu–37.5Mn–9.7Ni) was mixed with pure Ni powder to increase Ni content in the filler metal. This alloy is a good candidate for TLPB in Ni superalloy applications because of its high strength, low liquidus temperature, low cost and the absence of brittle intermetallic phases. Analysis of in situ braze joint formation with IN625 base metal using differential scanning calorimetry measurements allowed for optimization of Ni content, braze temperature and hold time conditions of the brazing process. Calculated ternary phase diagrams coupled with EDS analysis of braze joints identified a low-temperature copper-rich segregated region in braze joints, which, through interdiffusion with the base metal, increases melting temperature with increasing hold time. This work concludes that a 50-µm half braze joint can be formed through full isothermal solidification with an 80-min hold at 1050 °C. Solidification occurs primarily through interdiffusion of base metal and filler metal.

Analysis of Splitting and Martensitic Transformation of AlNi Intermetallic Obtained by Transient Liquid Phase Bonding

This study investigates the characteristics of the AlNi intermetallic formed in a Ni/Al/Ni couple obtained by the Transient Liquid Phase Bonding (TLPB) process. The crystal structure and orientation of the AlNi intermetallic phase were evaluated through SEM-EDS EBSD and its mechanical properties were analyzed by means of instrumented hardness. The results showed that AlNi intermetallic splits into two layers, with different Al content and the same crystal structure and orientation. EBSD mapping revealed that there is no grain boundary along the split line, suggesting that a chemical partition takes place without the need of nucleation, like in a spinodal decomposition. A martensitic layer formed at the Ni-rich AlNi split side was identified by indexing the measured Kikuchi patterns. Instrumented hardness showed that the mechanical properties of AlNi phase change markedly depending on its chemical composition. These results provide experimental data that contribute to the understanding of the solid-state transformations occurring in the central portion of the Al-Ni phase diagram under isothermal conditions.

Ultrasonic-accelerated metallurgical reaction of Sn/Ni composite solder: Principle, kinetics, microstructure, and joint properties.

The high-melting-point joints by transient-liquid-phase are increasingly playing a crucial role in the die bonding for the high temperature electronic components. In this study, three kinds of Sn/Ni composite solder pastes composed of different sizes of Ni particles were synthesized to accelerate metallurgical reaction among Sn/Ni interfaces under the ultrasonic-assisted transient liquid phase (U-TLP) soldering. The temperature evolution, microstructure and mechanical property in joints composed by these composite solder pastes with or without ultrasonic energy were systemically investigated. The intermetallic joint consisted of high-melting-point sole Ni3Sn4 intermetallic compound with a little residual Ni was obtained under the conditions of no pressure and lower power (200W) in a high-temperature duration of only 10s, its shear strength was up to 45.3MPa. Ultrasonic effects significantly accelerated the reaction among the interfaces of liquid Sn and solid Ni, which attributed to the temperature rise caused by acoustic cavitation because of large number of liquid/solid interfaces during U-TLP, resulting in accelerated solid/liquid interfacial diffusion and growth of intermetallic compounds. This intermetallic joint formed by U-TLP soldering has a promising potential for applications in high-power device packaging.

Investigating the transient liquid phase bonding (TLPB) behaviour of a Palladium containing Ni-B based braze filler metal

ABSTRACT A cyclic DSC method has been adapted to study the TLPB behaviour and isothermal solidification rate (IS) in an IN625/CPW475 base metal/filler metal joint at 1000°C. Analysis of the DSC data resulted in a prediction of the maximum brazing clearance (MBC) as a function of braze time. The rate of IS was determined to be linear with the square root of time with a rate constant k = 4.59 µm/min1/2, once the braze temperature and isothermal conditions were reached. During transient heating from the solidus temperature up to the braze temperature there is a region of rapid boron uptake by the base metal and an accelerated growth rate of the IS zone (ISZ). This initial ISZ layer, 2wISZi was predicted to be 29.5 µm. In the current case of an initial gap width of 80 µm, this initial ISZ represents 36% of the gap width.

