Transient Liquid Phase Process Research Articles

Tailoring thermal behavior and solid–liquid contact area of Sn/Cu composite solder during ultrasonic-assisted transient liquid phase process

Tailoring thermal behavior and solid–liquid contact area of Sn/Cu composite solder during ultrasonic-assisted transient liquid phase process

Effect of bonding temperature and bonding time on microstructure of dissimilar transient liquid phase bonding of GTD111/BNi-2/IN718 system

The effect of bonding temperature and bonding time on the microstructure of transient liquid phase (TLP) bonding named GTD111 and IN718 superalloys, using a commercial Ni–B–Cr filler alloy (BNi-2) interlayer were evaluated. The sandwich assembly was kept in a vacuum furnace at temperatures of 1050, 1100, and 1150 °C for 1, 15, 30, 45, 60, and 80 min until the TLP process occurred. Microstructural characterization was carried out via optical microscopy, scanning electron microscopy (SEM) equipped with field emission energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Microstructural assessments displayed those in little bonding times, the joint microstructure includes continuous eutectic intermetallic phases and longer times cause eutectic free microstructure. The bonding temperature affects the isothermal solidification rate, while, at low bonding temperatures microstructure of the joint centerline is controlled by diffusion of melting point depressant (MPD) elements. Despite, at high bonding temperature effect of base metal alloying elements on the joint microstructure development was more marked. The results showed that athermally solidified zone (ASZ) size reduces with increasing bonding temperature and time due to diffusion of boron into the base metal.

Hybrid additive manufacturing of an electron beam powder bed fused Ti6Al4V by transient liquid phase bonding

Hybrid Additive Manufacturing (HAM) is a production strategy enhancing the flexibility of the already versatile Additive Manufacturing (AM) techniques. AM of Ti6Al4V, on the other hand, has been of great interest to numerous research works, thanks to the unique corrosion, biomedical and mechanical properties of the alloy. Hence, this research marks the first report on the HAM of Ti6Al4V by Transient Liquid Phase (TLP) bonding of an Electron Beam Powder Bed Fused (EB-PBF) sample to a conventional one. A copper interlayer was used for bonding, and the TLP process was performed at 890 °C and 970 °C for 60 min. Shear strength test was carried out and the results showed the highest shear strengths of 579.3 and 662.5 MPa for TLP bonding at 890 °C and 970 °C, respectively. By increasing the bonding temperature to 970 °C, no Cu-rich phases were observed in the microstructure, as opposed to the 890 °C samples, and a complete isothermal solidification without intermetallic phases was achieved. Moreover, the 970 °C TLP sample was featured with a much better microstructural integrity and homogeneity in both the base metals and the bonded zone. TLP bonding at 970 °C resulted in a more ductile fracture surface than that bonded at 890 °C. The strong differences between the two TLP bonds were primarily attributed to the faster diffusion rate of elements along the joint and base metal at higher temperatures.

Mechanical performances of AlN/Al metallized ceramic substrates fabricated by transient liquid phase bonding and pre-oxidation treatment

For the miniaturization of high-power electronic components, AlN/Al is a promising metallized ceramic substrate due to its superior mechanical and thermal performances. Numerous bonding processes have been proposed for fabricating the metallized ceramic substrate. Unfortunately, the influences of various bonding techniques on the mechanical performance of AlN/Al metallized ceramic substrate remain undetermined to date. The objective of this study was thus to investigate the effects of the transient liquid phase (TLP) technique and pre-oxidation treatment on the bonding, microstructure, and mechanical strength of the AlN/Al metallized ceramic substrate.The results indicated that the three-layered AlN/Al/AlN specimen could be effectively bonded by the TLP process and pre-oxidation treatment. However, the bending strengths of the specimens fabricated by the two techniques were obviously divergent. The bending strength of raw AlN substrate was 333 MPa. In contrast, the bending strengths of the three-layered specimens with AlN substrates pre-oxidized at 1050 °C, 1150 °C, and 1250 °C were 292 MPa, 250 MPa, and 224 MPa, respectively. Raising the pre-oxidation temperature of the AlN substrate from 1050 °C to 1250 °C obviously increased the thickness of the Al2O3 layer and deteriorated the bending strength, for the fracture propagated along the Al2O3 layer and the Al2O3/AlN interface. For the TLP bonding, the Cu film deposited on the AlN substrate contributed to the generation of Al–Cu transient liquid and to bonding. The bending strength of the three-layered specimens fabricated by TLP at 650 °C was 417 MPa, which was 25% and 43% better than those of the raw AlN substrate and the three-layered specimens prepared by the pre-oxidation treatment, respectively.

