Low Temperature Transient Liquid Phase Research Articles

Interfacial intermetallic compounds growth kinetics and mechanical characteristics of Ga-Cu interconnects prepared via transient liquid phase bonding

Driven by continuous miniaturization, higher performance, and functional robustness in three-dimensional (3D) embodiments of electronic devices, Ga which possesses a low melting point (29.8°C) has shown its potential uses in electronics packaging and integration. The use of Ga metal as a solder material to achieve low-temperature transient liquid phase bonding (TLPB) has emerged as a viable solution for reliable interconnections of future-generation products. This study investigates the interfacial intermetallic compounds (IMCs) structure, growth kinetics, and mechanical characteristics of Ga-Cu TLPB joints in the temperature range of 150–220°C. The Cu9Ga4 phase were observed and the growth rate of Ga-Cu IMCs has been investigated. The research findings indicate that the growth mechanism of interface IMCs is primarily diffusion-controlled, with CuGa2 grains exhibiting anisotropic growth. Their shear strength has reached 23.8 MPa, with a porosity of 1.82±0.07%. Furthermore, the fracture interface is presented and correlated with the experimental observations.

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.

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.

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

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

Microstructure evolution and grain orientation of IMC in Cu-Sn TLP bonding solder joints

In this work, Cu/Sn/Cu solder joints were fabricated by low temperature transient liquid phase (TLP) bonding. The microstructure evolution and grain orientation of intermetallic compounds (IMC) in Cu-Sn solder joints were systematically investigated by using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). With the bonding time increased, a typically scallop-type Cu6Sn5 was formed at the interface between Sn/Cu and then gradually thickened to form a full-IMC solder joints. The higher the temperature, the faster the IMC grew. Besides, the asymmetry growth of IMC at both ends of the solder joints was clearly observed due to the temperature gradients. The corresponding EBSD results indicated that Cu6Sn5 and Cu3Sn grains showed the preferred orientation of (001) and (100), respectively. Measurement of the kinetic exponent of IMC suggested that the growth mechanism of Cu3Sn is controlled by grain boundary diffusion. In addition, the wettability and mechanical property of the solder joints were also studied. It was found that the wettability and shear strength of the solder joints improved as the bonding temperature and bonding time increased. The shear fracture analysis of the solder joints revealed that most fractures of the Cu-Sn solder joints have the characteristics of brittle fracture.

Low temperature transient liquid phase bonded Cu-Sn-Mo and Cu-Sn-Ag-Mo interconnects – A novel approach for hybrid metal baseplates

Transient liquid phase (TLP) bonding is one of novel techniques, which is an alternative to conventional soldering used in power electronics packaging. In this paper, we report on the formation and analysis of TLP bonded Cu-Sn-Mo and Cu-Sn-Ag-Mo systems for production of hybrid metal baseplates. The obtained bonds of intermetallic compounds are characterised by shear testing, optical microscopy and energy dispersive X-ray spectroscopy. Cu-Sn-Mo systems show low bond strength due to severe void and crack formation while Cu-Sn-Ag-Mo interfaces demonstrate high homogeneity and considerable shear strength. Thus, it is concluded that the low temperature TLP bonding using Cu-Sn-Ag-Mo can be a promising candidate for the fabrication of hybrid Cu-Mo baseplates. However, deeper studies of thermal reliability of these systems are required.

Microstructure evolution and mechanical properties of Cu/Sn/Ag TLP-bonded joint during thermal aging

Low-temperature transient liquid phase (TLP) bonding is a promising joining technology for high-temperature electronic devices packaging. However, the effect of thermal aging on the joint microstructure and performance has been rarely studied so far. In this paper, the relationship between microstructural evolution and mechanical property was developed for the Cu/Sn/Ag TLP-bonded joint during 350 °C aging. Experimental results show that an interfacial reaction occurs between the intermetallic compounds (IMCs) of Cu3Sn and Ag3Sn, which induces that a great amount of Cu3Sn particles or wedges embed into the Ag-Sn phase layer. The reaction process is controlled by a volume diffusion of Cu elements from the Cu substrate, and capable of promoting the transformation from Ag3Sn into Ag(Sn) solid solution. Cu41Sn11 phase can be formed at the expense of Cu3Sn and Cu, of which transformation process is completely achieved after aging for 480 h, and the Cu(Sn) solid solution intimately contacts with the opposite Ag(Sn) solid solution in many areas after aging for 960 h. The joint mechanical property initially declines due to the formation of Cu41Sn11 phase and Cu-Sn IMCs grain coarsening, and then increases because of the complete depletion of Cu3Sn and substantial precipitation of Cu(Sn) solid solution. After aging for nearly 1000 h, the joint still maintains a high shear strength of >40 MPa, indicating an excellent thermal reliability.

