Transient Liquid Phase Bonding Research Articles

Low-Temperature Transient Liquid Phase Bonding Technology via Cu Porous-Sn58Bi Solid–Liquid System under Formic Acid Atmosphere

This study proposes a low-temperature transient liquid phase bonding (TLPB) method using Sn58Bi/porous Cu/Sn58Bi to enable efficient power-device packaging at high temperatures. The bonding mechanism is attributed to the rapid reaction between porous Cu and Sn58Bi solder, leading to the formation of intermetallic compounds with high melting point at low temperatures. The present paper investigates the effects of bonding atmosphere, bonding time, and external pressure on the shear strength of metal joints. Under formic acid (FA) atmosphere, Cu6Sn5 forms at the porous Cu foil/Sn58Bi interface, and some of it transforms into Cu3Sn. External pressure significantly reduces the micropores and thickness of the joint interconnection layer, resulting in a ductile fracture failure mode. The metal joint obtained under a pressure of 10 MPa at 250 °C for 5 min exhibits outstanding bonding mechanical performance with a shear strength of 62.2 MPa.

Simultaneous intermetallics suppression and residual-stress relaxation of heat-resistant Nb-interlayer-inserted Ti-6Al-4V/Si3N4 joints via one-step transient liquid phase bonding and brazing

• One-step bonding of Ti-6Al-4V/Nb/Si 3 N 4 joints by simultaneous transient liquid phase bonding and brazing. • Suppression of intermetallics in Ti-6Al-4V/Nb side of joints by transient liquid phase bonding. • Tailoring of filler's thickness for sound interfacial bonding in Nb/Si 3 N 4 side of joints. • Enhancement of residual-stress relaxation with highly ductile Ag-rich filler. • Elevated temperature strength of joints determined by tradeoff between stress relaxation and filler's weakening. This work has successfully proposed a solution to produce robust Nb-interlayer-inserted Ti-6Al-4V/Si 3 N 4 joints optimized for a maximum operating temperature of 873 K; transient liquid phase bonding (TLPB) of Ti-6Al-4V/Nb side was carried out with Cu and Ni fillers to suppress brittle intermetallic compounds (IMCs), whereas brazing of Nb/Si 3 N 4 side was performed using a highly ductile Ti-added Ag-rich filler for effective residual-stress relaxation. A sound yet simple one-step bonding process incorporating simultaneous TLPB and brazing was achieved with a relatively short holding time of 10 min at 1213 K. TLPB of Ti-6Al-4V/Nb side with Cu and Ni foils of 2-µm-thick each as a laminated filler suppressed brittle Ti-based IMCs and developed a homogenized microstructure consisting mainly of (α + β)-Ti via isothermal solidification. Meanwhile, brazing of Nb/Si 3 N 4 side with 100-µm-thick SILVER-ABA filler (92.75Ag-5Cu-1Al-1.25Ti mass%) foil enhanced interfacial bonding with sufficient total Ti content and accommodated residual stress better than conventional eutectic Ag-Cu-based fillers, and it was verified by finite element analysis with consideration of materials’ temperature-dependent elasto-plastic properties. All joints with a bonding area of 10 mm × 10 mm were tested via symmetrical four-point bending from room temperature (RT) to 873 K fractured from Nb/Si 3 N 4 side. When re-heating the joints from RT to 673 K, fracture initiation gradually shifted from Si 3 N 4 towards interfacial-compounds/Si 3 N 4 interface and bending strengths maintained ∼220 MPa as weakening of SILVER-ABA filler was compensated by residual-stress relaxation in Si 3 N 4 . When tested at 873 K, joints fractured mainly across the Ag-rich solid solution in a ductile manner and bending strength degraded by ∼20% to 171 MPa as weakening of SILVER-ABA filler dominated.