Systematic investigation of the effect of Ni concentration in Cu-xNi/Sn couples for high temperature soldering

High temperature solders have experienced a marked increase in demand due to the boom of high power interconnects in electric vehicles and the miniaturisation of electronic devices. Transient liquid phase (TLP) bonding is a promising technology for these applications. Alloys from the Cu-Sn system widely used in conventional soldering processes have potential for high temperature applications if the relatively low melting point Sn phase can be consumed in a TLP process. This process results in a joint composed of Cu6Sn5 or Cu3Sn intermetallic compounds (IMCs) which have higher melting points than Sn. Alloying Ni to the Cu substrate promotes the growth rate of the (Cu,Ni)6Sn5 IMCs, significantly reduces the TLP processing time and costs, while suppressing the formation of (Cu,Ni)3Sn. However, the properties of the (Cu,Ni)6Sn5 formed also change significantly depending on the Ni concentration in the system. In this work, the composition, morphology, grain sizes and crystal structure of the (Cu,Ni)6Sn5 formed via the reaction of Sn and Cu-xNi substrates of different Ni concentrations are studied in detail via electron microscopy and synchrotron powder X-ray diffraction to provide a basis for further understanding of the physical, electrical and mechanical properties. The findings are further verified by density functional theory calculations. Additionally, the complex interplay of grain boundary diffusion rates and the IMC nucleation rate which accelerate with increasing Ni concentration, along with the Cu-xNi dissolution rate and the crystal growth rate which decrease with increasing Ni concentration are identified to be the main factors affecting the IMC growth rates.

Effect of Interlayer Composition on Microstructure and Properties of Titanium/Aluminum TLP Bond

Abstract The transient liquid phase bonding of titanium and aluminum alloys was investigated, in which the interlayer was a 30-µm-thick brass foil and pure silver foil. The optimized variables used in this work are bonding temperature and time of 600°C and 30 min, respectively. The bonding temperature of 600°C (higher than both Al–Ag and Al–Cu eutectic temperatures) was selected to ensure melting. In both cases, an interesting intergranular eutectic-like phase was observed along the Al2024 grain boundaries. The evaluation of the interfaces of joints via optical microscopy, scanning electron microscopy, line scan, and X-ray diffraction demonstrated that a high temperature in the transient liquid phase process leads to the diffusion of alloying elements to form the molten eutectic. By holding the joint at the bonding temperature, the diffusion of alloying elements resumes, and the liquid interlayer solidifies isothermally because of the changes in the composition of the melted interlayer. The formation of Al2Cu, MgZn2, and TiMg3Zn15 phases was confirmed in the specimen made with brass foil as the interlayer. The interface width is decreased significantly from 8 µm to 2 µm by using silver foil instead of Cu–Zn foil as the interlayer. It is suggested that applying silver interlayer was successful in controlling the diffusion between two base metals and resulted in the formation of Ag2Al and AgMg phases on the fracture surface. This joint gave a higher shear strength of 154.7 MPa. The significant reduction in the strength of the bond due to using Cu–Zn foil as an interlayer (70.2 MPa) can be attributed to aggregation and growth of the brittle intermetallic compounds (IMCs) in the vicinity of the joint interface.

