Conventional Transient Liquid Phase Research Articles

Ultrasonic-enhanced silver‑indium transient liquid phase bonding in advancing rapid and low-temperature packaging

Transient liquid phase (TLP) bonding technique is one of the promising solutions to third-generation wide-bandgap semiconductor power packaging. However, its prolonged heating process of the components restricts its application to some extent. To address this issue, the singular impact of acoustic cavitation induced by ultrasound emerges as a promising solution. For the first time, this study proposes the incorporation of ultrasound into the silver‑indium (Ag-In) TLP bonding process, enabling rapid and efficient bonding with 50 μm interlayers within seconds, while achieving a maximum shear strength of 60 MPa. A comprehensive comparative and analytical investigation of the interfacial microstructure, mechanical properties, and grain distribution has been carried out, in comparison to that of the conventional Ag-In TLP joint. The overall temperature profile was monitored utilizing a non-contact infrared camera and subsequently corroborated with finite element simulation outcomes. The phase analysis unveiled a predominant composition of the solid solution phase (Ag)-In within the bonding region, which contributes to a pronounced enhancement of the mechanical properties of the joints. Transmission electron microscopy (TEM) confirmed the complete degradation of the initial bonding interface, resulting in a bond with a bulk-like appearance. Finally, this work has elucidated the underlying mechanisms for the ultrasonic-enhanced transient liquid phase bonding (U-TLP) joint formation, where the influence of acoustic cavitation on element diffusion and the metallurgical bonding process has been discussed.

Transient liquid phase bonding using Cu foam and Cu–Sn paste for high-temperature applications

This study presents a new structure for transient liquid phase (TLP) bonding using Cu foam and Cu–Sn paste. Cu foam and Cu–Sn paste with large reaction surface areas were used to reduce the long bonding time, which is a disadvantage of the conventional TLP bonding process. After applying the Cu–Sn paste at an optimized ratio to the top and bottom of the Cu foam, pressure was applied to fill the pores inside the Cu foam with the paste, and a bonding process was performed at 260 °C under a pressure of 3 kgf. Bonded joints composed of only the Cu skeleton and Cu–Sn intermetallic compounds (IMCs) were confirmed within a bonding time of 5 min. In addition, the proportion of the Cu3Sn phase in the joint increased with the bonding time. After a bonding time of 40 min, the joint was mostly composed of a Cu skeleton and Cu3Sn, with a measured shear strength of approximately 39.2 MPa. After the joint analysis of the as-bonded samples, a high-temperature storage test (HTST) was conducted to evaluate their high-temperature long-term reliability. As a result of HTST, because most of the joint was converted to Cu3Sn after the bonding process, the shear strength did not decrease despite heat treatment at a high temperature of 240 °C, and a similar microstructure of the joint was maintained.

Effect of two-step heating transient liquid phase bonding on microstructure and mechanical behaviour of IN-738LC gas turbine components

The long isothermal solidification and homogenization time is the major concern in repairing turbine parts by the conventional transient liquid phase method. In this study, microstructure and morphology of the proposed two-step heating transient liquid phase joints were compared to results of the conventional ones conducted at the same temperature. After the isothermal solidification, deleterious precipitates such as Ni3Si, Ni3B and CrB were removed. The application of the two-step technique increased the shear strength of the joints up to 28% (516–674 MPa) due to collision of two non-planar interfaces and formation of the longer bonding line. The shear fracture surfaces of all bonds with the complete isothermal solidification stage represent a dimple pattern that is characterized by ductile fracture. The stress-rupture life of the joint at 982 °C was about 85% of the superalloy service life at this temperature.

Numerical Simulation Study of Temperature Gradient Transient Liquid Phase Bonding with Concentration-Dependent Diffusivity

A new numerical model is developed to study the kinetics of temperature gradient transient liquid phase (TG-TLP) bonding under concentration-dependent diffusivity by using a first-order implicit–explicit finite-difference numerical method and Landau coordinate transformation with adaptable spatial discretization. The model is validated with experimental data reported in the literature. The results of computational analysis by the new model show that the presence of solute concentration gradient in the liquid is the major factor that enables shorter solidification completion time in TG-TLP bonding compared to the conventional transient liquid phase bonding (C-TLP bonding). In contrast to the common assumption that solid-state diffusion can be ignored during the modeling of TG-TLP bonding, this work shows that solid-state diffusion plays a significant role, not only in controlling the transition in solidification behavior from bidirectional to unidirectional, but also affects the kinetics of the bonding process. Moreover, it is found that the anomalous increase in solidification completion time with increase in temperature that occurs during C-TLP bonding can be avoided by TG-TLP bonding. This is possible if the solute concentration gradient in the liquid is sufficiently high to enable adequate solidification kinetics to overcome increased volume of liquid that accompanies increase in bonding temperature.

