Transient Liquid Phase Bonding Research Articles

Phase Transformations in the Brazing Joint during Transient Liquid Phase Bonding of a γ-TiAl Alloy Studied with <i>In Situ</i> High-Energy X-Ray Diffraction

TiAl alloys are increasingly used as a lightweight material, for example in aero engines, which also leads to the requirement for suitable repair techniques. Transient liquid phase bonding is a promising method for the closure of cracks (in non-critical or non-highly loaded areas). The brazing solder Ti-24Ni was investigated for brazing the alloy Ti-45Al-5Nb-0.2B-0.2C (in at. %). After brazing, the joint exhibits different microstructures and phase compositions. The transient liquid phase bonding process was investigated in the middle of the joint region in situ to acquire time resolved information of the phases, their development, and thus the brazing process. These investigations were performed using high-energy X-ray diffraction at the “High-Energy Materials Science” beamline HEMS, located at the synchrotron radiation facility PETRA III at DESY in Hamburg, Germany. For this, we used an induction furnace, which is briefly described here. During the analysis of the diffraction data with Rietveld refinement, the amount of liquid was refined with Gaussian peaks and thus could be quantified. Furthermore, while brazing four different phases occurred in the middle of the joint region over time. Additionally, the degree of ordering of the βo phase was determined with two ideal stoichiometric phases (completely ordered and disordered). Altogether, the phase composition changed clearly over the first six hours of the brazing process.

The phase diagram Au-Ni-Si

Phase equilibria of Au-Ni-Si were investigated in the isothermal section at 700 °C using a combination of powder X-ray diffraction and quantitative phase analysis by SEM-EDX. Four vertical sections of the phase diagram at 10 at.% Au and at 15, 30 and 60 at.% Ni were studied by DTA. These data were combined with isothermal sections at 320° and 500° published earlier to develop a ternary reaction scheme and a liquidus surface projection. Fifteen ternary invariant reactions, a three-phase field maximum and a critical tie-line are proposed. The observed phase diagram is discussed regarding the possible use of Au-Si liquids for transient liquid phase (TLP) soldering of Nickel.

A Brief Review on High-Temperature, Pb-Free Die-Attach Materials

High-Pb solders have been used as die-attach and interconnect materials for decades in discrete power components. Because of the known harmful effects of Pb to human health and the environment, as well as the demand of the wide-band-gap SiC/GaN power devices serving under high power density and high junction temperature, alternative Pb-free solders/materials and solutions have been studied intensively. Pb-free alternatives to replace high-Pb solders are still in their infancy. The exemption of using high-Pb solders has been extended to 2021, although it may be terminated anytime if a new technology or material were to be accepted industry-wide. The potential high-temperature Pb-free materials, including solders (AuSn/AuSi/AuGe, ZnAl and BiAgX®), Ag-sintering material, and transient liquid-phase bonding (TLPB) material, are reviewed in this paper with an emphasis on BiAgX® solders. The extremely high cost of Au-based solders limits their applications, although they have been used in high-temperature electronics. Zn-Al has a high melting temperature, good mechanical properties, and good thermal performance. However, the high reactivity makes Zn-Al solders only available in wire and preform. BiAgX® solder (available in paste, wire and preform) has been developed to replace the high-Pb solders for use in low-power discrete components, with relatively low cost and better reliability than the high-lead counterpart. Ag-sintering material and TLPB material form joints by atomic diffusion (either solid diffusion for sintering or solid–liquid interdiffusion for TLPB). The challenges in mass-production and the cost have restricted their success to be widely used in industry. Although it is likely that none of these materials/technologies are ideal to satisfy all the requirements of the variety of high temperature Pb-free die attach applications in terms of processing/reliability/cost, each solution has the potential to satisfy a niche within this broader categorization.