Imaging of boron distribution in steel with neutron radiography and tomography

B distribution in steel was investigated using neutron imaging techniques, on a millimetric scale with a spatial resolution below 100 μm. Steel structural components joined using the transient liquid-phase bonding (TLPB) process, with Fe-based B-alloyed foils as filler material, were observed. By means of neutron radiography and with the proposed calibration method, a quantitative profile of B concentration was attained across the joint, in particular in the region where the completion of the TLPB process was achieved. It is noteworthy that B concentration did not exceed the typical specified range of B-alloyed steels. This result explains why prior austenite grain boundaries are boride-free. To test the validity of the measured B concentrations, they were compared with those of the numerically simulated TLPB process. As a result, the calculated and experimentally measured values are in good agreement, within the estimated error. On the other hand, in regions where the TLPB process was not finished, athermally solidified liquid (ASL) was found at the joint, in the form of borides. In addition, neutron tomography enabled us to attain a three-dimensional visualization of the size and distribution of ASL, highlighting its potential as a non-destructive testing technique of TLPB weldments.

Initial Boron Uptake and Kinetics of Transient Liquid Phase Bonding in Ni-Based Superalloys

The present work details a quantitative investigation into the braze gap width dependence on the initial boron uptake (IBU) phenomena—an event which manifests during the initial stages of brazing of boron-containing filler metals. This was accomplished using in situ cyclic DSC of both full- and half-joint TLPB couples, i.e., IN625/xBNi2/IN625 and IN625/xBNi2. All joints demonstrated a linear relationship between the isothermally solidified gap width with the square root of time which were in excellent agreement with conventional metallographic analysis methods. A conceptual model designed to quantify both the extent of isothermal solidification and IBU during TLPB in terms of the redistribution of boron as borides was implemented. The investigation demonstrated excellent agreement with microstructural and DSC results further supporting the crucial role boride formation has on the kinetics of TLPB.

Growth and mechanical properties of intermetallic compound between solid cobalt and molten tin

Transient liquid phase (TLP) bonding has gained attention due to the advantage of producing bond that has a higher melting point than the bonding temperature. Cobalt (Co) is a potential candidate for base metal in TLP bond. This work studied the interfacial reaction in binary cobalt–tin (Co–Sn) system, growth kinetics and mechanical properties of Co–Sn intermetallic compound (IMC) for transient liquid phase (TLP) joints. Dipping method was employed for the investigation of Co–Sn IMC growth. Solid Co was immersed into molten Sn statically at varying temperatures (250–300 °C) and durations (15–60 min). The IMC formed was characterized by field emission scanning electron microscope (FESEM) coupled with energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) tests and nanoindentation. The results showed the formation of a CoSn3 layer with plate-like morphology at temperature range of 250–300 °C. The thickness of the CoSn3 layer increases with dipping temperature and duration. The growth of CoSn3 is suggested to be controlled by only diffusion reaction at 250 °C. As temperature increases to 300 °C, the growth of CoSn3 is controlled by chemical reaction. The average nanohardness and Young’s modulus of CoSn3 phase reported in this work are 4.25 ± 0.6 GPa and 98.30 ± 9.0 GPa, respectively.

Hypereutectic Al-Si Alloys with Ag-28Cu Interlayer Jointed by Ultrasound-induced Transient Liquid Phase Bonding

Hypereutectic Al-Si Alloys with Ag-28Cu Interlayer Jointed by Ultrasound-induced Transient Liquid Phase Bonding

Interlayer Material Design Reducing Transient Liquid Phase Bonding Time

Power semiconductors require a large bonding area for die attachment. For this purpose, transient liquid phase bonding (TLPB) was applied using fabricated interlayers, namely Sn–3Cu and Sn–10Cu, to study the intermetallic compound (IMC) formation, and the results were compared with those obtained with a pure-Sn interlayer. The Sn–3Cu and Sn–10Cu interlayers exhibited primary IMC fraction of 0.06 and 0.24, respectively, before the TLPB. For a Cu/interlayer/Cu sandwich structure, the TLPB was applied at 250 °C over various time periods (1–4 h). A reduction in the bonding time was more significant for a Sn–10Cu interlayer with a larger amount of primary IMCs than for the Sn–3Cu interlayer. The time exponent of the IMC fraction with respect to the bonding time was approximately 0.3 for all interlayers. This implies that the IMC growth mechanism is governed by a liquid channel or wet grain boundary diffusion. The nearly constant fraction and increasing size of the primary IMCs produced during the TLPB indicate that the primary IMCs coalesced during this process. The primary IMCs preferentially coalesced with the interface IMCs produced during the TLPB when they had the same crystalline orientation.

