Transient Liquid Phase Sintering Research Articles

Effects of Cu/SnAgCu Powder Fraction and Sintering Time on Microstructure and Mechanical Properties of Transient Liquid Phase Sintered Joints.

The effects of the sintering duration and powder fraction (Ag-coated Cu/SnAgCu) on the microstructure and reliability of transient liquid phase sintered (TLPS) joints are investigated. The results show that two main intermetallic compounds (IMCs, Cu6Sn5 and Cu3Sn) formed in the joints. The Cu6Sn5 ratio generally decreased with increasing sintering time, Cu powder fraction, and thermal treatment. The void ratio of the high-Cu-fraction joints decreased and increased with increasing sintering and thermal stressing durations, respectively, whereas the low-Cu-fraction counterparts were stable. We also found that the shear strength increased with increasing thermal treatment time, which resulted from the transformation of Cu6Sn5 and Cu3Sn. Such findings could provide valuable information for optimizing the TLPS process and assuring the high reliability of electronic devices.

Analysis of initial stage of liquid-phase sintering by combined multi-phase-field/discrete-element method

A combined multi-phase-field/discrete-element method in two dimensions is applied to the liquid phase sintering process of blended elemental powders. The boundary migration, diffusion of elements, and phase transformation between two dissimilar particles are calculated using a multi-phase-field method for sintering based on Gibbs energy functions. The sintering shrinkage is computed with the discrete-element method taking account of the neck size and sintering force, which are obtained from a multi-phase-field simulation. In this study, a pair or small cluster consisting of two elemental powders in a binary eutectic alloy is used as a model system. First, the formation of a liquid phase around the contact area between two dissimilar particles above the eutectic temperature of the system, and the motions of the particles due to grain boundary sliding along the liquid films are confirmed. Second, an Sn–Bi system is selected as the real eutectic system, and the sintering behavior of powder compacts with various mixing ratios of Sn and Bi particles is demonstrated. Finally, transient liquid-phase sintering of Sn–Bi system is simulated, that is, the generation and extinction of the liquid phase due to interdiffusion between Sn and Bi particles are calculated.

Textured Pb(Mg1/3Nb2/3)O3 −Pb(In1/2Nb1/2)O3 −PbTiO3 ceramics with enhanced piezoelectric properties and high Curie temperature prepared by low-temperature sintering

A low-temperature sintering is crucial for the fabrication of multilayer piezoceramic actuators through low-cost internal metal electrodes. Herein, Pb(Mg1/3Nb2/3)O3−Pb(In1/2Nb1/2)O3 −PbTiO3 (PMN−PIN−PT) textured ceramics were prepared by a combined use of refined matrix powder and CuO sintering additive. When the matrix powder was refined by planetary ball-milling for 10 h, the sintering temperature of the textured 0.37PMN−0.29PIN−0.34PT ceramics was lowered to 1175 ºC, and a subsequent addition of 1 mol% CuO further lowered it to 950 ºC. The textured 0.37PMN−0.29PIN−0.34PT ceramics with 3 vol% BaTiO3 (BT) template sintered at 950 ºC for 10 h yielded excellent piezoelectric properties, such as a Lotgering factor (LF) of 97%, a unipolar strain of 0.23% at 2 kV/mm, and a Curie temperature (TC) of 231 ºC. These enhanced piezoelectric properties and high TC result from the high texture, transient liquid phase sintering, Cu2+ ion doping, and minimum BT template content.

ZrB2–MoSi2 ceramics: Kinetics of compaction during hot pressing in СО environment, phase interactions at interfaces and static fatigue

Ceramics in the ZrB2–MoSi2 system pass through different stages during hot pressing in CO-CO2 environment. The first stage follows a smooth parabolic curve up to 1600 °C, then, a sharp compaction to full density occurs by transient liquid-phase sintering in the 1610–1850 °C temperature range. Ceramics sintered in this range have the primary compounds, ZrB2 and MoSi2, disappeared and new phases, such as (ZrB2)SS, SiC, B4C and ZrC, formed. The high sintering rate are associated to exothermic phase transformations, which significantly increase the temperature in the diffusion zones. In the 1850–2150 °C range, complete homogenization of the solid solution based on ZrB2 occurs and the sintering curve follows that typical of solid-state densification, i.e. increases continuously and slowly. Materials sintered at 1370–1850 °C have matrix grain size around 0.6–1.8 µm, while those sintered at 1940–2150 °C have grain size around 8–10 µm. Hot compression tests showed that the ZrB2-MoSi2 ceramic sintered at 1870 °C had the highest creep resistance compared with a ZrB2-SiC baseline.