Perspectives of High-Temperature Pb-Free Bonding Materials

Abstract High lead solders have been used as die-attach and interconnect materials in discrete power packages. Due to the demand of SiC devices serving the high-power market and the harmful effects of Pb to human health and the environment, alternative Pb-free solders, novel bonding materials, as well as solutions have been studied extensively in recent years. The exemption of using high-Pb solders has been extended again to 2021, although it could be terminated at any time if a new technology or material were to be accepted by the industry. This paper presents potential materials and technologies for high-temperature Pb-free die-attachment, focusing on alternative solders. Sintering materials and transient liquid phase bonding (TLPB) materials have been briefly covered as well. AuSn, AuSi, and AuGe solders have shown to be exceptionally high in cost, which limited their application. BiAg- and BiCu-based solders—the BiAgX® family including solder paste, solder wire, and solder preform—improved wetting and exhibited remelting temperatures of 262°C and 270°C, respectively. The acceptable reliability performance on temperature cycling and thermal aging, as well as low material cost, has made them the most competitive candidates for low-power discrete die-attach devices. SnSbAgCu, with well-designed compositions in recent studies, offers a remelting temperature above 320°C. SnSbAgCu is targeted in markets for mid-to-high power devices. Reliability testing for other recently designed SnSbAgCu pastes for various die-attach vehicles is being studied. ZnAl has a remelting temperature above 380°C and an extremely low material cost (comparable to or even lower than the high-lead solders). Although the bonding process is stringent, the excellent thermomechanical behavior and the superior thermal/electrical conductivity have allowed ZnAl to be a potential candidate for high-temperature/high-power die-attach that is competitive with AuSi and AuGe solders. Sintering materials form bonds through solid state interdiffusion, while TLPB materials create a joint through solid-liquid interdiffusion, in which the remelting temperature is enhanced by forming massive IMCs. The desired high thermal/electrical/mechanical/melting performances, as well as the relatively low processing temperature (<350°C), are shining the sintering materials (especially Ag-sintering materials). The intrinsic high porosity (>20%) and the evolution of pores from pressureless sintering may overshadow the reliability. In addition, the immaturity of the processing (time/temperature/pressure/atmosphere/equipment availability, etc.) may deter the industrial adoption of sintering materials. So far, none of these materials or technologies is ideal to satisfy all the requirements of the variety of high-temperature, Pb-free die-attach applications in terms of processing, reliability, and cost. However, each material and solution has the potential to be a niche within this broader categorization.

Understanding of Void Formation in Cu/Sn-Sn/Cu System During Transient Liquid Phase Bonding Process Through Diffusion Modeling

Transient Liquid Phase (TPL) bounding of Sn foil sandwiched between two Cu foils involves, in the temperature range above the melting point of Sn (232 °C) and below 350 °C, the formation and the growth of two intermetallic compounds (IMCs) Cu6Sn5 and Cu3Sn and mostly unintended micro-pores. The present study aims to analyze the mechanism of void development during the soldering process through an experimental and modeling approach of diffusion-controlled IMC transformation. This modeling couples the diffusion process and the interface motion with the volume shrinkage induced by the difference of partial molar volumes of atoms between each phase. We also consider two types of inter-diffusion transports: (i) inter-diffusion based on the exchange of Cu and Sn atoms and (ii) inter-diffusion of Sn atoms with vacancies allowing Kirkendall void formation. The simulations of IMC growth performed correspond to a sequence of planar phase layers, where the distinctive scallop morphology of the Cu6Sn5 layer is described through an analytical function allowing to quantify the grain boundary diffusion pathway. We take into account of the volume diffusion mechanism for Cu3Sn intermetallic. For Cu6Sn5 intermetallic two mechanisms are considered, volume diffusion and grain boundary diffusion, limited by grain growth. The simulations of IMC growth kinetics, for different transport scenarios, are compared to the experimental evolving morphologies to determine the most likely mechanism of micro-void formation.

