Au-Sn Eutectic Alloy Research Articles

2430% Superplastic strain in a eutectic Au-Sn alloy with micrometer-sized grains maintained by spinodal-like decomposition

Polycrystalline metals with fine grains are highly possible to possess superplasticity via grain boundaries sliding (GBS). However, the mechanism of stabilizing the fine grains and releasing grain boundary strain incompatibilities to maintain GBS has puzzled the researchers for several decades. Here, a classic example is represented by the Au-Sn eutectic (ζ-Au5Sn + δ-AuSn) alloy with micrometer-sized equiaxed grains, which achieved 2430% tensile strain at 473 K. Characterization at nanoscale reveals, unexpectedly, the universal occurrence of spinodal-like decomposition in one of the eutectic phases (δ-AuSn phase) in the Au-Sn alloy. The occurrence of the spinodal-like decomposition is energetically favorable. Therefore, the system's free energy was minimized by the spinodal-like decomposition rather than grain growth. Micrometer-sized grains were thus maintained at relatively high temperatures for maintaining GBS. In addition, the stacking faults (SFs) were generated in the spinodal-like decomposed substructures during deformation. SFs coordinated the strain incompatibilities during GBS and contributed to the plastic flow during superplasticity. In the present Au-Sn alloy, the spinodal-like decomposition is the root cause to stabilize the fine grains, eventually leading to outstanding superplasticity. This study enriches our fundamental understanding of the relation between spinodal-like decomposition and mechanical performance. It also provides new insights into the design of polycrystalline metals to achieve superplasticity.

Microstructural evolution and orientation-correlated origin of the coarsening behaviors in Au–Sn eutectic alloys

Au–Sn eutectic alloys are widely used in lead-free sealings and solders. Hot rolling, a process of the combination of deformation and heat, remains an attractive method to thin the solder strips for bulk ingots of Au–Sn eutectic alloys, but the mechanisms governing the hot rolling process of Au–Sn eutectic alloys is not yet clear. Solder strips thinning is thus not reached easily. In this study, the heat induced coarsening origin and spheroidization mechanisms in Au–Sn eutectic alloys were uncovered in terms of the microstructural evolution and orientation relationship. Microstructure evolution of lamellar colony of Au–Sn eutectic alloy revealed that the initiation sites of coarsening occurs at the boundary of the lamellar colony structure and the end of the lamellar fault inside the colony structure. Termination migration was found to dominate the origin of coarsening mechanism of Au–Sn eutectic alloy, with involving large scale nano-lamellae, paired lamellar faults and a large number of coarsening structure. And another new coarsening mechanism, lamellae splitting, was proposed based on “groove” morphology, interface structure and crystallographic orientation relationship, to create massive lamellar faults for termination migration. The mechanisms will provide a new insight into coarsening origin. And the results pave the way of hot rolling Au–Sn eutectic alloys in terms of parameter selection, microstructure evolution and coarsening mechanisms.

A Review of Developments in Au-Sn Eutectic Alloy Electrodeposition

Background: The problems in the electrochemical synthesis of Au-Sn alloy from aqueous and non-aqueous solutions concerning Au : Sn ratio and phase composition control, as well as the stability of the solutions and mechanisms of the alloying are discussed. The information is presented on the methods of Au-Sn eutectic synthesis by layer-by-layer gold and tin deposition and by simultaneous metals electrochemical reduction. The importance of this alloy synthesis is stipulated by its unique properties determining application as a solder in electronics and instrumentation. Method: The analysis is performed on the effect of tin and gold compounds composition and concentration in the solutions for electrochemical gold, tin and Au-Sn alloy deposition, the nature of a solvent, the value of pH in aqueous solutions, the availability of different additives on the stability of electrolytes, current efficiency, the rate of coatings deposition, and their chemical and phase composition. Results: The reasons for the instability of aqueous electrolytes such as Sn(II, IV) compounds hydrolysis, and Au(I, III) compounds reduction in the solution together with the causes of small tin content in the alloys which is largely stipulated by a great difference between gold and tin electrode potentials, are analyzed. The known methods of these drawbacks of diminution are described. Conclusion: The conceptions about mechanisms of electrochemical simultaneous reduction of Sn(II, IV) and Au(I, III) from aqueous and non-aqueous electrolytes are considered. The advantages and weak aspects of the usage of non-aqueous electrolytes based on glycols, mixtures of choline chloride with urea and/or glycols are shown.