Pressureless transient liquid phase bonding using SAC305 with hybrid Ag particles and its reliability under high-temperature storage test

Transient liquid phase (TLP) bonding has attracted much attention as a high-temperature interconnection method. TLP is a bonding method that converts entire joints into heat endurable intermetallic compounds (IMCs) by solid–liquid interdiffusion between high-melting-point (high-MP) parent metals and low-MP bonding metals. However, conventional TLP bonding requires high bonding pressure and long process time because IMC reactions occur only at the interfaces between the high-MP parent metals and the low-MP bonding metals.We fabricated TLP paste using SAC305 with 10 wt% Ag particles to induce IMC reactions to take place across an entire joint simultaneously. A TLP bonding process was performed at 300 °C for 2 h in air without added pressure. SAC residue was found in the SAC305 joint, while the whole SAC305–10 wt% Ag joint was converted into Cu–Sn and Ag–Sn IMCs. The SAC305–10 wt% Ag joint also demonstrated 2.5 times higher maximum shear strength compared with the SAC305 joint. In addition, the bonding reliability was evaluated by high-temperature storage test at 200 °C for 1000 h. The SAC305–10 wt% Ag joint maintained its bonding strength, while the SAC305 joint mechanically deteriorated.

Reliable single-phase micro-joints with high melting point for 3D TSV chip stacking

As micro solder-joints downsize to less than 10 μm in 3-dimensional (3D) TSV (Through Silicon Via) chip stacking, rapidly fabricating reliable micro-joints without collapse in chip stacking is regarded as a critical issue. A practicable method with a suitable temperature gradient (TG) and Ni/Sn(10 μm)/Ni interconnection structure was developed compared with conventional transient liquid phase (TLP) bonding. Using this method, the micro-joint fully composed of Ni3Sn4 intermetallic was fabricated within 17 min bonding time. It is nearly three times faster than that under conventional TLP bonding, which may overcome this technology limitation caused by longer bonding times. This fast TG-TLP method provided an asymmetry growth characteristic with thicker Ni3Sn4 at the cold end and thinner Ni3Sn4 at the hot end. The basic micro-joint formation mechanisms were suggested and experimentally verified. Furthermore, the properties of the fabricated Ni3Sn4 micro-joint were evaluated in this study. The results demonstrated its desirable features such as high melting temperature, stable single-phase constitution, stronger mechanical strength and low thermal resistance. Finally, the collapsed micro solder-joint issue was completely solved resulting in higher reliability.

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.

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.

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.

Influence of liquid state diffusion on joint microstructure developed during transient liquid phase bonding of dissimilar superalloys

Theoretical prediction of the influence of liquid-state diffusion on the joint microstructure developed during transient liquid phase (TLP) bonding of dissimilar superalloys, based on asymmetric numerical simulation model, is experimentally verified. A new understanding of the diffusional solidification mechanism during TLP bonding has enabled the formation of a single-crystal (SX) joint microstructure between dissimilar polycrystalline and SX substrates, which is considered unachievable by conventional TLP bonding theoretical models.

Grain refinement of transient liquid phase bonding zone using ODS insert foil

Joint strengthening of oxide dispersion strengthened (ODS) martensitic steel has been attained by a transient liquid phase (TLP) bonding utilizing a newly developed ODS insert foil. The ODS insert foil (Fe–9Cr–2W–0.2Ti–0.35Y2O3–0.5C–3B–2Si) was fabricated using mechanical alloying and a spark plasma sintering method. Compared to conventional TLP bonding with a non-ODS insert foil (Fe–0.5C–3B–2Si), the microstructure of the melted zone consists of finer grains in the joint with the newly developed ODS insert material, and the grain size is almost one third of that in the conventional insert material. This increases the hardness by ΔHv=100 in the region of the joints. Oxide particles that are coherent to the ferritic matrix in the ODS insert foil could be responsible for the grain refinement, which is explained in terms of enhanced nucleation of consolidation matrix at the oxide particles.