Thermal reliability investigation of Ag-Sn TLP bonds for high-temperature power electronics application

Transient liquid phase (TLP) bonding is a potential joining technology for high-temperature power electronics packaging. In this study, the thermal reliability of Ag-Sn TLP bond was investigated through high-temperature storage test, in which the relationship between microstructure evolution and mechanical property was developed during 350 °C aging. After aging for 120 h, the Ag3Sn or ζ phase within the joint evolved totally into Ag(Sn) solid solution, which fractured with a ductile characteristic during shear test. The Ag-Sn TLP bond was confirmed to operate at ambient temperature of 350 °C for nearly 1000 h without serious failure, still possessing high shear strength of about 40 MPa. During thermal aging, a great amount of irregular Ni-Sn intermetallic particles were precipitated and dispersed adjacent to the interface between the Cu substrate and the Ag-Sn phases once the Ag metallization layer was consumed entirely. Moreover, a relatively thick Ni buffer layer should be carefully considered to prevent continuous cracks propagating along the Cu/Ag(Sn) interface, which was induced by the significant outflow of Cu flux from the interfacial zone, like a Kirkendall voiding process.

Transient Liquid Phase Bonding of Semi-Solid Metal 7075 Aluminum Alloy using ZA27 Zinc Alloy Interlayer

Transient Liquid Phase Bonding (TLPB) process of semi-solid metal 7075 aluminum alloys (SSM7075) using 50 μm thick of ZA27 zinc alloys as interlayers for the experiment were carried out under bonding temperatures of 480 and 540 °C and bonding times of 30, 60, 90 and 120 min respectively. In the bonding zone, the semi-solid state of ZA27 zinc alloy interlayers were diffused into the SSM7075 aluminum alloy. Examination of the bonding zone using Scanning Electron Microscope (SEM) and Energy-dispersive X-ray spectroscopy (EDS) showed that the precipitation of the intermetallic compound of η(Zn–Al–Cu), β(Al2Mg3Zn3), T′(Zn10Al35Cu55) and MgZn2 were formed in the bonding zone. The better homogenized microstructure in the bonding zone was formed when increasing bonding time and bonding temperature. The highest bonding strength was recorded at 17.44 MPa and average hardness was at 87.67 HV with the bonding time of 120 min and temperature at 540 °C. Statistically, the coefficient of determination analysis of bonding strength data was at 99.1%.

Transient Liquid Phase Bonding of AlN to AlSiC for Durable Power Electronic Packages

Conventional power electronic modules employ a direct bonded copper (DBC) substrate and multiple interface layers to dissipate heat. However, reliability issues arise due to the coefficient of thermal expansion (CTE) mismatch that exists between the metal, ceramic, and semiconductor materials in the conventional module. Significant performance enhancement can be achieved by eliminating the DBC and developing an integrated substrate/cold plate with a low CTE mismatch throughout the package. To address this need, we have demonstrated the ability to directly bond the aluminum nitride (AlN) substrate to an AlSiC heat sink through transient liquid phase bonding using a Cu–Al binary system. Fabricated samples are found to have good interfacial adhesion. The novel bond material exhibits properties analogous to AlSiC and is analyzed for thermal, mechanical, and metallographic properties. The novel structure demonstrated in this work will enable smaller, lighter, and more reliable power modules, when compared to traditional configurations.

Microstructural evolution of Cu–Sn–Ni compounds in full intermetallic micro-joint and in situ micro-bending test

This study focuses on the microstructural evolution process of Cu–Sn–Ni intermetallic compounds (IMCs) interlayer in the micro-joints, formed from the initial Ni/Sn (1.5 µm)/Cu structure through transient liquid phase (TLP) soldering. Under the bonding temperature of 240 °C, the micro-joints evolve into Ni/(Cu, Ni)6Sn5/(Cu, Ni)3Sn/Cu structure, where the interfacial reactions on Cu/Sn and Sn/Ni are suppressed by the atoms diffusing from the opposite side. The thickness of (Cu, Ni)3Sn layer on plated Cu layer still increases with the prolonged dwell time. When the bonding temperature was elevated to 290 °C, the phase transformation of (Cu, Ni)6Sn5 into (Cu, Ni)3Sn has been accelerated, thus the majority of IMCs interlayer is constituted with (Cu, Ni)3Sn. However, a small amount of Ni-rich (Cu, Ni)6Sn5 phases still remain near the Ni substrate and some of them close to the center-line of IMCs interlayer. The state between (Cu, Ni)6Sn5 and the adjacent (Cu, Ni)3Sn tends to reach equilibrium in Ni content based on the observation from Transmission Electron Microscope (TEM). In addition, the Cu–Sn–Ni IMCs micro-cantilevers were fabricated from these micro-joints using Focus Ion Beam (FIB) for the in situ micro-bending test, the results indicate a high ultimate tensile strength as well as the brittle fracture in the inter- and trans-granular modes.