Thermal chemical vapor deposition of tin oxide nanowires in a hydrogen reduction atmosphere

We fabricated tin oxide nanowires (NWs) on Au/Si3N4/Si(001) substrates using thermal chemical vapor deposition (CVD) in a hydrogen reduction atmosphere. Unlike typical thermal CVD conditions, a mixture of H2 and Ar gas was used as the carrier gas, and a catalyst powder such as graphite was not used. The hydrogen gas maintained the reducing atmosphere and easily vaporized the SnO2 powder without a catalyst, to supply vapors of Sn and O. This process enables vapor-liquid-solid (VLS) growth of SnO2 NWs at temperatures below 600 °C. The formation of a eutectic AuSn alloy that acts as a liquid seed during the VLS process contributes to lowering the growth temperature. X-ray diffraction and transmission electron microscopy analysis show that SnO2 NWs and SnO NWs coexist in the as-prepared sample. The oxygen-deficient atmosphere results in nucleation and growth of SnO NWs.

Formation of alloys upon the simultaneous electrochemical deposition of gold and tin from ethylene glycol and aqueous electrolytes

An ethylene glycol (EG) solution containing Au(III) and Sn(IV) compounds, and conditions for the electrochemical deposition of Au–Sn alloy based on AuSn and Au5Sn intermetallics with total tin content of 30–55 at % are proposed. Fundamental difficulties of the deposition of alloys with high tin content, (including eutectic Au–Sn alloy) from aqueous electrolytes are revealed. It is determined via voltammetry that the simultaneous deposition of gold and tin from aqueous and EG electrolytes proceeds with the depolarization effect of both Au(III) and Sn(IV) as a result of the formation of the alloy, the increase in the rate of tin cathodic reduction being more noticeable in case of EG solution. Formation of SnCl2EG(H2O)2+ complex upon the dissolution of SnCl4 · 5H2O in glycol, the stability of the composition of tetracyanoaurate ions upon the dissolution of K[Au(CN)4], and the weakening of intermolecular interactions in EG with small amounts of water were revealed via IR spectroscopy. It is suggested that the depolarization effect is due not only to alloy formation, but also to the formation of SnCl2EG(H2O)2+ cations, their association with Au(CN)4- anions, and a change in the mechanism of Au(III) and Sn(IV) reduction.

Thermodynamic reassessment of Au–Cu–Sn ternary system

The Au-20wt.%Sn|Cu diffusion couples were annealed at 320°C for 100s and 2500s and at 150°C for 6600h. The microstructures of these samples were studied with SEM+EDS technique. It was found out that the interface between Cu and near eutectic AuSn alloy consisted of AuCu3/AuCu/Au5Sn layers after annealing at 150°C. For having a better understanding on the formation and evolution of metal bonding interconnection microstructures, the Au–Cu–Sn ternary system was thermodynamically reassessed on the basis of the experimental results from this work as well as by using the data available in the literature and the thermodynamic descriptions of the binary systems. The thermodynamic parameters from all the three binary systems were directly cited in this work, except for those of Cu6Sn5_HT and Cu6Sn5_LT. In the present work, the compounds AuSn and Cu6Sn5_HT were described as a one phase due to the reported complete solid solubility at 200°C and 360°C. A set of self-consist thermodynamic parameters for Au–Cu–Sn system was obtained in the present work. This thermodynamic description can reproduce most of the experimentally determined values, i.e. both the thermochemical quantities and phase equilibria.

Thermodynamic reassessment of Au–Ni–Sn ternary system

Abstract For having a better understanding on the formation and evolution of metal bonding interconnection microstructures the Au–Ni–Sn ternary system was reassessed on the basis of experimental results and the recently reported thermodynamical description for Au–Sn and Ni–Sn systems. In this paper, the thermodynamic parameters of Ni 3 Sn 4 phase were modified in order to achieve a better agreement with experimentally determined phase boundaries. Further, a self-consistent set of thermodynamic parameters for the Au–Ni–Sn system were obtained, which were able to reproduce most of the available experimental data. The Ni|80Au20Sn (wt%) diffusion couples were annealed at 320 °C for 10,000 s and at 150 °C for 2500 h. The microstructures of these samples were studied with SEM+EDS technique. The reaction interface between Ni and near eutectic AuSn alloy consisted of Ni 3 Sn/(Ni, Au) 3 Sn 2 /AuSn/Au 5 Sn layers. This experimentally observed diffusion path of Ni against Au–20 wt% Sn solder at 150 °C was rationalized on the basis of the thermodynamically calculated isothermal section.