In-situ observation of high-temperature Pb-free electric interconnections by synchrotron microradiography

Despite a large amount of research on transient liquid phase (TLP) bonding methods for high operating temperature Pb-free solder electric interconnections using Cu6Sn5 intermetallics (IMCs), two fundamental difficulties remain, namely, slow TLP process kinetics and uncontrolled void formation. Here we observe the TLP processes for rapid Cu6Sn5 growth using a recently developed technique via real-time synchrotron X-ray imaging. The technique enables in-situ characterization of the growth kinetics and real-time observation of the IMC evolution at the molten solder/solid substrate interfaces and the formation of voids and cracking during TLP bonding, which will contribute to the development of products suitable for high reliability and high-temperature applications, e.g., electric vehicles.

Unravelling the metal borides evolution in the transient liquid phase bonding of Ni-based alloys via high-throughput transmission electron microscopy and first-principles thermo-kinetic calculations

The understanding of temperature and time-dependent metal borides precipitation/dissolution is crucial for the design of the transient liquid phase (TLP) bonding process of Ni-based alloys. It however remains elusive despite substantial research efforts for many years mainly owing to the uncertainty on the precipitated metal borides and the complex thermo-kinetics in the process. In this paper, we have unambiguously constructed the micro/nanoscale map of the precipitated metal borides in the TLP bonded Ni-based Inconel 718 superalloy via a high-throughput transmission electron microscopy (TEM) analysis. Five types of metal borides were found to precipitate in the diffusion affected zone (DAZ) when the isothermal solidification is completed. They are the M5B3 with stacking faults, Ti-rich M3B4, and Nb-rich M3B4 at the grain boundaries as well as single-crystalline M5B3 and M3B2 inside the grains. Notably, the crystal structure of the faulted M5B3 was rationalized by a hybrid modelling approach integrating first-principles calculation and TEM experiments. The sublattice model, with the optimized thermodynamic model parameters, was used to reproduce the metal borides precipitation map in the TLP process using DICTRA software. Coupling with multiscale simulation and experimental data, the present work built a modified thermo-kinetic model, which enables the design of the TLP bonding process of Ni-based alloys that often involves complicated time-temperature schedules and the precipitation/dissolution of a variety of different phases. The strategy can be applied to the design of the brazing process of other alloys or hybrid materials such as ceramics and metal.

Effect of the isothermal solidification completion on the mechanical properties of Inconel 625 transient liquid phase bond by changing bonding temperature

Transient liquid phase (TLP) bonding as an effective and precise bonding process attracted enormous attentions. However, there are several parameters affecting the mechanical properties of the joint zone which are crucial to be studied. In this work, two pieces of Inconel 625 alloy was bonded via TLP process using a BNi-2 interlayer. TLP process was done in different bonding temperatures (e.g., 1025 °C, 1100 °C, and 1175 °C) in a vacuum furnace in 10−5 bar pressure for 65 min. It is found that there is a meaningful relationship between process temperature and width of the eutectic phase zone. Results suggested that the rate of diffusion of melting point depressant atoms was increased by increasing the process temperature, leading to increasing the width of isothermal solidification zone (ISZ). It is found that the silicon and boron-rich eutectic compounds are responsible for large hardness value and weak the shear strength. To inhibit the formation of these hard and brittle phases, and to complete the ISZ region, the samples were bonded at 1175 °C and as a result, the ISZ region could be completed and mechanical properties were improved (e.g., the maximum shear strength at 1175 °C was measured to be 55.43% and 138.39% higher than 1100 °C and 1025 °C respectively). In addition to it, fracture cross-section was containing semi-cleavage and dimple shaped features which verified the completion of ISZ.