Kinetics of Cu6Sn5 and Cu3Sn intermetallic compounds growth and isothermal solidification during Cu-Sn transient liquid phase sintering process

The kinetics of Cu-Sn intermetallic compounds (IMCs) growth relates to the materials preparation, process regulation and reliability prediction in the new energy field and electronic packaging industry. As a typical instance that Cu-Sn IMCs are involved with, this work investigated the kinetics of Cu-Sn IMCs growth and isothermal solidification of Cu-Sn transient liquid phase sintering (TLPS) process for high temperature resistant packaging of the third-generation semiconductors power devices. A three-dimensional kinetics model of Cu/Sn solid/liquid interfacial reaction and the corresponding kinetics equations of Cu6Sn5 and Cu3Sn growth was established. Based on Cu-Sn IMCs growth kinetics, a parameter ξSn called the residual rate of Sn was defined to characterize the isothermal solidification kinetics of Cu-Sn TLPS process. By measuring the volume fraction of Cu and Sn, the quality fraction of residual Cu and Sn and produced Cu-Sn IMCs were determined, and thus the kinetics of Cu-Sn IMCs growth and the isothermal solidification of Cu-Sn TLPS process were experimentally studied to validate the theoretical model. The results showed that the theoretical results predicted by the model were well-matched with experimental results. Besides rising temperature, the kinetics of Cu-Sn IMCs growth and the isothermal solidification can be remarkably accelerated by decreasing Cu particle size. Reducing Sn content does not affect the kinetics of Cu-Sn IMCs growth but helps to shorten the isothermal solidification time of Cu-Sn TLPS process.

High-Performance Skutterudite/Half-Heusler Cascaded Thermoelectric Module Using the Transient Liquid Phase Sintering Joining Technique.

Thermoelectric (TE) materials have made rapid advancement in the past decade, paving the pathway toward the design of solid-state waste heat recovery systems. The next requirement in the design process is realization of full-scale multistage TE devices in the medium to high temperature range for enhanced power generation. Here, we report the design and manufacturing of full-scale skutterudite (SKD)/half-Heusler (hH) cascaded TE devices with 49-couple TE legs for each stage. The automated pick-and-place tool is employed for module fabrication providing overall high manufacturing process efficiency and repeatability. Optimized Ti/Ni/Au coating layers are developed for metallization as the diffusion barrier and electrode contact layers. The Cu-Sn transient liquid phase sintering technique is utilized for SKD and hH stages, which provides a high strength bonding and very low contact resistance. A remarkably high output power of 38.3 W with a device power density of 2.8 W·cm-2 at a temperature gradient of 513 °C is achieved. These results provide an avenue for widespread utilization of TE technology in waste heat recovery applications.

Study on the preparation process and sintering performance of doped nano-silver paste

AbstractAfter tin–lead solders are banned, the widely used electronic packaging interconnect materials are tin–silver, tin–copper, and other alloy solders. With the application of high-power devices, traditional solders can no longer meet the new requirements. Nano-silver (Ag) paste, as a new solder substitute, exhibits excellent properties, such as excellent thermal and electrical conductivity, sintering at lower temperatures, and service at high temperatures. However, many organic devices still cannot withstand this temperature, and are often not suitable for the connection of nano-Ag paste packaging materials, therefore, it is very urgent to further reduce the sintering temperature of nano-silver paste. Based on the transient liquid phase sintering technology, by doping the nano-Ag paste with the nano-tin paste with a lower melting point to make the two uniformly mix, pressureless sintering at low temperature can be realized, and a sintered joint with a connection strength greater than 20 MPa can be formed. Considering that tin is easy to be oxidized, and the core–shell material can prevent the oxidation of tin, and at the same time ensure the uniform distribution of tin in silver, the doping scheme of the core–shell structure is determined. Heterogeneous flocculation method refers to the particles with different properties of charges which attract each other and agglomerate. It is a continuous reduction method for preparing core–shell materials. This method has the advantages of mild reaction conditions and less equipment investment, so the heterogeneous flocculation method is selected to prepare Sn@Ag core–shell nano paste. And research its sintering performance and strengthening mechanism.