Solidification and interfacial interactions in gold–tin system during eutectic or thermo-compression bonding for 200 mm MEMS wafer level hermetic packaging

In this work, Au–Sn eutectic bonding and Au–Sn thermo-compression bonding are studied for applications in hermetic packaging at wafer level. Eutectic bonding experiments were performed under vacuum or pure nitrogen at temperatures between 300 and 350 °C while thermo-compression bonding experiments were performed under vacuum at 270 °C. During these experiments, the solidification of electrodeposited Au–Sn alloy as well as the interaction of this alloy with W2N layers are studied. Some supplementary specific brazing experiments were performed using commercial sheets of eutectic Au–Sn alloy in order to understand the mechanisms of interactions between the Au–Sn alloy and the W2N layer and of solidification of the Au–Sn eutectic alloy. The melting and solidification process of eutectic Au–Sn alloy were studied by differential scanning calorimetry under different geometrical configurations such as commercial eutectic Au–Sn sheets alone, brazing joints performed by commercial eutectic Au–Sn alloy and samples made by thermo-compression bonding. Bonded wafers with good mechanical properties were characterized by cross-section scanning electron microscopy using energy dispersive X-ray mode. Some samples were characterized by transmission electron microscopy. The mechanical strength of the seal was checked by shear tests.

Reaction dynamics of diffusion soldering with the eutectic Au–Sn alloy on copper and silver substrates

The dynamics and the morphology of the intermetallic phase evolution for isothermal diffusion soldering with the eutectic Au–Sn alloy as the solder material on copper and silver substrates was investigated. The phases involved and the reaction parameters like the rate of the intermetallic layer growth and the activation energy could be determined by in-situ X-ray diffraction. Transmission electron microscopy and energy dispersive X-ray spectroscopy bring further insight into the morphology changes that occur during the process. The results contribute to a better understanding of this advanced joining method extensively used in semiconductor packaging.

Fabrication and Microstructures of Sequentially Electroplated Sn-Rich Au-Sn Alloy Solders

Three Sn-rich, Au-Sn alloy solders with eutectic, hypoeutectic, and hypereutectic Sn compositions were fabricated by sequential electroplating of Au and Sn and then the dual-layer films were reflowed at 250°C. The microstructures and phase compositions of the deposited Au/Sn dual-layer film and the reflowed Sn-rich Au-Sn alloys were studied. Microhardness values of the different phases or phase zones for the reflowed alloys were also tested. Finally, two Si wafers were bonded together with the eutectic Sn-rich Au-Sn alloy solder. For as deposited Au/Sn dual-layer films, reaction between Au and Sn occurs at room temperature leading to the formation of Au5Sn, AuSn, and AuSn2 at the Au/Sn interface. After reflowing at 250°C, two phases remain, Sn and AuSn4, with the morphology and phase distribution depending on the original solder composition. In the Sn-rich, eutectic Au-Sn alloy, AuSn4 particles are distributed uniformly in the Sn matrix. In the Sn-rich hypoeutectic/hypereutectic Au-Sn alloys, the proeutectic phase, AuSn4 (Vickers hardness, Hv 125) or Sn (Hv 14.2), is larger in size and is surrounded by the eutectic zone (Sn + AuSn4) (Hv 16.1). In all cases, the TiW adhesion and barrier layer remains intact during annealing. After reflowing at 250°C under a pressure of 13 kPa, two Si wafers are joined by the Sn-rich eutectic Au-Sn alloy solder, without crack or void formation at the Si wafer/solder interface or within the solder.

Au–Sn flip-chip solder bump for microelectronic and optoelectronic applications

As an alternative to the time-consuming solder pre-forms and pastes currently used, a co-electroplating method of eutectic Au–Sn alloy was used in this study. Using a co-electroplating process, it was possible to plate the Au–Sn solder directly onto a wafer at or near the eutectic composition from a single solution. Two distinct phases, Au5Sn (ζ-phase) and AuSn (δ-phase), were deposited at a composition of 30 at.%Sn. The Au–Sn flip-chip joints were formed at 300 and 400°C without using any flux. In the case where the samples were reflowed at 300°C, only an (Au,Ni)3Sn2 IMC layer formed at the interface between the Au–Sn solder and Ni UBM. On the other hand, two IMC layers, (Au,Ni)3Sn2 and (Au,Ni)3Sn, were found at the interfaces of the samples reflowed at 400°C. As the reflow time increased, the thickness of the (Au,Ni)3Sn2 and (Au,Ni)3Sn IMC layers formed at the interface increased and the eutectic lamellae in the bulk solder coarsened.