Effect of Temperature and Time on the Joint Properties of AISI420 Steel to SAF2507 Steel Produced by Transient Liquid Phase Process

Effect of Temperature and Time on the Joint Properties of AISI420 Steel to SAF2507 Steel Produced by Transient Liquid Phase Process

Effect of transient liquid phase bonding time on the microstructure, isothermal solidification completion and the mechanical properties during bonding of Inconel 625 superalloy using Cr-Si-B-Ni filler metal

The effect of bonding time on microstructure and mechanical properties of a transient liquid phase (TLP) bonded Inconel 625 superalloy, using a BNi-2 interlayer, was investigated. The superalloy was kept in a vacuum furnace at a pressure of 10−5 torr and a temperature of 1025 °C for 35, 65 and 95 min until TLP process occurred. Microstructure analyses were performed using optical microscopy (OM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Microhardness examinations and shear tests were performed to determine the effect of bonding time on the mechanical behavior of TLP bonded joints. By increasing the bonding time, the diffusion of melting point depressant (MPD) increased and the isothermal solidification zone (ISZ) was completed. Isothermal solidification was completed at 1025 °C by 95 min bonding time. Also, by increasing the bonding time to 95 min, that caused the reduction of the eutectic phase, the ISZ widened and the hardness increased. At the bonding time of 35 min the amount of shear strength was 208.7 MPa, while by increasing the bonding time to 95 min, this amount increased to 448.9 MPa. So, the stress concentration factors decreased by reduction of the eutectic phases, which caused an increase in the shear strength.

Two-step heating transient liquid phase bonding of Inconel 738LC

The high-temperature short-time heating step was followed by the low-temperature isothermal solidification step to join IN-738LC superalloy. The shorter bonding time can be applied in comparison to the conventional transient liquid phase process. A dendritic-cell like initial interface was observed which is different from a planar interface of conventional TLP bonds. This led to non-uniform distribution of voids along the joint. By completion of the isothermal solidification, quantity and size of the voids were decreased. A homogenization heat treatment produced the microstructure similar to that of the base metal.

An Investigation on High-Temperature Oxidation and Hot Corrosion Resistance Behavior of Coated TLP (Transient Liquid Phase)-Bonded IN738-LC

Despite all achievements to improve nickel-based superalloy, these classes of alloys are still prone to degradation via high-temperature oxidation and hot corrosion. Repairing damaged parts could decrease the life cycle, cost of equipment, and a transient liquid phase (TLP) bonding is a favorable method that has successfully been used for this purpose. One way to increase the lifetime of the repaired parts and the main body is to utilize protective coating. In the current study, aluminized coating was applied on IN738-LC which was first bonded by TLP process. Coating performance on the joint centerline compared to the other parts of the sample was investigated using a scanning electron microscope (SEM and FESEM) and X-ray diffraction method (XRD). The oxidation test result showed that coating provided less protection on the joint centerline due to coating’s chemical composition difference in this area: particularly Fe and Cr. XRD results showed that at the initial time of oxidation, all (α, γ, δ and θ)-Al2O3 were formed and by prolonged exposure were transformed to α-Al2O3. The hot corrosion test also proved that the joint centerline and the diffusion-affected zone were less resistant to the corrosion attack of 3Na2SO4 + NaCl salts and severity of damage in these zones were clearly distinguished from microscopic images.

The role of TLP process variables in improvement of microstructure and mechanical properties in TLP joints of GTD-111/Ni-Cr-Fe-B-Si/GTD-111 system

In this research microstructure and mechanical properties of the transient liquid phase (TLP) joint of GTD-111 superalloy using Ni-7.6Cr-3.4Fe-3.6Si-3B amorphous interlayer were investigated. The effect of joining parameters such as temperature, time and interlayer thickness on the microstructure and mechanical properties of the joint was studied. The results showed that eutectic and intermetallic compounds e.g. nickel-rich and chromium-rich borides, Ni–Si–B compound and eutectic-γ form continuously in the joint region during cooling. The thickness and continuity of the intermetallic phases decreased with increasing joining time. Moreover, increasing joining temperature to 1180 °C led to the elimination of intermetallic phases. The phases were reappeared as the joining temperature was further increased up to 1200 °C. In addition, increasing the interlayer thickness led to a thicker eutectic structure in the joint region. Shear strength of the joints increased with increasing holding time. Increasing the bonding temperature led to an increase in shear strength (except 1200 °C) which was in agreement with hardness variation. Furthermore, it was shown that homogenization of the samples after the joining process led to a more uniform hardness profile in the joint and significant increase in the shear strength up to 97% of the base metal strength. According to the obtained results the optimum time, temperature, and interlayer thickness for the TLP bonding of GTD-111 are 105 min, 1120 °C and 25 μm, respectively.

