Transient Liquid Phase Soldering Process Research Articles

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.

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.

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.

Imaging of boron distribution in steel with neutron radiography and tomography

B distribution in steel was investigated using neutron imaging techniques, on a millimetric scale with a spatial resolution below 100 μm. Steel structural components joined using the transient liquid-phase bonding (TLPB) process, with Fe-based B-alloyed foils as filler material, were observed. By means of neutron radiography and with the proposed calibration method, a quantitative profile of B concentration was attained across the joint, in particular in the region where the completion of the TLPB process was achieved. It is noteworthy that B concentration did not exceed the typical specified range of B-alloyed steels. This result explains why prior austenite grain boundaries are boride-free. To test the validity of the measured B concentrations, they were compared with those of the numerically simulated TLPB process. As a result, the calculated and experimentally measured values are in good agreement, within the estimated error. On the other hand, in regions where the TLPB process was not finished, athermally solidified liquid (ASL) was found at the joint, in the form of borides. In addition, neutron tomography enabled us to attain a three-dimensional visualization of the size and distribution of ASL, highlighting its potential as a non-destructive testing technique of TLPB weldments.

Transient Liquid Phase Bonding of Ti-6Al-4V and Mg-AZ31 Alloys Using Zn Coatings.

Ti-6Al-4V and Mg-AZ31 were bonded together using the Transient Liquid Phase Bonding Process (TLP) after coating both surfaces with zinc. The zinc coatings were applied using the screen printing process of zinc paste. Successful bonds were obtained in a vacuum furnace at 500 °C and under a uniaxial pressure of 1 MPa using high frequency induction heat sintering furnace (HFIHS). Various bonding times were selected and all gave solid joints. The bonds were successfully achieved at 5, 10, 15, 20, 25, and 30 min. The energy dispersive spectroscopy (EDS) line scan confirmed the diffusion of Zn in both sides but with more diffusion in the Mg side. Diffusion of Mg into the joint region was detected with significant amounts at bonds made for 20 min and above, which indicate that the isothermal solidification was achieved. In addition, Ti and Al from the base alloys were diffused into the joint region. Based on microstructural analysis, the joint mechanism was attributed to the formation of solidified mixture of Mg and Zn at the joint region with a presence of diffused Ti and Al. This conclusion was also supported by structural analysis of the fractured surfaces as well as the analysis across the joint region. The fractured surfaces were analyzed and it was concluded that the fractures occurred within the joint region where ductile fractures were observed. The strength of the joint was evaluated by shear test and found that the maximum shear strength achieved was 30.5 MPa for the bond made at 20 min.

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

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

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

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

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

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

Grain morphology and mechanical strength of high-melting-temperature intermetallic joints formed in asymmetrical Ni/Sn/Cu system using transient liquid phase soldering process

The grain morphology and mechanical strength of the high-melting-temperature intermetallic joints formed in the asymmetrical Ni/Sn/Cu system using a transient liquid phase (TLP) soldering process were investigated. The grain morphology of the (Cu, Ni)6Sn5 compounds subsequently exhibited fine rounded, needlelike and coarse rounded shapes from the Ni side to the Cu side, which was highly in accordance with the Ni concentration gradient across the intermetallic joints. The thermodynamic analysis indicates that the critical Ni concentration which determined the grain morphology between the needlelike shape and the coarse rounded shape is approximately 11 at% at the soldering temperature of 260 °C. The growth of the Cu3Sn compounds was suppressed and exhibited the grain morphology of fine rounded shape. The melting temperature and the shear strength of the intermetallic joints are 418.4 ± 0.1 °C and 49.8 ± 0.3 MPa, respectively. The shear fracture was inclined to occur in the area of (Cu, Ni)6Sn5 grains with the coarse rounded shape.