Nanomechanical Responses of an Intermetallic Compound Layer in Transient Liquid Phase Bonding Using Indium

Low-temperature transient liquid phase bonding (TLPB) can be achieved using indium, and the joints thus formed can withstand high temperatures. Since the joints that are formed with TLPB entirely consist of intermetallic compounds (IMCs), this study investigates the phase evolution and mechanical properties of IMCs that are obtained from isothermal reactions between In and commonly used substrate metals, Cu, Ag and Au. Using nanoindentation, the relationships between the yield strain, work hardening exponent and strain rate sensitivity of In-bearing and other frequently observed IMC phases were compared. Notably, the strain-induced precipitation of Ag3In occurred in the indent of Ag9In4.

Fabrication of Highly Reliable Joint Based on Cu/Ni/Sn Double-Layer Powder for High Temperature Application

Abstract A highly reliable three-dimensional network structure joint was fabricated based on Cu/Ni/Sn powder with double-layer coatings and transient liquid phase bonding (TLPB) technology for high temperature application. The Cu/Ni/Sn joint is characterized by Cu metal particles embedded in the matrix of (Cu,Ni)6Sn5/Ni3Sn4 intermetallic compounds (IMCs), with a low void ratio, and can be reflowed at low temperatures (<260°C), but it can reliably work at a high temperature up to 415°C. Cu/Ni/Sn double-layer powders with different Sn layer and Ni layer thickness were was fabricated and compressed as preform used for TLPB joint bonding. The microstructure and phase composition evolution for Cu/Sn and Cu/Ni/Sn systems during reflow and aging were comparatively studied. Two kinds of interfacial structure designs were made, and corresponding interfacial microscopic morphology was analyzed and compared under once and twice reflow soldering processes. The results indicated that the Sn-coating layer was completely consumed to form (Cu,Ni)6Sn5/Ni3Sn4 IMCs, and the Cu/Ni/Sn joint had a lower void ratio and a higher shear strength than those of Cu/Sn. The mechanism of the Ni-coating layer inhibiting phase transformation was studied. The high reliable three-dimensional network structure joint based on Cu/Ni/Sn double-layer powder was fabricated for high temperature application.

On-Chip Diffusion Bonding creates Stable Interconnections Usable at Temperatures over 300°C

Abstract In this paper, we present a conceptual design of an on-chip solder stack to connect silicon devices faster and more reliable. Almost all electronic devices rely on solder layers to provide electrical, mechanical, and thermal connections between components. We improve the solder connection with industry-standard solder parameters of 300°C and some minutes of soldering time. An ideal solder connection is composed of intermetallic phases (IMPs) at the interfaces between device and solder, and substrate and solder. Typically, a thin region of Sn-based solder remains between the two IMP layers at the interfaces. IMPs of copper (Cu) and tin (Sn) are Cu6Sn5 and Cu3Sn. The formation of IMPs is decisive for a good mechanical connection because of their high melting point and mechanical stability. To achieve these requirements, we implement the solder stack as a transient liquid phase bonding (TLPB) system. To realize durable interconnections, we use the diffusion of a high-melting first component in a second component, which is liquid at solder process temperature. Ongoing diffusion leads to the formation of IMPs with a melting point above process temperature, resulting in a solidification of the connection at constant temperature. By this isothermal solidification, the solder connection becomes more durable against mechanical and thermal load and is usable at temperatures exceeding 300°C.

Au-Sn Solders Applied in Transient Liquid Phase Bonding: Microstructure and Mechanical Behavior

Transient liquid phase bonding offers one option to generate a robust lead-free die attach with sound thermal and electrical conductivity in microelectronic packages. However, it remains a challenge to characterize the microstructure of the bonding layer because of its ultra-thin thickness and its nano-sized constituents. In this work, we characterized a Au-Sn soldering system of transient liquid phase bonding process in detail using various advanced techniques which successfully identified the nano-constituents and mapped their distribution in the sub-layers. On top of that, the mechanical behavior of the bonding layer has been investigated by micro-cantilever bending testing.