Microstructural and mechanical characteristics of Cu-Sn intermetallic compound interconnects formed by TLPB with Cu-Sn nanocomposite

Transient liquid phase bonding (TLPB) is a promising technology for three-dimensional integration of circuits (3D IC), but it can be slow and less productive. A novel Cu-Sn nanocomposite interlayer (Cu-Sn NI) composed of Sn matrix with an embedded Cu nanowire array prepared by electrodeposition can significantly accelerate the bonding process, approximately by 20 times. Bonding time with a Cu-Sn NI can be as short as ~2 min to achieve a full Cu-Sn intermetallic compound (IMC) joints, whereas it can take ~60 min with a pure Sn interlayer of the same thickness under the same bonding conditions (250 °C). Unlike the columnar Cu 6 Sn 5 grains commonly formed with Sn interlayer, refined equiaxed Cu 6 Sn 5 grains with an average size of ~1.6 µm are found to be formed with Cu-Sn NI. Such grain refinement has significantly contributed to the improvement of shear strength of IMC joints formed with Cu-Sn NI (23.1 ± 3.3 MPa), higher than those bonded with pure Sn interlayer (17.9 ± 2.1 MPa). The underlying mechanisms of the new TLPB process and the formation of finer microstructure when bonding with Cu-Sn NIs are also illuminated and validated based on the experimental observation. • A unique Cu-Sn nanocomposite interlayer with Cu nanowire array embedded in Sn. • TLPB with this layer is >20 times faster than conventional Cu-Sn TLPB. • IMC joint bonded with this layer possesses refined equiaxed Cu 6 Sn 5 grains. • Strength of IMC joint bonded with this layer is higher than pure Sn layer. • Clarified the growth mechanisms of IMC in the joints bonded with this layer.

Transient liquid phase bonding with Ga-based alloys for electronics interconnections

Significant demands on electronics packaging are driven by miniaturization, energy efficiency, high performance, and minimal carbon footprint. Liquid Ga and Ga-based alloys as interconnect materials exhibit a great potential in these applications due to the benefits including low melting points, non-toxicity, and stable high melting point intermetallic compounds (IMCs) formed with other metals such as Cu. Taking advantages of transient liquid phase bonding (TLPB) technology, the uses of Ga-based alloys can be attractive as a more efficient and environmentally friendly method for interconnections. This study aims to address the technical challenges in the applications of Ga and Ga-based alloys for Cu bonding through the TLPB process. These include solderability of Ga-based solders on Cu, interfacial reactions during bonding, as well as the microstructure and properties of the resultant joints. In addition, the porosity and shear strength of the joints have been evaluated for better understanding of the reliability of the joints. The results show that Ga-based solders present an acceptable solderability given their wetting behavior observed. The constituent of the resultant joints consists of θ − CuGa2 and γ3 − Cu9Ga4 IMC phases as identified, which are formed as the results of interfacial reactions between liquid Ga-based alloys and Cu substrate. Indium, as an alloying element in the solders, has been found to be able to accelerate γ3 − Cu9Ga4 formation and reduce the porosity in the joints. It is also found that the pressure applied during TLPB also helps the reduction of porosity, as well as the increase of mechanical strength of the joints. These results provided not only new insight into the mechanism of the interfacial reaction between Ga-based liquid metals (GLMs) and solid Cu systems, but also a new manufacturing route for electronics integration.

Evaluation of the Forming Limit Diagram (FLD) for St-Mg-St Multilayer Sheet Manufactured by Transient Liquid Phase (TLP) Bonding

Evaluation of the Forming Limit Diagram (FLD) for St-Mg-St Multilayer Sheet Manufactured by Transient Liquid Phase (TLP) Bonding

Measurement and simulation of residual stresses in transient liquid phase bonded ferritic steels

Ferritic steel bars—25 mm in diameter—were welded by transient liquid phase bonding (TLPB) using Fe-based amorphous metallic foils as filler material. The resulting residual stress (RS) field shows a low peak magnitude—of 147 MPa—as measured by neutron diffraction. The most distinctive feature of TLPB is the heat input delivered simultaneously at the whole joint that allows much lower cooling rates compared with arc welding (AW). Therefore, the elapsed time between 800 and 500 °C (t 8/5) was particularly long reaching 390 s. As a result, a low RS peak magnitude (147 MPa) was obtained in the as-welded condition. This value is well below the RS peak magnitude obtained with AW which typically attain the yield strength of the base metal (276 MPa). The numerical simulation of RS at the welded bars was performed by a thermal and mechanical analysis. It shows that TLPB produced a large austenized region, low cooling rates and a remarkable t 8/5. Consequently, the large volume in which the heat input is delivered is the driving force to reduce RS peak magnitudes. From the mechanical analysis, it was found that the simulated RS was in good agreement with the measured RS. Therefore, the proposed numerical simulation model can be used to predict RS in TLPB weldments.Graphical abstract

Low Temperature Transient Liquid Phase Bonding of Alumina Ceramics with the Bi2O3-ZnO Interlayer.