A Comparative Study on the Microstructure and Mechanical Properties of Cu6Sn5 and Cu3Sn Joints Formed by TLP Soldering With/Without the Assistance of Ultrasonic Waves

In this study, the microstructure and mechanical properties of Cu6Sn5 and Cu3Sn intermetallic joints, formed by the transient liquid phase (TLP) soldering process with and without the assistance of ultrasonic waves (USWs), were compared. After the application of USWs in the TLP soldering process, Cu-Sn intermetallic compounds (IMCs) exhibited a novel noninterfacial growth pattern in the molten solder interlayer. The resulting Cu6Sn5 and Cu3Sn joints consisted of refined equiaxed IMC grains with average sizes of 3 and 2.3 µm, respectively. The Cu6Sn5 grains in the ultrasonically soldered intermetallic joints demonstrated uniform mechanical properties with elastic modulus and hardness values of 123.0 and 5.98 GPa, respectively, while those of Cu3Sn grains were 133.9 and 5.08 GPa, respectively. The shear strengths of ultrasonically soldered Cu6Sn5 and Cu3Sn joints were measured to be 60 and 65 MPa, respectively, higher than that for reflow-soldered intermetallic joints. Ultrasonically soldered Cu6Sn5 and Cu3Sn joints both exhibited a combination of transgranular and intergranular fractures during shear testing.

Microstructure and Properties of the Al-27Si/Cu/Al-50Si Joint Brazed by the Partial Transient Liquid Phase Bonding

Al-27Si and Al-50Si were brazed by using a thin Cu interlayer. The metallurgical bonding without obvious defects is achieved, and a wide brazing seam consisting of fine eutectic structures and coarse Si particles is formed in the Al-27Si/Cu/Al-50Si joint. The deposition of Si element in the liquid phases during solidification results in the formation of the larger Si particles and ultra-small Si particles in the brazing seam. The shear strength of the joint reaches 63 MPa.

On the Effect of Liquid-State Diffusion on Isothermal Solidification Completion Time During Transient Liquid-Phase Bonding of Dissimilar Materials

A study of transient liquid-phase bonding (TLPB) of dissimilar materials consisting of a nickel-based superalloy and a cobalt material is performed. In contrast to the assumption that rapid atomic diffusion in the liquid would reduce the processing time, tf, required to completely eliminate the liquid phase from the joint region through diffusional solidification, the occurrence of liquid-state diffusion significantly prolonged the processing time tf. This finding is crucially important to the TLPB of dissimilar materials.

Study of the Fracture Mechanisms of Electroplated Metallization Systems Using In Situ Microtension Test

This framework assesses the mechanical behavior of some potential thin/thick metallization systems in use as either ohmic contacts for diamond semi-conductors or for metallization on copper double bounded ceramic substrates present in the next-generation power electronics packaging. The interesting and unique characteristic of this packaging is the use of diamond as a semi-conductor material instead of silicon to increase the lifetime of embedded power converters for use in aeronautical applications. Theoretically, such packaging is able to withstand temperatures of up to 300 °C without breaking the semi-conductor, provided that the constitutive materials of the packaging are compatible. Metallization is very important to protect the chips and substrates. Therefore, we address this issue in the present work. The tested metallization systems are Ni/Au, Ni/Cr/Au and Ni/Cr. These specific systems were studied since they can be used in conjunction with existing bonding technologies, including AuGe soldering, Ag–In Transient liquid Phase Bonding and silver nanoparticle sintering. The metallization is achieved via electrodeposition, and a mechanical test, consisting of a microtension technique, is carried out at room temperature inside a scanning electron microscopy chamber. The technique permits observations the cracks initiation and growth in the metallization to locate the deformation zones and identify the fracture mechanisms. Different failure mechanisms were shown to occur depending on the metallic layers deposited on top of the copper substrate. The density of these cracks depends on the imposed load and the involved metallization. These observations will help choose the metallization that is compatible with the particular bonding material, and manage mechanical stress due to thermal cycling so that they can be used as a constitutive component for high-temperature power electronics packaging.