Thermal processing strategies enabling boride dissolution and gamma prime precipitation in dissimilar nickel-based superalloys transient liquid phase bond

The microstructure of dissimilar transient liquid phase bond between Hastelloy X and IN792 nickel-based superalloys is featured by lack of sufficient formation of γ′ precipitates in the bond-centerline and extensive in-situ precipitation of boride second phases in the diffusion affected zones (DAZ). This paper investigates the impact of two thermal processing strategies, using standard solution treatment and aging of IN792 (STA strategy) and using solution treatment of IN792 followed by a post-bond heat treatment utilizing solution treatment of Hastelloy X and aging treatment of IN792 (SPTA strategy) on the joint microstructure and mechanical properties. The boride precipitates in the DAZ of Hastelloy X showed higher resistance to dissolution rather than those in IN792 which can be attributed to the higher content of the boride former elements in Hastelloy X. Both thermal processing routs resulted in the formation of γ′ strengthened bonds with overmatched hardness compared to the Hastelloy X. However, STPA strategy enabled complete dissolution of borides in DAZ of IN792, significant dissolution of borides in DAZ of Hastelloy X, and development of bimodal γ′ distribution in the IN792 base material. Nevertheless, the grain growth in the Hastelloy X after STPA strategy limits the achievable joint shear strength.

Fabrication of highly reliable joint based on Cu@Ni@Sn double-layer powder for high temperature application

Abstract A highly reliable three-dimensional network structure joint was fabricated based on Cu@Ni@Sn core-shell powder and transient liquid phase bonding (TLPB) technology for high temperature application. Cu@Ni@Sn joint is characterized by Cu metal particles embedded in the matrix of (Cu,Ni)6Sn5/Ni3Sn4 intermetallics (IMCs), low level of voiding is achieved, they can be reflowed at a low temperatures (<260°C), but reliably working at high temperature up to 415°C. Cu@Ni@Sn double-layer microparticles with different Sn layer and Ni layer thickness were fabricated and compressed as preform used for TLPB joint bonding, the microstructure and phase composition evolution for Cu@Sn and Cu@Ni@Sn system were comparatively studied during reflowing and aging process. Different kinds of interfacial structure designs were made, interfacial microscopic morphology was analyzed and compared under once and twice reflowing soldering process. Results indicated that Sn coating layer was completely consumed to form (Cu,Ni)6Sn5/Ni3Sn4 IMCs, Cu@Ni@Sn bondline have lower void rate and higher shear strength than that of Cu@Sn. The mechanism of Ni coating layer inhibit Cu atom diffusing towards Cu6Sn5 to form Cu3Sn was studied. The high reliable three-dimensional network structure joint based on Cu@Ni@Sn double-layer powder was fabricated for high temperature application.

Influence of bonding time on the transient liquid phase bonding behavior of Hastelloy X using Ni-Cr-B-Si-Fe filler alloy

The effects of different transient liquid phase (TLP) bonding times on the microstructure and mechanical properties of Hastelloy X joints made by Ni–Cr–B–Si–Fe filler alloy were investigated. The specimens were TLP bonded at 1070 °C for holding times of 5, 20, 80, 320, and 640 min. The electron probe microanalysis (EPMA) results revealed that the main eutectic phases observed at the joints following incomplete isothermal solidification were Ni-rich borides, Ni-rich silicides, Ni–Si eutectic, and some Cr-rich borides. A high density of plate-like, blocky, and acicular (Mo and Cr)-rich borides were observed in the diffusion-affected zone (DAZ) of the samples; however, increasing the holding time decreased the contents of these phases. The solid-state diffusion was found to be a more effective transportation phenomenon than base metal dissolution at longer holding times. The increased DAZ thickness and the complete isothermal solidification as a result of the improved solid-state diffusion helped increase the uniformity of the hardness profile of the TLP bond at higher holding times (320 and 640 min). The results showed reverse relationship between the athermally solidified zone (ASZ) width and the bonding strength. The highest tensile strength (∼617 MPa) was achieved for the sample bonded at a holding time of 320 min; this strength was more than 80% of the base metal strength. A fractographic analysis of the tensile failure revealed a cellular fracture surface, exhibiting the characteristics of both brittle and ductile fractures. The sites prone to stress concentration and crack initiation were reduced with the completion of isothermal solidification.