Effect of milling time on microstructural and mechanical properties of nano-Al2O3P/6061Al composites prepared by transient liquid-phase sintering

In this study, 5 vol% Al2O3 nanoparticles reinforced 6061 Al matrix nanocomposite was prepared by mechanical alloying the corresponding powders followed by hot-pressing. The effect of the milling time on the morphology of powder as well as the microstructure and mechanical properties of resulting nanocomposites was studied. The result shows that the effect of the milling time on the mechanical properties of nanocomposites comes from its effect on the distribution of Al2O3 nanoparticles in Al matrix. The long milling time is required to uniformly disperse Al2O3 nanoparticles within Al grains, in order to obtain good mechanical properties. The nanocomposite made from the powder milled for 50 h exhibits a tensile strength of 602 MPa and an elongation to failure of 8.8%, which are higher than those ever reported for the similar materials. The higher mechanical properties are mainly attributed to Orowan strengthening, grain boundary strengthening, and high dislocation density.

Powder-metallurgy fabrication of ZrB2–hardened Zr3Al2 intermetallic composites by high-energy ball-milling and reactive spark-plasma sintering

A powder metallurgy route combining high-energy ball-milling (HEBM) of elemental powders and reactive spark-plasma sintering (SPS) is proposed for the controlled fabrication of novel composites based on a Zr–Al intermetallic matrix hardened with a ceramic second-phase. As proof-of-concept, its suitability is demonstrated on ZrB2–hardened Zr3Al2. Specifically, commercially available powders of ZrH2, Al, and B were first combined in molar ratios of 2:1:1 to give an intermetallic–ceramic composite nominally formed by ∼76.8 vol.% Zr3Al2 plus 23.2 vol.% ZrB2, and were intimately mixed and mechanically activated by HEBM in the form of dry shaker milling for 30 min, next identifying by a dilatometric SPS test at 50 MPa pressure that the densification window of these composites is ∼975–1275 °C. Subsequent densification SPS tests at 50 MPa pressure in that temperature interval, and also at 1350 °C, plus the microstructural and mechanical characterisations of the resulting materials, established 1175 °C as the optimal SPS temperature. It was also identified that densification takes place by transient liquid-phase sintering with molten Al, and that it occurs gradually, not abruptly, because most molten Al disappears in a flash by reacting with Zr to form in situ the intermetallic. It is also shown that the combination of HEBM plus reactive SPS yields Zr3Al2+ZrB2 composites with fine-grained microstructures formed essentially by multitudinous ZrB2 nanograins dispersed within a matrix of submicrometre, or nearly submicrometre, Zr3Al2 grains. Importantly, these intermetallic–ceramic composites were found to be very hard (i.e., ∼11.5 GPa), attributable to the hardening provided by the ZrB2 nanograins, and fairly tough (i.e., ∼4.5 MPa·m1/2), and therefore potential candidate materials for a multitude of structural–tribological applications. Finally, implications for future study are discussed.