Alumina ceramics were joined by a transient liquid phase (TLP) bonding method at relatively lower temperatures, using mixed powders of Bi2O3 and ZnO with different weight ratios as interlayers between the ceramic components. Bonding was achieved at 750 °C for several of the prepared interlayer mixtures, which makes the applied approach attractive due to the relatively lower joining temperature and potentially low fabrication costs. Measurements by SEM and EDX were used to study the microstructure and chemical analysis of the obtained joints. It also allowed us to investigate the diffusion mechanism occurring in the systems, which resulted in the hypothesis that Zn2+/ZnO diffuses through the ceramics. XRD and Raman spectra were acquired to examine the reaction products that formed during the thermal treatment. The results showed that both ZnO and Bi2O3 react with each other as well as with alumina to form spinel and other products.

Defect formation and mitigation in Cu/Cu joints formed through transient liquid phase bonding with Cu-Sn nanocomposite interlayer

Joining based on transient liquid phase bonding (TLPB) has a great prospect in microelectronic packaging. However, this process is plagued with its low throughput. In this work, we designed and utilised a preformed Cu-Sn nanocomposite interlayer (Cu-Sn NI) to speed up the TLPB process. Comparing with the uses of a Sn interlayer only, the preformed Cu-Sn NI enabled the bonding process to be accelerated by >20 times. Furthermore, instead of columnar Cu6Sn5 grains, Cu-Sn intermetallic compounds joints formed with Cu-Sn NIs were mostly filled by refined equiaxed Cu6Sn5 grains with an average size of ~1.6 μm. As a result, the shear strength of IMC joints achieved with Cu-Sn NI was found to be higher than those bonded with Sn interlayer. However, various kinds of defects, one of the main drawbacks in TLPB, were still found from this unique bonding process, which is likely to deteriorate the performances and long-term reliability of resultant TLPB joints. This study aims to observe and analyse various defects formed during TLPB with Cu-Sn NI, hence propose possible mitigation solutions to prevent their occurrence and consequently improve the reliability of the joints obtained by TLPB with Cu-Sn NIs.

Conventional vs. Temperature-Gradient Transient Liquid Phase Bonding of Stainless Steel 304 Using a Multi-component (Fe–Ni–Mo–B) Filler Metal

Conventional vs. Temperature-Gradient Transient Liquid Phase Bonding of Stainless Steel 304 Using a Multi-component (Fe–Ni–Mo–B) Filler Metal

Transient liquid phase bonding (TLPB) of Cu to Cu using Sn interconnect solder reinforced by submicron Al particles

As an intermediate layer of transient liquid bonding, Sn interconnect solder is widely used in 3D chip stack interconnection due to its lower cost and excellent wettability. This study focuses on the effect of submicron Al particles on the interfacial microstructure and properties of Sn interconnect solder. Adding submicron Al particles efficiently restrained the growth of interfacial IMC and improved the wettability and mechanical properties of Sn interconnect solder, but it has little effect on the melting characteristics. When the 0.3 wt% submicron Al particles are added into Sn interconnect solder, the spreading area, shear strength and microhardness were improved by 69.5 %, 45.6 % and 20 %, respectively. Based on the content optimization of Al particles, the microstructure evolution and bonding strength of Sn-0.3Al interconnect solder as transient liquid bonding intermediate layer at different bonding times were studied. It was found that the growth rate of IMC and the number of voids in Sn-0.3Al TLPB joints were lower than those in the Sn TLPB joints. The bonding strength of TLPB joints increased with the increase of bonding time, and the bonding strength of Sn-0.3Al TLPB joints was always higher than that of TLPB joints without submicron Al.

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.

Recent Studies of Transient Liquid Phase Bonding Technology for Electric Vehicles

Recently, interest in power modules is increasing in eco-friendly mobility such as electric vehicles (EVs) and fuel cell electric vehicles (FCEVs). The power module is a key component of the eco-friendly mobility that drives a motor by converting direct current (DC) supplied from the battery into alternating current (AC). However, a lot of heat is generated in the power module during its operation. To ensure the reliability of power modules, it is essential to form a joint suitable for high-temperature operation. Many pieces of research have been published on various bonding methods such as soldering, Ag sintering, Cu-Cu direct bonding, and transient liquid phase (TLP) bonding. The TLP bonding has many advantages, such as significantly less cost and a high melting temperature joint. In this review paper, various TLP bonding technologies with different materials and methods have been introduced and compared.