Elimination of pores in Ag–Sn TLP bonds by the introduction of dissimilar intermetallic phases

Low-temperature transient liquid phase (TLP) bonding is a very promising technology for achieving die attachment in high-temperature power devices. However, the Ag–Sn TLP-bonded joint has high sensitivity to shrinkage pores, which will lead to the deterioration of mechanical, thermal, and electrical properties. In this study, we have proposed two novel methods to solve the problem of pores in Ag–Sn TLP bonding through the introduction of second phases. The first method is replacing Ag substrate on one side with Cu substrate to create dual layers of dissimilar intermetallic compounds (IMCs). Consequently, the dual layers of Cu6Sn5 (or Cu3Sn) and Ag3Sn IMCs emerge on the cross-sections of bonded joint, resulting in the effective elimination of pores, which can be attributed to the change of the microstructure and the interface migration between two dissimilar IMC layers (e.g., Cu6Sn5/Ag3Sn). The second method is coating a thin Cu film on the Ag substrate to introduce Cu–Sn IMC particles. As a result, a great number of Cu6Sn5 particles disperse in the middle of the Ag3Sn layer, filling the micropores efficiently, such that a significant decrease in shrinkage pores is achieved. Both methods have been experimentally verified to improve the mechanical properties, and they have high potential to be implemented in other TLP systems.

Microstructure characterization and mechanical behavior for Ag3Sn joint produced by foil-based TLP bonding in air atmosphere

Low-temperature transient liquid phase (TLP) bonding for Ag-plated substrates was systematically investigated by using foil-based interlayer of pure Sn foil or preformed Sn/Cu/Sn sandwich structure in air atmosphere. The influences of bonding process, such as bonding temperature, bonding time and foil thickness, on the microstructure characterization and mechanical behavior of TLP joint were discussed. Experimental results show that Ag-plated substrates can be successfully TLP bonded in air atmosphere by the protection of flux. The formation of pores in intermetallic compounds (IMCs) is a serious problem for Ag/Sn/Ag TLP bonding, which is attributed to the volume shrinkage of isolated Sn areas during isothermal solidification. Prolonging homogenization time, properly increasing bonding pressure, decreasing temperature, or reducing interlayer thickness can effectively reduce the shrinkage porosity, but is still incapable of eliminating pores thoroughly. Both shear bands and intergranular facets are simultaneously observed on the fracture surface of Ag3Sn joint. Since the micro voids distributing along Ag3Sn grain boundaries weaken the cohesion strength between two neighboring Ag3Sn grains in some areas. Using preformed Sn/Cu/Sn interlayer is available to enhance the mechanical integrity, which is strongly depended on the Cu thickness and Sn thickness. The joint shear strength can be increased by even more than 100% by the introduction of Cu foil. Moreover, the remained Cu layer in the IMCs can act as a buffer layer during fracture process, leading to the improvement of the ductility of TLP joint.

Interfacial reaction and mechanical properties for Cu/Sn/Ag system low temperature transient liquid phase bonding

Low temperature transient liquid phase (LTTLP) bonding is a promising technology to enable in high temperature electronic packaging. In this study, interfacial reaction and mechanical characterizations for Cu/Sn/Ag system LTTLP bonding at temperatures ranging from 260 to 340 °C for various time were investigated. Experimental results showed that Cu and Ag substrate independently reacted with molten Sn, and the growth of IMCs on one side was hindered by the opposite IMCs layer after scalloped Cu6Sn5 contacted with the Ag3Sn, and there was no ternary alloy phase formed all the time. Pores were found and distributed at the Cu6Sn5/Ag3Sn interface or between grain boundaries after the residual Sn was fully consumed, however, they gradually disappeared with continuing reaction of that Cu6Sn5 phase converted into Cu3Sn phase. Shear strength of the LTTLP joints increased with increasing bonding time, and the adhesive strength of Cu6Sn5/Ag3Sn interface was weaker than that of the Cu3Sn/Ag3Sn interface. The rupture behaviors were also discussed with a fracture model. As follow, cracks initiated in the pore and mainly propagated along the Cu6Sn5/Ag3Sn interface for the joint consisted of layered Cu3Sn, Cu6Sn5 and Ag3Sn IMCs, however, failure path only passed through the Cu3Sn layer after Cu6Sn5 islands were completely transformed.