Novel transient liquid phase bonding through capillary action for high-temperature power devices packaging

A novel transient liquid phase (TLP) bonding was achieved by using a hybrid solder preform containing porous Cu interlayer, which was prepared by compacting a thin layer of Cu powders with Sn foils. This method combines the advantages of the conventional TLP bonding and sintering, and it can accelerate the reaction rate, suppress the voids formation, and be performed with lower pressure. During bonding process, liquid Sn on the two interfaces flows rapidly into the gaps between neighbored Cu particles through a strong capillary action, resulting in the densification of the microstructure. The TLP process can be accomplished at a temperature of 250 °C within 20 min, sequentially producing a high heat-resistant joint, which is comprised primarily of Cu-Sn intermetallic compounds (IMCs) matrix and dispersive Cu particles. In this study, the microstructure evolution, mechanical property, thermal reliability, and the effects of Cu particle size and morphology on the bonding process were investigated systematically.

Grain structure modification of Cu-Sn IMCs by applying Cu-Zn UBM on transient liquid-phase bonding in novel 3D-IC technologies

This study focuses on the formation mechanism of impinged-grain of Cu6Sn5 and interfolded-grain of Cu6(Sn,Zn)5 in Cu/Sn-3.5Ag/Cu and Cu/Sn-3.5Ag/Cu-15Zn (wt %) transient liquid-phase (TLP) bonding after reflow, respectively. Due to the application of TLP process in 3D-IC technology, the Cu-Sn intermetallic compounds (IMCs) rapidly form from the Cu pads and occupy the entire joint. These Cu-Sn IMCs with large volume fraction weaken the bond reliability resulting from its homogeneous grain structure and brittleness. It is demonstrated that doping Zn into one of the Cu substrates effectively modify the homogeneous grain structure into interfolded structure. The interfolded grain structure of Cu6(Sn,Zn)5 acts as a barrier for crack propagation and therefore enhances the bond reliability. The mechanisms involved are explained in details using thermodynamic related theories, First-principle simulation, FE-SEM observation and electron backscatter diffraction (EBSD) analyses. Owing to the effective modification of grain structure, Cu/Sn-3.5Ag/Cu-15Zn TLP bonding is potentially useful in strengthening the interconnections for novel 3D-IC technologies.

Rapid Ag/Sn/Ag transient liquid phase bonding for high-temperature power devices packaging by the assistance of ultrasound

Rapid transient liquid phase (TLP) bonding process on Ag/Sn/Ag system is achieved in air by the assistance of ultrasonic, which has great potential to be applied to high-temperature power devices packaging. In this study, the influence of ultrasonic effect on the morphology and growth kinetics of Ag3Sn grains, and the joint microstructure, mechanical property and thermal reliability were systematically investigated. Experimental results indicated that the rapid consumption of the “dynamic” transient liquid phase was attributed to the accelerated dissolution of Ag substrate and the extrusion of liquid Sn, which were entirely induced by the complex sonochemical effects on the liquid/solid intermetallic compounds (IMCs) interface. An elongated scallop-like morphology of Ag3Sn grains was developed during Ag/Sn interfacial reaction with ultrasonic, accompanied by widening of grooves between neighbored grains. This phenomenon is called as a strengthening thermal grooving, in which the grooves at grain boundaries provide stable molten channels for Ag atoms diffusion from the substrate. Consequently, the improved elemental diffusion was evaluated through the growth kinetics of Ag3Sn IMCs, with conservative estimation of 6–16.5 times faster than the traditional TLP process. In addition, both excellent mechanical property and thermal reliability of the Ag-Sn intermetallic joint were experimentally verified by shear test and high-temperature storage test, respectively.