Fluxless Flip-Chip Solder Joint Fabrication Using Electroplated Sn-Rich Sn-Au Structures

A fluxless flip-chip bonding process in hydrogen environment using newly developed Sn-rich Sn-Au electroplated multilayer solder bumps is presented. Cr/Au dual layer is employed as the plating seed layer and the underbump metallurgy (UBM). This UBM design, seldom used in the electronic industry, is explained in some details. To realize the fluxless possibility, proper intermetallic growth over the composite structure is needed. In this connection, we like to point out that it is much harder to achieve fluxless bonding using Sn-rich Sn-Au design than the familiar Au-rich 80Au20Sn eutectic design. This is so because Sn-rich Sn-Au alloys have numerous Sn atoms on the surface that can get oxidized easily while the Au-Sn eutectic alloy at thermal equilibrium consists of only Au5Sn and AuSn compounds. Intermetallic nucleation and growth mechanism of sequential electroplating of Au over thick Sn layer is studied with scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction method (XRD). It is found that Au-Sn intermetallic forms as Au is plated over the Sn layer and acts as a barrier that prevents the oxidation of the inner Sn layer, making fluxless possibility a reality. It is found that the SnAu intermetallic compounds are randomly distributed in the Sn rich joint making the joint strong. The resulting joints contain few voids as examined by an SEM and a scanning acoustic microscope (SAM) and have a remelting temperature of 217degC-222degC. The plated Sn-Au solder bumps on silicon with 50 mum in height are flip-chip bonded to borosilicate glass substrate. This new fluxless flip-chip bonding process is valuable in many applications where the use of flux is prohibited

Microstructural Evolution of Joint Interface between Eutectic 80Au–20Sn Solder and UBM

The soldering behaviour of the eutectic Au-Sn alloy on two kinds of under bump metallurgy was studied in relation to time and temperature. For a Ni substrate, two types of the intermetallic compounds were observed at the joint: (Au,Ni) 3 Sn 2 and (Au,Ni) 3 Sn. As the soldering temperature increased, the shape of the (Au,Ni) 3 Sn 2 grains generally changed from a long, thin rod-type to a short, thick type. The degree of buildup of the interfacial intermetallic compounds was similar up to 32 min, even if the soldering was conducted at three different temperatures between 300°C and 400°C. In addition, the reaction of the eutectic Au-Sn solder with the sputtered under bump metallurgy (Al/ Ni(V)/Au) was studied at 300°C. By 20 s of soldering, the protective Au layer was dissolved away and the Ni(V) layer started to dissolve into the solder. Thus, some of the Au reacted with the Al underlayer to form the Au 8 Al 3 phase, which was accompanied by volume expansion at the joint. The (Au,Ni) 3 Sn 2 layer was then lifted up, and several interlocked (Au,Ni) 3 Sn 2 grains were broken and separated at weak points along the joint interface. In this way, the joint interface was separated from the Si chip, and a resultant failure occurred in the device.

A high-frequency, high-stiffness piezoelectric actuator for microhydraulic applications

Abstract A piezoelectric actuator capable of high-frequency, high-stiffness actuation in microhydraulic systems was fabricated and experimentally characterized to frequencies in excess of 100 kHz. This high-frequency, high-stiffness actuation is enabled through integration of bulk piezoelectric cylinders (1 mm ×1 mm ) beneath a silicon micromachined tethered piston structure using a thin-film AuSn eutectic alloy as the piezoelectric-to-silicon bond layer. The actuator structure, with dimensions 10 mm ×10 mm ×2 mm , is comprised of a bonded stack of micromachined silicon, silicon-on-insulator (SOI), and borosilicate glass layers. Devices were successfully fabricated and tested that incorporate polycrystalline PZT-5H and single-crystal PZN-PT piezoelectric material arranged in different configurations. The microfabrication, device assembly, experimental testing procedures, and actuator performance are discussed in this paper. Issues such as piezoelectric material preparation, requisite dimensional tolerancing, micromachining of the silicon tethered structures, and integration of different arrangements of piezoelectric elements within the actuator structure are detailed.