Strength optimization of low-pressure transient liquid phase bonded Ti–6Al–4V similar joint with Cu–Ni multi-interlayer

The pressure reduction of transient liquid phase bonding (TLPB) technique with sound Ti–6Al–4V similar joint is reported. Eliminating brittle intermetallic compounds (microstructural homogenization) and voids at the interface (void closure) is critical for sound joints with bending strength above the yield strength of Ti–6Al–4V. These two goals are achieved by adjusting the holding time (0.5–12 h) and bonding pressure (0.001–0.3 MPa) at a fixed bonding temperature of 950 °C below the phase transus of the substrates. The maximum bending strength of 1570 MPa is obtained with joint having a homogenized microstructure without any voids under joining condition held for 12 h with 0.03 MPa corresponding to 3% of the conventional TLPB technique. The holding time for microstructural homogenization is reduced with 0.3 MPa bonding pressure by liquid push-off from the interface. Low-pressure TLPB is recommended for cost reduction and ensuring shape accuracy of complex geometry components.

Fast and Reliable Ag\u2013Sn Transient Liquid Phase Bonding by Combining Rapid Heating with Low-Power Ultrasound

Ag–Sn transient liquid phase (TLP) bonding is capable of producing joints with excellent mechanical and thermal properties, and with operation temperatures at or even above the processing temperature. However, reduction of the comparatively long processing times required to achieve high-quality joints is highly desirable. To this end, the use of fast heating and low-power ultrasonic (US) pulses for accelerated Ag–Sn TLP bonding is investigated with particular focus on defect generation and avoidance. Employing rapid heating with 20,{text {K s}}^{-1}, the process time can indeed be strongly reduced, yielding a high average shear strength of (58 pm 22) MPa. The large variance of the shear strength is caused by gas entrapment and pronounced lateral squeeze-out of liquid Sn, facilitated by the suppression of {text{Ag}}_{3} {text{Sn}} formation upon rapid heating. Introduction of a weak US pulse leads to enhanced {text{Ag}}_{3} {text{Sn}} formation and efficient removal of entrapped gas, and thus to avoidance of large-scale bond defects, which results in an enhanced shear strength of (104 pm 8) MPa.

Effect of post bonding heat treatment on the local strain evolution of transient liquid phase bonded RAFM steel

In this paper, post bonding heat treatments to transient liquid phase bond reduced activation ferrite/martensitic steel were conducted, with a Fe–Si–B amorphous foil as the interlayer. The results show that, the post bonding heat treatments decreased the strength difference between different regions of the transient liquid phase bonded joint and altered the shape and the amount of Cr2B along the interface between the diffusion affected zone and base metal. In particular, it is found that the tensile fracture position of the as-bonded joint has been altered by post bonding heat treatment, from the interface between the diffusion affected zone and the base metal to where inside the base metal. In order to unveil such deformation differences between transient liquid phase bonded joints with and without post bonding heat treatment, digital image correlation method was adopted to study the influence of post bonding heat treatment on the local strain evolution of transient liquid phase bonded joint from both macro- and micro-scale. The non-uniform strain distribution upon the nominal surface of the tensile samples during deformation showed that, the strain of the as-bonded joint was always localized at the interface of the diffusion affected zone, resulting brittle fracture eventually. While for the joint after post bonding heat treatment, the strain localization gradually transferred from the interface to the base metal during the tensile test, which is attributed to the changing deformation resistance of these regions during straining.

Development of 3D Wafer Level Hermetic Packaging with Through Glass Vias (TGVs) and Transient Liquid Phase Bonding Technology for RF Filter.