Magnetoelectric macro fiber composite

This paper describes the fabrication and performance results of a magnetoelectric macro fiber composite (ME MFC). The magnetoelectric composite was fabricated by bonding a magnetostrictive layer to a piezoelectric layer using a novel approach of low temperature transient liquid phase (LTTLP) bonding. The composite was diced into 150 micron wide fibers and bonded to a custom designed copper flexible circuit using a spin coated low viscosity room temperature curing epoxy. ME MFC’s with varying ferrite thicknesses of 0.6mm and 0.5mm were fabricated and characterized for energy harvesting. The composite with 0.6mm ferrite thickness achieved an open circuit voltage of 101mV (ME voltage coefficient of 6740mV/cmOe) and peak power of 3.1nW across 356kΩ matching load at 264Hz.

LT-TLPS Die Attach for High Temperature Electronic Packaging

Low temperature transient liquid phase sintering (LT-TLPS) can be used to form high-temperature joints between metallic interfaces at low process temperatures. In this paper, process analyses and shear strength studies of paste-based approaches to LT-TLPS are presented. The process progression studies include DSC analyses and observations of intermetallic compound (IMC) formation by cross-sectioning. It was found that the sintering process reaches completion after sintering times of 15 min for process temperatures approximately 50°C above the melting point of the low temperature constituent. For the shear studies, test samples consisting of copper dice and copper substrates joined by sintering with a variety of sinter pastes with different ratios of copper and tin have been assessed. A fixture was designed for high temperature enabled shear tests at 25°C, 125°C, 250°C, 400°C, and 600°C. The influence of the ratio of the amount of high melting-point constituent to the amount of low melting-point constituent on the maximum application temperature of the sinter paste was analyzed. Ag20Sn and Cu50Sn pastes showed no reduction in shear strength up to 400°C, and Cu40Sn pastes showed high shear strengths up to 600°C. It was shown that LT-TLPS can be used to form high temperature stable joints at low temperatures without the need to apply pressure during processing.

Strength and Reliability of High Temperature Transient Liquid Phase Sintered Joints

Low-temperature transient liquid phase sintering (LT-TLPS) can be used to form high-temperature joints between metallic interfaces at low process temperatures. In this paper, we will describe the processing and shear strength, along with the shock fatigue resistance of sintered joints made by this process. Joints made from different ratios of Ni and Cu high melting temperature constituents paired with Sn-based low melting temperature constituents have been evaluated. For the shear studies, the softening behavior of test samples joined by Ni-Sn3.5Ag and (Ni,Cu)-Sn3.5Ag sinter pastes have been assessed using a fixture designed for high temperature shear testing up to 600°C. The reliability of sinter paste joints in drop-shock environments will be discussed. It is shown that joints formed from these sinter pastes possess improved drop-shock reliability compared to Sn3.5Ag solder joints and melting temperatures considerably higher than those of conventional high temperature solders (e.g. Pb5.0Sn2.5Ag).

LT-TLPS Die Attach for High Temperature Electronic Packaging

Low temperature transient liquid phase sintering (LT-TLPS) can be used to form high-temperature joints between metallic interfaces at low process temperatures. In this paper, process analysis and shear strength studies of paste-based approaches to LT-TLPS are presented. The process progression studies include DSC-analyses and observations of intermetallic compound (IMC) formation by cross-sectioning. It was found that the sintering process reaches completion after sintering times of 15 minutes for process temperatures around 50°C above the melting point of the low temperature constituent. For the shear strength studies, test samples consisting of copper dice and copper substrates joined by sintering with a variety of pastes having different ratios of copper and tin have been assessed. A fixture was designed for high temperatures enabled shear tests at 25°C, 125°C, 250°C, 400°C, and 600°C. The influence of the ratio of the amount of high melting-point constituent to the amount of low melting-point constituent on the maximum application temperature of the sinter paste was analyzed. Ag20Sn and Cu50Sn pastes showed no reduction in shear strength up to 400°C, Cu40Sn pastes showed high shear strengths up to 600°C, and extended curing further increased the joint strength. It was shown that LT-TLPS can be used to form high temperature stable joints at low temperatures without the need of applying pressure during processing.