Effect of Bonding Temperature on Microstructure and Mechanical Properties during TLP Bonding of GH4169 Superalloy

The effect of bonding temperature on the microstructure and mechanical properties of transient liquid phase (TLP) joints of GH4169 superalloy was investigated. Joining processes were carried out at 1040–1100 °C for 30 min using BNi-2 solder paste. The results showed that three distinct microstructural zones were formed in the joint region: an athermal solidification zone (ASZ), consisting of eutectic compounds; an isothermal solidification zone (ISZ), consisting of γ solid solution; and a diffusion affected zone (DAZ), consisting of Ni-Cr rich boride and Cr-Nb-Mo-rich boride compounds. With increasing bonding temperature, the amounts of eutectic compounds in ASZ first decreased and then increased. A eutectic-free joint centerline was obtained at 1080 °C. The maximum bonding shear strength reached 728.03 MPa due to the completion of isothermal solidification. Fractographic studies revealed that the boride compounds in ASZ and the intermetallic compounds in DAZ were the main causes for the failure of joints. The fracture mode of the sample bonded at 1040 °C was brittle, and the fracture path was along the ASZ. However, the fracture mode of the sample bonded at 1080 °C was ductile, and the fracture occurred along the DAZ.

Transient Liquid Phase Bonding of Ti-6Al-4V and Mg-AZ31 Alloys Using Zn Coatings.

Ti-6Al-4V and Mg-AZ31 were bonded together using the Transient Liquid Phase Bonding Process (TLP) after coating both surfaces with zinc. The zinc coatings were applied using the screen printing process of zinc paste. Successful bonds were obtained in a vacuum furnace at 500 °C and under a uniaxial pressure of 1 MPa using high frequency induction heat sintering furnace (HFIHS). Various bonding times were selected and all gave solid joints. The bonds were successfully achieved at 5, 10, 15, 20, 25, and 30 min. The energy dispersive spectroscopy (EDS) line scan confirmed the diffusion of Zn in both sides but with more diffusion in the Mg side. Diffusion of Mg into the joint region was detected with significant amounts at bonds made for 20 min and above, which indicate that the isothermal solidification was achieved. In addition, Ti and Al from the base alloys were diffused into the joint region. Based on microstructural analysis, the joint mechanism was attributed to the formation of solidified mixture of Mg and Zn at the joint region with a presence of diffused Ti and Al. This conclusion was also supported by structural analysis of the fractured surfaces as well as the analysis across the joint region. The fractured surfaces were analyzed and it was concluded that the fractures occurred within the joint region where ductile fractures were observed. The strength of the joint was evaluated by shear test and found that the maximum shear strength achieved was 30.5 MPa for the bond made at 20 min.

Transient liquid phase bonding of carbon steel components using Ni-based foils – A comprehensive joint characterization

The transient liquid phase bonding (TLPB) process is one of the selected joining technologies to replace threaded connections in solid expandable tubulars. In particular, for expandable hot-rolled seamless carbon steel tubular products using Ni-based amorphous metallic foils as filler material.In this work, a comprehensive mechanical properties and microstructural characterization was carried out in TLP-bonded bars for this base metal/filler material combination. Both the joint and the heat affected zone exhibited a strength which compares well with the base metal, and a ductility in accordance with that which is typically specified for steel arc-welded joints. A coalesced lath-like bainitic microstructure was found at the joint. In addition, and by means of orientation imaging, a parent austenite grain - which is shared by the joint and an adjacent ferrite grain from the base metal - was found, which demonstrates the epitaxial nature of the TLPB solidification process.Also, a cell-block-like structure at ferrite grains next to the joint was detected, due to the plastic deformation developed in tensile-tested samples at room temperature.

Transient Liquid Phase Bonding of Copper Using Sn Coated Cu MWCNT Composite Powders for Power Electronics

In this paper, a novel transient liquid phase bonding material was fabricated by consequent electroless plating of Cu and Sn on a multi-walled carbon nanotube (MWCNT). The resulting Sn-Cu-MWCNT composites were used to join the Cu interconnects at 260°C. After 8 min of reflow time, a complete transformation of Cu3Sn intermetallic compound (IMC) occurred, leaving a Cu/MWCNT-Cu3Sn /Cu joint capable of withstanding the high operating temperature. Due to flake-like morphology, the Sn-Cu-MWCNT composite particles were well packed with lesser voids. The shear strength of the Cu/Cu3Sn-MWCNT/Cu joint was measured as 35.3 MPa, thus exhibiting the scope for replacing conventional transient liquid phase (TLP) powders in the future.