Understanding non-parabolic solidification kinetics in Ni-based alloys during TLP bonding via thermo-kinetic modelling

Through a diffusion controlled solidification process, transient liquid phase (TLP) bonding is widely exploited to repair or join materials in different industry areas for decades. However, a lack of modeling on the TLP process of multi-component systems makes design of the TLP process an expensive trial and error process. Here, by coupling with experiment, we developed a set of self-consistent kinetic model parameters, which are used to simulate the TLP process of AWS A5.8 BNi-9 (filler)/Inconel 718 (base metal) at 1090 °C such as the solidification kinetics, diffusion profile and the precipitated borides in the Diffusion Affected Zone (DAZ). Consistent with experiment results by X-ray diffraction (XRD) and scanning electron microscopy (SEM), our simulation demonstrate four types of boride precipitated in the DAZ: MB2, MB, M5B3 and M3B2, which consequently results in the non-parabolic solidification behaviors in the brazing system. Based on the calculations using the developed models, we predict that a post heat treatment at 1090 °C for 3500s will homogenize the boride phase distribution in the DAZ, thus increasing the strength of the brazing joint. The developed model parameters may be extended to the other Ni-based alloys.

Effect of Process and Service Conditions on TLP-Bonded Components with (Ag,Ni–)Sn Interlayer Combinations

Transient liquid phase (TLP) bonding of Cu substrates was conducted with interlayer systems with the stacking sequences Ag–Sn–Ag (samples A), Ni–Sn–Ni (samples B), and combined Ag–Sn–Ni (samples C). Because of the low mismatch of the coefficients of thermal expansion, characteristics of the TLP process and mechanical and thermal behavior of TLP-bonded samples could be investigated without interference from thermally induced residual stresses. An ideal process temperature of 300°C, at which the number of pores was lowest, was identified for all three layer systems. It was verified experimentally that formation of pores resulted from volume contraction during isothermal solidification of liquid Sn into intermetallic compounds (IMC). Temperature and interlayer-dependent growth characteristics of IMC accounted for the increasing size and number of defects with increasing process temperature and for different defect positions. The shear strength was measured to be 60.4 MPa, 27.4 MPa, and 40.7 MPa for samples A, B, and C, respectively, and ductile fracture features were observed for Ag3Sn IMC compared with the purely brittle behavior of Ni3Sn4 IMC. Excellent thermal stability for all three layer systems was confirmed during long-term annealing at 200°C for up to 1200 h, whereas at 300°C the microstructure was driven toward Ag–Sn solid solution, accompanied by Cu diffusion from the substrate along grain boundaries and Cu3Sn IMC formation (A), and toward Ni-rich IMC phases (B). Combined IMC interlayers (C) tended to be transformed into Ni-based IMC when held at 300°C; intermixing into (Ni,Cu)3Sn was accompanied by pore formation.

Reduced Temperature Metal Based Bonding Processes for Wafer Level Packaging and 3D Integration

Metal based bonding processes are commonly selected because of the need for a conductive interface along with a high degree of hermeticity. Metal based bonding process can be based on solid diffusion which is commonly called thermo-compression or can be based on liquefying part or all of the metal in the bond interface. Thermo-compression bonding has the primary challenges of being slow because it is based on solid diffusion and has very poor step coverage. The most common liquid metal bonding processes are commonly called solder or eutectic bonding. The solder or eutectic bonding process allows some step coverage and the time required for the process is driven by the maximum temperature required, the residence time at this temperature and the maximum ramp rate. The process temperature and the required process time can be reduced by selecting an alloy with a low melting point; however this reduced melting point reduces the maximum post bond working temperature. TLP (Transient Liquid Phase) also known as SLID (Solid Liquid Inter Diffusion) bonding decouples the bonding temperature from the post bond maximum working temperature which allows the selection of alloys that reduce the bonding temperature while maintaining a high post bond maximum working temperature. For example a Cu-Sn TLP process can be performed at 280°C that will survive post bond temperature of up to 415°C. The fundamentals of TLP bonding will be reviewed and data will be presented for Cu-Sn TLP bonding