Spark-plasma sintering of boron carbide–silicon carbide composites at 1400 °C from B4C+Si: Densification and sintering/reaction mechanisms

The feasibility of fabricating novel boron carbide–silicon carbide composites by spark-plasma sintering (SPS) of B4C+Si powder mixtures at only 1400 °C was investigated. First, it is shown that B4C can be fully densified at 1400 °C if ~20 vol% Si aids are used, leading to bi-particulate composites constituted by boron carbide (major phase) and SiC (minor phase). The formation of these composites is due to the fact that Si acts as a reactive sintering additive during SPS. Lower and higher proportions of Si aids are not optimal, the former leading to porous bi-particulate composites and the latter to dense triplex-particulate composites with some residual free Si. Importantly, it is also shown that these novel boron carbide–SiC composites are fine-grained, nearly-ultrahard, moderately tough, and more affordable to fabricate, a combination that makes them very appealing for many engineering applications. Second, it is demonstrated that during the heating ramp of the SPS cycles a eutectic melt is formed that promotes full low-temperature densification by transient liquid-phase sintering if sufficient Si aids are used. Otherwise, a subsequent stage of solid-state sintering is required at higher temperatures once the eutectic liquid has been consumed in the in-situ formation of SiC. And third, it is demonstrated that during SPS the original B4C undergoes a gradual isostructural crystallographic transition towards a Si-doped carbon-deficient boron carbide that is more relevant with increasing proportion of Si aids, and it is identified that the carbon source for the formation of SiC is almost exclusively the carbon exsoluted from the B4C crystals themselves during their isostructural transition. Finally, implications of interest for the ceramic and hard-material communities are discussed.

Microstructural evolution and performance of high-tin-content Cu40Sn60 (wt. %) core/shell powder TLPS bonding joints

This paper investigated the microstructure evolution and performance of high tin content Cu@Sn core/shell (60 wt% Sn) powder transient liquid phase sintering bonding. The formation and evolution mechanism of voids were analyzed. Results showed that, after consuming Sn, the joint was mainly composed of Cu6Sn5, Cu3Sn, and Cu, raising the remelting temperature of the joint to 400 °C. The porosity in the joint was decreased firstly, then increased, and finally decreased with bonding time. The maximum porosity was 0.876% and 1.052% at 300 °C for 150 min and 340 °C for 120 min. With the bonding time increased, the room temperature (RT) shear strength was increased first, then decreased, and finally remained stable. In contrast, the high-temperature (350 °C) shear strength was increased first, then remained stable with bonding time. The joint was possessed excellent RT (20.79 MPa) and high-temperature (18.46 MPa) shear strength. The electrical resistivity of joints was increased first, then remained stable with bonding time. The electrical resistivity was reached its maximum value of 1.83 × 10−5 Ω·cm at 300 °C for 150 min. The thermal conductivity of the joint was 28.72 W/(m·K), 26.83 W/(m·K), and 25.99 W/(m·K) at 30 °C, 250 °C, and 350 °C, respectively.

Effect of Milling Time on Microstructural and Mechanical Properties of Nano-Al2o3p/6061al Composites Prepared by Transient Liquid-Phase Sintering

Effect of Milling Time on Microstructural and Mechanical Properties of Nano-Al2o3p/6061al Composites Prepared by Transient Liquid-Phase Sintering

Kinetics of Cu6sn5 and Cu3sn Intermetallic Compounds Growth and Isothermal Solidification During Cu-Sn Transient Liquid Phase Sintering Process

Kinetics of Cu6sn5 and Cu3sn Intermetallic Compounds Growth and Isothermal Solidification During Cu-Sn Transient Liquid Phase Sintering Process

Microstructure, shear properties and corrosion resistance of Cu/Cu-In/Cu solder joints

Transient liquid phase sintering can successfully fabricate reliable high-temperature solder joints at a relatively low joining temperature by forming intermetallic compounds. In this work, microstructure, shear properties and corrosion resistance of Cu/Cu-In/Cu solder joints prepared by TLPS process were investigated. The shear strength of the Cu-In solder joints significantly increased during the initial aging period and then slightly decreased due to the increase of pores after continuous 24 h aging. The corresponding microstructure is that part of the Cu2In in the Cu-In solder joints transforms to Cu7In3 in the meanwhile. After continuous aging, the interfacial microstructure remained stable and contributed to good shear strength that exceeds 20 MPa after 96 h aging at 250 °C and 350 °C, which was 20.8 MPa and 23.4 MPa, respectively. Regarding corrosion resistance, the Cu-In interfacial layer (−0.24 V) has similar corrosion resistance to Cu substrate (−0.22 V) and shows superior to the commonly used Sn63Pb37 solder (-0.40 V) and Sn95.5Ag3.8Cu0.7 solder (−0.42 V).