Non-interfacial growth of Cu3Sn in Cu/Sn/Cu joints during ultrasonic-assisted transient liquid phase soldering process

The non-interfacial growth of Cu3Sn intermetallic compounds (IMCs) in a Cu/Sn/Cu interconnection structure during the ultrasonic-assisted transient liquid phase (TLP) soldering process was investigated. In the traditional TLP soldering process, the Cu3Sn phase always nucleated at Cu6Sn5/Cu interfaces and grew at the expense of the Cu6Sn5 phase and towards the joint centre, eventually forming a full intermetallic joint consisting of a single Cu3Sn phase with columnar grains. While in the ultrasonic-assisted TLP soldering process, the Cu3Sn phase nucleated and grew randomly within the whole joint, the resulting intermetallic joint consisted of a single Cu3Sn phase with equiaxed grains. This anomalous behaviour can be wholly ascribed to the effects ultrasonic on the nucleation and growth mechanisms of Cu3Sn.

Mössbauer characterization of joints of steel pieces in transient liquid phase bonding experiences

Joining of seamless, low carbon, steel tubes were performed by means of Transient Liquid Phase Bonding process employing a foil of Fe-Si-B metallic glass as filler material. The influence of the main parameters of the process was evaluated: temperature, holding time, pressure and post weld heat treatment. Powder samples were obtained from the joint of tubes and characterized employing Mossbauer Spectroscopy in transmission geometry. The sampling was performed both in tubes successfully welded and in those which show joint defects. The results obtained are correlated with the obtained microstructure and the diffusion of Si and B during the process.

Interfacial reaction in Cu/Sn/Cu system during the transient liquid phase soldering process

A thin interlayer of pure Sn foil has been sandwiched between two pieces of Cu foil base metal and reflowed at 260, 300 and 340 °C, under 2%H 2/98%N 2 forming gas, for 5 – 480 min. We report here: (i) the interfacial microstructures to show Cu 6Sn 5 scallops and Cu 3Sn columnar crystals; (ii) the pronounced difference in the thicknesses of the Cu 6Sn 5 and Cu 3Sn layers formed at the two original boundary planes in the Cu/Sn/Cu samples; (iii) the diffusion-kinetic constants for the Cu 6Sn 5 and Cu 3Sn growth derived from the Cu/Sn/Cu samples for a range of Sn interlayer thicknesses; and (iv) a type of grain boundary/molten channel-controlled growth of Cu 6Sn 5 with a time dependence similar to that for the volume diffusion-controlled growth. These results provide new insight into the mechanism and kinetics of the interfacial reaction between liquid Sn and solid Cu, and other similar metallic liquid/solid systems.

Kinetics of Ag 3Sn growth in Ag–Sn–Ag system during transient liquid phase soldering process

The kinetics of the interfacial reaction of a thin layer of Sn sandwiched between two pieces of Ag foil has been investigated at temperatures of 260 °C, 300 °C and 340 °C. A time dependence of the form t 1/ n with n = 3 was obtained for the kinetics of both the consumption of the Sn remaining and the thickening growth of the Ag 3Sn scallops formed between Sn and Ag. Such a result can be explained well using the model of grain boundary/molten channel-controlled growth of intermetallic compounds. In this case, the diffusion of Ag atoms through the molten channels existing between the previously formed Ag 3Sn scallops is the controlling mechanism for the kinetics. We also report here the derived kinetic constants including reaction constants and the associated activation energy for guiding the practical transient liquid phase soldering of the Ag–Sn–Ag system.

Isothermal Solidification Behavior During the Transient Liquid Phase Bonding Process of Nickel Using Binary Filler Metals

Transient liquid phase (TLP) bonding process of Ni using Ni–11.0 mass%P binary filler metal was simulated by using a mathematical model based on diffusion analysis. In the model, diffusion-controlled transformation was assumed, and the base metal dissolution in the early stage and the subsequent isothermal solidification stage of the TLP bonding process were simulated. The calculated width of the eutectic structure at the bonding region agreed well with the experimental result and showed the validity of the model. In order to obtain the condition to reduce isothermal solidification time, the effect of factors, such as the width of filler metal, diffusivity of solute element in Ni and partition coefficient of element, were investigated by numerical simulation. The simulation showed that isothermal solidification time remarkably decreases with increasing partition coefficient of solute element. In order to confirm the numerical prediction, the TLP bonding experiment of Ni using Cu filler metal was carried out (the partition coefficient of Cu in Ni is close to unity). The experimental result showed that Cu filler metal remarkably shortened the isothermal solidification time and showed the validity of the prediction.