Transient Liquid Phase Bonding of Magnesium Alloy AZ31 Using Cu Coatings and Cu Coatings with Sn Interlayers

Transient liquid phase bonding (TLP) of AZ31 samples has been investigated using Cu coatings and Cu coatings with Sn interlayer. Copper coatings were used for one set of the bonds, and a combination of Cu coatings and Sn interlayer was used for the other set of bonds. The bonding temperature was fixed at 520 °C, and various bonding times were applied. This study shows that the bonds produced using only Cu coatings have shown weaker bonds compared to the bonds made using Cu coatings and Sn interlayer. The Cu2Mg particles were detected at the joint region of both bonds made by Cu coatings and Cu coatings with Sn interlayer by X-ray diffraction (XRD). However, it has been observed that the joint region was dominated by solid solution which is rich in Mg. Sn interlayer was not contributed to the intermetallic compound (IMC) at the joint region, and therefore it was diffused away through the Mg matrix. Within the joint interface, a slight increase of micro-hardness was observed compared to Mg base metal alloy. This was attributed to the formation and presence of IMC’s within the joint region. It was noticed that the presence of the Sn interlayer improved the joint strength by reducing the pores at the joint region. Pores were clearly observed for those bonds made using Cu coatings—especially for region where the fracture occurs; this was accomplished by scanning electron microscope (SEM).

Effect of Bonding Temperature on Microstructure Evolution during TLP Bonding of a Ni3Al based Superalloy IC10

Transient liquid phase (TLP) bonding of Ni3Al-based superalloy IC10 was carried out using the interlayer based on the base metal which added B and Hf as the melting point depressant elements. The effect of bonding temperature (1250 – 1270 °C) on the microstructure evolution of bonding joints were investigated. Microstructure of bonding joint composed of isothermally solidification zone (ISZ) formed γ’ phase and athermally solidified zone (ASZ) which consists of newly formed γ+γ’ reticular eutectic among with borides and carbides. Boride precipitates are not formed in diffusion affected zone (DAZ) and the boundary between ASZ and ISZ become not obvious. Isothermally solidification rate decreases as the increase of the bonding temperature.

Homogeneous (Cu, Ni)6Sn5 intermetallic compound joints rapidly formed in asymmetrical Ni/Sn/Cu system using ultrasound-induced transient liquid phase soldering process

Homogeneous (Cu, Ni)6Sn5 intermetallic compound (IMC) joints were rapidly formed in asymmetrical Ni/Sn/Cu system by an ultrasound-induced transient liquid phase (TLP) soldering process. In the traditional TLP soldering process, the intermetallic joints formed in Ni/Sn/Cu system consisted of major (Cu, Ni)6Sn5 and minor Cu3Sn IMCs, and the grain morphology of (Cu, Ni)6Sn5 IMCs subsequently exhibited fine rounded, needlelike and coarse rounded shapes from the Ni side to the Cu side, which was highly in accordance with the Ni concentration gradient across the joints. However, in the ultrasound-induced TLP soldering process, the intermetallic joints formed in Ni/Sn/Cu system only consisted of the (Cu, Ni)6Sn5 IMCs which exhibited an uniform grain morphology of rounded shape with a remarkably narrowed Ni concentration gradient. The ultrasound-induced homogeneous intermetallic joints exhibited higher shear strength (61.6 MPa) than the traditional heterogeneous intermetallic joints (49.8 MPa).