Transient Liquid Phase Sintering of PM Steel—A Matter of the Heating Rate

Powder metallurgy (PM) offers several variants to introduce alloying elements for establishing the desired final composition. One route is the master alloy (MA) approach. The composition and the elements contained in the MA can be adjusted to obtain a liquid phase that penetrates through the interconnected pore network and thus enhances the distribution of the alloying elements and the homogenization of the microstructure. Such a liquid phase is often of a transient character, and therefore the amount of liquid formed and the time the liquid is present during the sintering are highly dependent on the heating rates. The heating rate has also an impact on the reaction temperatures, and therefore, by properly adjusting the heating rate, it is possible to sinter PM-steels alloyed with Fe-Cr-Si-C-MA at temperatures below 1250 °C. The present study shows the dependence of the melting regimes on the heating rate (5, 10, 20, 120 K/min) represented by “Kissinger plots”. For this purpose, liquid phase formation and distribution were monitored in quenching dilatometer experiments with defined heating up to different temperatures (1120 °C, 1180 °C, 1250 °C, 1300 °C) and subsequent quenching. Optimum sintering conditions for the materials were identified, and the concept was corroborated by C and O analysis, CCT diagrams, metallographic sections, and hardness measurements.

High-temperature stability of Ni-Sn intermetallic joints for power device packaging

In this study, the feasibility and high-temperature stability of Ni-Sn bonded joints fabricated by transient liquid phase sintering (TLPS) were investigated for high-temperature power electronics applications. A 30Ni-70Sn (wt%) TLPS paste was fabricated with micro-sized pure Ni and Sn powders, and chip bonding was performed on a direct bonded copper (DBC) substrate. During TLPS bonding, Sn particles melted and reacted with Ni particles, which resulting in the formation of stable Ni-Sn TLPS intermetallic joints. During aging treatments at 150 and 200 °C, the formed Ni3Sn4 phases were transformed into the Ni3Sn2. With increasing aging treatments, the reactions between Ni3Sn4 intermetallics and remaining Ni particles increased, which resulted in the formation of the Ni-rich Ni-Sn intermetallics. Even after 1000 h at an aging temperature of 200 °C, there is no significant difference in the stable and dense microstructure of Ni-Sn TLPS joints. In addition, their mechanical strengths did not vary significantly even after long-term treatment for 1000 h at 150 and 200 °C. This means that the Ni-Sn TLPS joints have a good long-term microstructural stability and mechanical reliability at high temperatures up to 200 °C. Therefore, we conclude that the Ni-Sn paste is a promising candidate as a die-attach material for high-temperature electronic assemblies.

Microstructure Evolution and Shear Property of Cu-In Transient Liquid Phase Sintering Joints

Transient liquid phase sintering (TLPS) is a promising joining technology that can achieve high temperature resistant solder joints at low temperature, showing excellent potential in power electronics. In this work, Cu/Cu-In/Cu solder joints were successfully prepared by TLPS process. The effects of bonding pressure and holding time on the microstructure and shear strength of Cu-In TLPS joints at 260 and 320°C were studied. The results showed that as bonding pressure increased from 0.1–0.6 MPa, the porosity decreased and shear strength increased significantly. No obvious change was found as bonding pressure continued to increase to 1 MPa. As holding time increased at 260°C, Cu11In9 was formed and gradually transformed to Cu2In that can withstand elevated temperature. Meanwhile, the porosity decreased while shear strength increased. It was calculated that volume expansion (12.74%) occurred during the phase transition from Cu11In9 to Cu2In. When bonding temperature increased to 320°C, only Cu2In was detected and then gradually transformed to Cu7In3 with the growing holding time. As holding time reached 120 min, their porosity increased and lead to weak shear strength due to volume shrinkage (15.43%) during the phase transition from Cu2In to Cu7In3.