Transient liquid phase bonding of aluminium alloy using two-step heating process

5A02 aluminium alloy was joined by transient liquid phase (TLP) bonding using the two-step heating process as follows: the faying area was first heated to 600°C and kept for 5 s and then cooled down to 595°C and kept for 3 min. The microstructure and properties of the joint were investigated and compared with that of conventional TLP bond at 595°C for 3 min. The results show that the two-step heating process can produce a wave bond line that is different from the planar interface associated with conventional TLP bonding at a constant temperature. The defects at the bond line are greatly reduced, and metal to metal contacts are established along the wave bond line using the two-step heating process. Therefore, the tensile strength and bending ductility of the joint are dramatically improved.

Effects of boron and silicon on microstructure and isothermal solidification during TLP bonding of a duplex stainless steel using two Ni–Si–B insert alloys

The influence of B and Si on microstructure and isothermal solidification during transient liquid phase (TLP) bonding of a duplex stainless steel using MBF-30 (Ni–4·5Si–3·2B, wt-%) and MBF-35 (Ni–7·3Si–2·2B, wt-%), was investigated. Based on experimental studies, the formation of Ni3B within the joint centreline is dependent on B content; the morphology of Ni3Si is dominated by Si concentration. There was a deviation between the times for complete isothermal solidification obtained by the experiment and the conventional TLP bonding model. Isothermal solidification of the liquid dependent on different solidification regimes is suggested to be a controlling factor contributing to the change in the rate of isothermal solidification observed using the two filler metals.

Effect of solidification mechanism on microstructure and mechanical properties of joint in TLP bonded Al2024‐T6 alloy

Transient liquid phase (TLP) bonding of Al2024‐T6 alloy, using gallium (Ga) interlayer, has been investigated. Bonding process was carried out at 470°C for 6 min, and homogenising temperature and time were 495°C and 2 h respectively. Conventional TLP bonding using Ga interlayer was not an appropriate method for joining of Al2024. In this method, the boundary between two Al2024 specimens was not fully eliminated during bonding because of solidification with planar front. In addition, bonding zone was depleted of copper, and as a result, tensile and shear strength of joint decreased to 200 and 110 MPa respectively. TLP bonding under temperature gradient offered very good results in bonding of Al2024. In this method, solidification mechanism change from planar to dendritic, and tensile and shear strength of joint increased to about 460 and 220 MPa respectively. Microstructure of bonding zone changed basically by changing solidification mechanism.

Microstructural Evolution and Bonding Behavior during Transient Liquid-Phase Bonding of a Duplex Stainless Steel using two Different Ni-B-Based Filler Materials

Microstructural evolution and bonding behavior of transient liquid-phase (TLP) bonded joint for a duplex stainless steel using MBF-30 (Ni-4.5Si-3.2B [wt pct]) and MBF-50 (Ni-7.5Si-1.4B-18.5Cr [wt pct]) were investigated. Using MBF-30, the microstructure of the athermally solidified zone was dependent on B diffusion at 1333.15 K (1060 °C). Ni3B and a supersaturated γ-Ni phase were observed in this zone. BN appeared in the bonding-affected zone. However, using MBF-50, the influences of base metal alloying elements, particularly N and Cr as well as Si in the filler material, on the bond microstructure development were more pronounced at 1448.15 K (1175 °C). BN and (Cr, Ni)3Si phase were present in the bond centerline. The formation of BN precipitates in the bonding-affected zone was suppressed. A significant deviation in the isothermal solidification rate from the conventional TLP bonding diffusion models was observed in the joints prepared at 1448.15 K (1175 °C) using MBF-50.