The development of 5G mobile communication created the need for high-frequency communication systems, which require vast quantities of radio frequency (RF) filters with a high-quality factor (Q) and low inband losses. In this study, the packaging of an RF filter with a through-glass via (TGV) interposer was designed and fabricated using a three-dimensional wafer-level package (3D WLP). TGV fabrication is a high-yielding process, which can produce high precision vias without masking and lithography and reduce the manufacturing cost compared with the through silicon via (TSV) solution. The glass interposer capping wafer contains Cu-filled TGV, a metal redistribution layer (RDL), and the bonding layer. The RF filter substrate with Au bump is bonded to the capping wafer based on Au-Sn transient liquid phase (TLP) bonding at 280 °C with a 40 kN (approximately 6.5 MPa) bonding force. Experimental results show that shear strengths of approx. 54.5 MPa can be obtained, higher than the standard requirement (~6 MPa). In addition, a comparison of the electrical performance of the RF filter package after the pre-conditional level three (Pre-Con L3) and unbiased highly accelerated stress (uHAST) tests showed no difference in insertion attenuation across the passband (<0.2 dB, standard value: <1 dB). The final packages passed the reliability tests in the field of consumer electronics. The proposed RF filter WLP achieves high performance, low cost, and superior reliability.

Asymmetric Cu Diffusion in Transient Liquid Phase (TLP) Bonding of Inconel 625 to AISI 316L Stainless Steel with Cu Interlayer

Asymmetric Cu Diffusion in Transient Liquid Phase (TLP) Bonding of Inconel 625 to AISI 316L Stainless Steel with Cu Interlayer

Strength optimization of two-step-bonded Ti-6Al-4V/Si3N4 joint with Nb interlayer via transient-liquid-phase bonding and active-metal brazing

A novel two-step bonding of Ti-6Al-4V/Si3N4 joint was developed with Nb interlayer as residual-stress reliever via low-pressure transient-liquid-phase bonding (TLPB) of Ti-6Al-4V/Nb side prior to active-metal brazing of Nb/Si3N4 side. While 1.75 mass% of Ti in a 50-µm-thick CUSIL-ABA® filler was sufficient for sound bonding at Nb/Si3N4 side when brazed at 1103 K for 10 min, one-step-brazed joints with bonding area of 10 × 10 mm2 were prone to failure at the Ti-6Al-4V/Nb side due to brittle Cu-Ti intermetallic compounds (IMCs). Replacing brazing of Ti-6Al-4V/Nb side with TLPB using pure Cu and Ni foils as filler at 1213 K for 180 min eliminated the formation of brittle IMCs via homogenization of (α + β)-Ti; bending strength increased to 193 MPa with residual-stress-induced failure from Si3N4 ceramics. Finally, effectiveness of stress-accommodation via Nb interlayer and filler’s plastic flow was quantitatively verified with reasonable fidelity by finite-element analysis incorporating temperature-dependent elasto-plastic properties.

Bubble formation and growth during Transient Liquid Phase Bonding in Cu/SnAg system for microelectronic packaging

Bubble formation and growth during Transient Liquid Phase Bonding in Cu/SnAg system for microelectronic packaging

Ultrasonically enhanced flux-less bonding with Zn-5Al alloy under ambient condition for high-temperature electronics interconnects

Zn-5Al eutectic alloy is a promising Pb-free solder suitable for high-temperature electronics packaging. However, the soldering with Zn-5Al alloy is undermined by its poor oxidation resistance and wettability in ambient atmosphere, which also demands significant time to complete if transient liquid phase soldering (TLPS) is required. In this work, the TLPS assisted by ultrasonic vibration (USV) under ambient condition without flux has been performed to enhance and accelerate interfacial reactions between Zn-5Al and Cu or Ni substrate. It has been found that the resultant full intermetallic compounds (IMCs) joints with uniform microstructure can be formed within 1 min and 3 min on Cu and Ni substrate, respectively, without flux or protective inert gases. The IMCs resulted from the interfacial reactions have been identified after the completion of TLPS process. For the ultrasonic-assisted TLPS (U-TLPS) of Zn-5Al onto Cu substrate, the joints consist of three IMC layers, CuZn, Cu5Zn8, and CuZn with the residual oxide fragments remained at the central line inside of IMC Cu5Zn8, where Al is rich likely due to the segregation of element Al during the TLPS process. For the U-TLPS of Zn-5Al onto Ni substrate, three IMC layers, Ni5Zn21, NiAl and Ni5Zn21 IMC were also observed, with the porous Ni5Zn21 IMC layers present in adjacent to the Ni substrate. Both residual oxide fragments in the Zn-5Al/Cu joints and porous Ni5Zn21 IMC in the Zn-5Al/Ni joints could potentially deteriorate mechanical integrity of the joints, which is yet to be understood.