In-situ synthesis and low-temperature fabrication of B4C/TiB2-based multilayer graded composites with B4C and Ti–Al intermetallics mixtures through transient liquid phase sintering

The seven-layer B4C/TiB2-based graded composites was prepared with B4C and Ti–Al intermetallics through stepped laminating processing and transient liquid phase spark plasma sintering. The sintering strategy of the graded composites was proposed based on the sintering products of monolayer materials with different contents of Ti–Al intermetallics from 5 wt% to 60 wt%. The top three layers and bottom three layers were sintered respectively at 1650 °C and 1500 °C, and then the middle layer was used as the binder to joint the as-preserved two sections at 1550 °C. The apparent density of the as-prepared B4C/TiB2-based multilayer graded composites was 2.94 g/cm3, which was lower than that of most advanced ceramics. With the increase in the addition of Ti–Al intermetallics, the hardness of B4C/TiB2-based multilayer graded composites decreased from 31 GPa (B4C-riched) to 25 GPa (TiB2-riched), whereas the fracture toughness increased from 3.8 MPa·m0.5–6.02 MPa·m0.5. The compressive strength was up to 1100 MPa, displaying the jagged stress-strain curve. Crack propagation resistance mechanisms such as deflection and bridging enhanced the fracture toughness. The B4C/TiB2-based multilayer graded composites fabricated at low temperature possess high front hardness, high rear toughness, high overall strength and low density, and has promising applications in impact-resistant fields such as lightweight ceramic armor.

Effect of mechanically alloyed sintering aid on sinterability of TiB2

High-melting-point borides are attractive for structural and functional materials used in extreme environments. However, their poor sinterability due to their high melting point and strong covalent bonding have prevented their further applications. In this study, a new sintering process, called transient liquid-phase diffusion sintering (TLPDS), has been developed for high-melting-point borides. TiB2 compacts were fabricated by spark plasma sintering at 50 MPa for 15 min at temperatures of 1300 and 1600 °C via TLPDS of TiB2 powder with a eutectic TiB–Ti powder as the sintering aid, prepared via mechanical alloying (MA). Differential scanning calorimetry results indicated that the melting temperature of the obtained sintering aid was lower than its equilibrium melting point. MA of the sintering aid suppressed the open pore fraction to one-third of that in the compact sintered with the aid prepared without MA when sintering at 1300 °C. We also propose a possible mechanism for TLPDS of TiB2.

Mechanical properties of refractory castables bonded with hydratable magnesium carboxylate-boric acid

Hydratable magnesium carboxylate (HMC), which is similar to the properties of cement, can be used as a potential binder for refractory castables. However, its decomposition may lead to poor mechanical properties at medium temperatures (300 °C–1100 °C). This work investigated the effects of boric acid on the mechanical properties and microstructural evolution of castables bonded with hydratable magnesium carboxylate. The mechanical strength, bulk density, apparent porosity, thermal shock resistance, and sintering properties of the castables were evaluated. The results showed that the mechanical properties of HMC-bonded castables (HMCC) at various temperatures can be improved by adding boric acid. Boric acid reacts with HMC to form magnesium carboxylate borate ester (MCBE), which improves the bonding strength between HMC molecules. Thus, the cold modulus of rupture of HMCC containing boric acid dried at 110 °C are higher than that of calcium aluminate cement-bonded refractory castables (CACC). The decomposition temperature of MCBE is 77 °C higher than that of HMC, so MCBE can endow castables with better mechanical properties at 110 °C–500 °C. The B4C obtained by MCBE pyrolysis could form a boron-rich liquid phase, which can accelerate the structural densification of castables via transient liquid phase sintering, thus improving the mechanical properties of castables at 500 °C–1100 °C. Moreover, boric acid can improve the thermal shock resistance of HMCC. The residual strength rate first increases and then decreases with an increasing boric acid, and reaches a maximum value of 29.7% (1 wt% boric acid is added), which is 2.3 times that of the CACC. The nanoindentation test showed that the microcracks in the matrix of 1 wt% boric acid castables are easy to initiate but difficult to propagate, so the microcracks are many and wavy.