On the Extension of Processing Time with Increase in Temperature during Transient-Liquid Phase Bonding

Transient-liquid phase (TLP) bonding of a nickel-based superalloy, IN 738, was performed. Contrary to conventional TLP bonding analytical models, which assume a parabolic relationship between liquid/solid interface migration and holding time, deviation from this law was observed experimentally and by numerical simulation. The deviation, which is caused by reduction in solute concentration gradient below a critical value, is suggested as an alternate phenomenon responsible for anomalous extension of processing time required to produce an eutectic-free joint with increase in bonding temperature. A decrease in the filler gap size and the use of a melting-point depressant (MPD) solute with higher solubility in the base material could reduce the occurrence of the anomalous behavior during a high-temperature TLP joining process.

Effect of two step heating process on joint microstructure and properties during transient liquid phase bonding

45MnMoB steel was joined by transient liquid phase (TLP) bonding using three two step heating processes. The joining area was first heated up to 1250°C and kept for 5 s, and then cooled down to 1210, 1220 and 1230°C named second holding temperature respectively and kept for 120 s. The interface morphology of the bond made using two step heating process was also compared with that of conventional TLP bond at a constant temperature. The results show that two step heating process can produce a cellular interface at the initial stage, which is different from the planar interfaces associated with conventional TLP bonding. No bond interface can be found in the final joint by two step heating process and the microstructure of joint is similar to that of base metal. With the increase in second holding temperature, oxide scale particles and porosities in the bond region are decreased noticeably both in size and amount, and the bond strength is increased. The bond fails at the base metal and no failure occurs when the joint is tensioned and bent to 180° at 1250 and 1230°C respectively. Therefore, two step heating process can produce a homogenous joint, free of bond interface and identical in composition and properties to that of the base metal.

Half-transient liquid phase diffusion welding: An approach for diffusion welding of SiCp/A356 with Cu interlayer

Aluminum matrix composite SiCp/A356 was welded by half-transient liquid Phase diffusion welding (HTLPDW) with a Cu interlayer. The effects of welding parameters and interlayer thickness on the properties of the welded joint were investigated, and the optimal welding parameters were subsequently put forward. The relationship between the tensile strength of the joint and the microstructure was studied by analyzing the microstructure of joint using a scanning electron microscope (SEM) and an electron probe micro-analysis (EPMA). Results confirmed the success of welding aluminum matrix composite SiCp/A356 utilizing HTLPDW method with a Cu interlayer. Shorter welding time was a prominent characteristic of HTLPDW as compared with conventional transient liquid phase (TLP) diffusion welding. Furthermore, its welded joints had a tensile strength almost 72% of its parent matrix composites, evidently signifying the suitable application of half-transient liquid phase diffusion welding in welding composite engineering structures.

Influence of process parameters on microstructure of transient liquid phase bonded Inconel 738LC superalloy with Amdry DF-3 interlayer

The effect of bonding temperature and time on microstructure of diffusion brazed joint of nickel base superalloy Inconel 738LC using Amdry DF-3 filler, alloy was investigated. It was observed that the formation of eutectic microconstituents, within the joint regions, was significantly influenced by the brazing temperature and time. A deviation from the conventional transient liquid phase (TLP) bonding diffusion models was observed in samples brazed above 1175 °C. The rate of isothermal solidification was substantially reduced at this brazing temperature, and also resulted in the formation of a centerline eutectic microconstituent, which was different from that observed at lower bonding temperatures. It is suggested that a probable factor contributing to the change in isothermal solidification rate and the formation of a different type of eutectic microconstituent from that observed at lower temperatures, is the considerable enrichment of the liquated insert with Ti atoms from the base alloy matrix, since they normally exhibit a lower solidification partition coefficient in nickel based alloys.

Effect of bonding temperature on isothermal solidification rate during transient liquid phase bonding of Inconel 738LC superalloy

A commercial cast Inconel 738 superalloy was transient liquid phase (TLP) bonded using a commercial Ni–Cr–B filler alloy, Nicrobraz 150. In contrast to the solidification behavior predicted by the current TLP models, isothermal solidification occurred under two separate regimes, depending on the bonding temperature. The rate of isothermal solidification in the first regime was faster than in the second regime. This led to a deviation in isothermal solidification completion time from that predicted by a conventional TLP model. The change in solidification rate was attributed to the substantial enrichment of the liquid interlayer with the base alloy solute elements and its continuous modification during isothermal solidification. These factors were also influenced in the nature of the phases formed in the centerline eutectic constituents subsequent to the incomplete isothermal solidification.