High Temperature Sintered Interconnects Formed by Transient Liquid Phase Sintering

Low Temperature Transient Liquid Phase Sintering (LT-TLPS) enables the formation of joints robust to high temperatures at low process temperatures. TLPS systems consist of one or more low temperature constituents (i.e. Sn) and one or more high temperature constituents (i.e. Cu). The sinter joints are formed by intermetallic compound formation between these constituents. In this paper a paste based LT-TLPS approach is demonstrated. The organic binders and fluxes used to mix the pastes prevent the metal particles from oxidation and facilitate a vacuum-free process in air without the need of a reducing atmosphere. Pastes based on the Cu-Sn system have been developed enabling a completely pressure-less process. Furthermore sinter pastes for LT-TLPS at low pressure (<0.5MPa) applied during the initial stage of the sintering process have been developed which form almost void free joints. To assess the strength of the sintered joints a high-temperature shear fixture has been designed. Shear tests have been performed at 25°C, 400°C, and 600°C to characterize the influence of high temperature conditions on the joint performance. The shear strength of the joints formed without pressure has been assessed for different Cu-to-Sn ratios at all temperature levels. It is shown that the maximum application temperature and shear strength depends on the ratio of low melting temperature and high melting temperature constituents. The pastes introduced here can be used to form joints resilient to application temperatures of up to 600°C. They show the potential to form joints for reliable operation under extreme temperature conditions.

Low temperature transient liquid phase bonding of Au/Sn and Cu/Sn electroplated material systems for MEMS wafer-level packaging

This paper presents the development of a low temperature transient liquid phase bonding process for 8ź wafer-level packaging of micro-electro-mechanical systems. Cu/Sn and Au/Sn material systems have been investigated under varying bonding temperatures from 240 to 280 °C and different dwell times from 8 to 30 min. The used bond frame had a width of 80 μm and lateral dimensions of 1.5 mm × 1.55 mm. The sealing frame of the cap wafer consisted of Au and Cu, respectively, and Sn. The MEMS wafer only holds the parent metal of Au or Cu. High quality bonds were confirmed by shear tests, cleavage analysis, polished cross-section analysis using optical and electron microscope, energy dispersive X-ray spectroscopy and pressure cocker test. The samples showed high shear strength (>80 MPa), nearly perfect bond regions and no main failure mode in the cleavage analyses. Non-corroded Cu test structures confirmed the hermeticity.

Kinetic study of low-temperature transient liquid phase joining of an aluminum-sic composite

The transient liquid phase (TLP) joining of an aluminum 6061-SiC composite using a gallium interface has been investigated. The observed kinetics of this process suggest that diffusion occurs along interphase and subgrain boundaries at low temperatures (<723 K). An existing TLP model has been modified to account for this behavior.

Low Temperature Transient Liquid Phase (LTTLP) Bonding for Au/Cu and Cu/Cu Interconnections

The Low Temperature Transient Liquid Phase Diffusion Bonding (LTTLP) process has been used to reduce the residual stresses and hence, improve the thermal fatigue performance of bonds between dissimilar electronic packaging materials. Aluminum oxide plated with either gold or copper has been bonded to copper heatsinks at temperatures less than 160°C, using In-Sn eutectic solders. After two hours at the bonding temperature, Cu/In-Sn/Cu joints exhibit a remit temperature in excess of 470°C and provide static strengths up to four times greater than the original solder bonds. Au/In-Sn/Cu joints do not exhibit a high remit temperature due to the formation of a stable Au-In-Sn phase which melts below 130°C. Both types of joints exhibited acceptable thermal fatigue performance for the conditions which were studied.

In Situ Reaction of B2O3 with AlN and/or Si3N4 to Form BN‐Toughened Composites

BN‐toughened oxide matrix composites were formed by the in situ reaction of B2O3 with Si3N4 and/or AlN. A lowtemperature transient liquid phase aids densification at &lt;1000°C, and the process tends to produce an intrinsically homogeneous microstructure. Mechanical properties and microstructure of the composites formed in situ were compared to those of composites prepared by conventional means from oxide and BN powders. Fracture toughness and flexural strength of the nearly isotropic in situ formed composites ranged from 2.82 to 3.66 MPa · m1/2 and 130 to 320 MPa, respectively, with Young's moduli of 100 to 110 GPa. Densities achieved ranged from 90% to 97% of estimated theoretical densities. The strength and toughness values are intermediate to the extreme values for the anisotropic composites formed by hot‐pressing mixed powders.