Effect of Joining Atmosphere in Transient Liquid Phase Bonding of Inconel 617 Superalloy

In the present paper, transient liquid phase (TLP) bonding of Inconel 617 alloy under the different furnace atmospheres of vacuum, argon gas, and air was investigated. The experiments were carried out at 1408 K (1135 °C) for 300 minutes by using BNi-1 filler metals with different thicknesses of 50 and 100 µm. The microstructure, chemical analysis, shear strength, and microhardness of the resultant joints were studied. After 300 minutes, complete isothermal solidification was obtained for both the interlayers having 50 and 100 µm thicknesses under all the furnace atmospheres. The specimens bonded under vacuum condition showed the highest shear strength, failure energy, and ductility. Also, the joint produced under argon gas atmosphere using 100-µm-thick filler metal showed a good strength and may be acceptable in many applications. However, the specimens bonded under air atmosphere were weak, due to their further cavities and non-dissolved carbides. The presence of non-dissolved carbides in the diffusion-affected zone resulted in increasing the hardness of the base metal near the interface. The carbides were dissolved and the hardness of the diffusion-affected zone was reduced under vacuum condition compared to argon and air atmospheres. The best result was obtained for the specimen joined using the 50-µm-thick interlayer under vacuum atmosphere, with about 95.8 pct joint efficiency.

Identification of damage and fracture modes in power electronic packaging from experimental micro-shear tests and finite element modeling

Micro-shear tests are performed in order to characterize the mechanical behavior and the fracture of the chip/metallized ceramic substrate assemblies of power electronic devices. These assemblies are elaborated using three types of junctions: AuGe solder/Au or Ag finish, transient liquid phase bonding (TLPB) AgIn/Ag finish and Ag nanoparticles/Au or Ag finish. The experiments are associated to finite element simulations of both nano-indentation and micro-shear tests. The mechanical behavior of the different assembly interfaces is represented using an in-built cohesive zone model (CZM) available in the user friendly finite element code Abaqus®. It is worth noting that the fracture mechanisms observed during the test and service periods of the power electronic packaging are not only due to the debonding at the interfaces but also to the initiation and growth of voids in the joint. Therefore, in addition to the CZM model, Gurson-Tvergaard-Needlmann (GTN) damage model is used in combination with the Rice bifurcation theory to correctly describe the fracture in the joint and, therefore the overall fracture mechanism of the entire junction. The simulation results are compared with the experimental force displacement curves and the SEM observations in order to assess the implemented model.

Grain morphology evolution and mechanical strength change of intermetallic joints formed in Ni/Sn/Cu system with variety of transient liquid phase soldering temperatures

The grain morphology evolution and mechanical strength change of the intermetallic joints formed in a Ni/Sn/Cu system by the transient liquid phase (TLP) soldering at 260°C, 300°C and 340°C were investigated. The grain morphology evolution of the (Cu, Ni)6Sn5 intermetallic compounds (IMCs) was highly correlated with the Ni concentration gradient across the joints and strongly affected by the soldering temperatures, which was analyzed in detail from the aspect of thermodynamics. The shear strengths of the three types of intermetallic joints were 49.8MPa, 50.3MPa and 42.7MPa, respectively. The correlation between the grain morphology evolution and the mechanical strength change of the intermetallic joints was revealed.

Transient Liquid Phase Bonding of Nickel-Base Single Crystal Alloy with a Novel Ni-Cr-Co-Mo-W-Ta-Re-B Amorphous Interlayer

AbstractA novel Ni-Cr-Co-W-Mo-Ta-Re-B alloy consisting of plate γ and M23B6 phases was prepared as interlayer for the transient liquid phase (TLP) bonding of Rene’ N5 nickel-base single crystal superalloy. The molten Ni-Cr-Co-W-Mo-Ta-Re-B alloy exhibited an excellent wettability on the nickel-base superalloy. The TLP bonding experiment has been carried out in vacuum furnace at 1,240 ℃ for 12 h and followed by post-weld heat treatment (PWHT) at 1,305 ℃ for 4 h. PWHT eliminated the intermetallic compounds and promoted the formation of γ´ precipitates in the bonding region. A more uniform microhardness profile of TLP joint was found after PWHT. The shear strength of the joint after PWHT significantly increased to 533.4 MPa compared with the value of 437.2 MPa without PWHT.