An effective diffusion barrier metallization process on copper

Copper substrates have been widely used to mount semiconductor device chips due to their high electrical and thermal conductivity. To achieve high heat conduction, solders are utilized to bond the chip on the substrate. Soft and ductile solders such as indium and lead–tin alloys are often used to help release the stress developed in a bonded structure through plastic deformation and strain. In the solder joint, a major concern is the formation of intermetallics caused by the diffusion of copper atoms from the substrate to the joint. The growth of intermetallics makes the solder joint less ductile and less capable of releasing stresses. Prevention of copper diffusion is needed in applications where the solder joint must remain soft and ductile. On the other hand, in applications where creep movement cannot be tolerated, hard solders such as AuSn eutectic alloy should be considered. In this paper, we report a barrier metallization technique on copper substrates to block copper atoms from diffusing into the solder. Indium was selected as the solder medium in this study because of its increasing applications in many photonic devices such as laser diodes and photonic switches. Other solders such as lead–tin alloys can be also used with the metallization process. The metallization consists of Ni buffer layer and Cr blocking layer. The solder joint is fabricated by the fluxless bonding technology. After accelerated aging test at 130°C for 100 h, no sign of copper diffusion through the metallization is detected using energy dispersive X-ray (EDX). If 90°C is the maximum temperature of the solder joint during device operation, the corresponding time for no copper diffusion is 107 500 h, or 12.3 years. This result shows that the metallization process developed in this research is, indeed, very effective.

Plasma sprayed copper-nickel-indium coatings on titanium alloys: A. Koutsomichalis, H. Badekas (National Technical University of Athens, Athens, Greece). Mater. and Manuf. Processes, vol 10, no 5, 1995, 1087–1092

Copper substrates have been widely used to mount semiconductor device chips due to their high electrical and thermal conductivity. To achieve high heat conduction, solders are utilized to bond the chip on the substrate. Soft and ductile solders such as indium and lead–tin alloys are often used to help release the stress developed in a bonded structure through plastic deformation and strain. In the solder joint, a major concern is the formation of intermetallics caused by the diffusion of copper atoms from the substrate to the joint. The growth of intermetallics makes the solder joint less ductile and less capable of releasing stresses. Prevention of copper diffusion is needed in applications where the solder joint must remain soft and ductile. On the other hand, in applications where creep movement cannot be tolerated, hard solders such as AuSn eutectic alloy should be considered. In this paper, we report a barrier metallization technique on copper substrates to block copper atoms from diffusing into the solder. Indium was selected as the solder medium in this study because of its increasing applications in many photonic devices such as laser diodes and photonic switches. Other solders such as lead–tin alloys can be also used with the metallization process. The metallization consists of Ni buffer layer and Cr blocking layer. The solder joint is fabricated by the fluxless bonding technology. After accelerated aging test at 130°C for 100 h, no sign of copper diffusion through the metallization is detected using energy dispersive X-ray (EDX). If 90°C is the maximum temperature of the solder joint during device operation, the corresponding time for no copper diffusion is 107 500 h, or 12.3 years. This result shows that the metallization process developed in this research is, indeed, very effective.

Alignable liftoff transfer of device arrays via a single polymeric carrier membrane

The Letter reports epitaxial liftoff and direct bonding of GaAs/AlxGa1−xAs heterostructure device arrays and continuous films, using a single transparent polymer membrane to support the material during etch of the sacrificial layer and to manipulate it into position for bonding. Au–Sn eutectic alloy bonding leads to a metallurgical bond in minutes. Photoluminescence and current–voltage characteristics show that material quality is not substantially degraded in the transfer process.

AuSn alloy phase diagram and properties related to its use as a bonding medium

AuSn eutectic alloy has been successfully used in microelectronic packaging for high reliability applications where a hard solder as well as a low processing temperature are required. A new multilayer bonding technology not only has produced nearly perfect bonding but also has reduced the processing temperature even below the eutectic melting point. Knowledge of the different phases of the alloy and their formation, as well as the interdiffusion that occurs, thus becomes important in studying the bonding principle and the long-term reliability. In this paper, we review a large number of publications on the AuSn system and summarize the important properties. We hope that this summary would further enhance the development of new AuSn bonding methods as a result of an overall understanding of oxidation and diffusion properties.

Thermal expansion and elastic properties of high gold-Tin Alloys

The Au-Sn eutectic alloy, composed of the ζ and δ phases, has been used for many years in the sealing of hermetic parts. Recently, it was shown that the ζ phase undergoes an ordering transformation to ζ′. Since this transformation may affect hermeticity, measurements of density, linear thermal expansion coefficient, elastic moduli, and Poisson’s ratio were made on ζ-phase, eutectic, and δ-phase specimens. Measurements were made over a temperature range that includes the ζ → ζ′ transformation. Abrupt changes in these thermophysical properties, near the phase transformation temperature, are noted for ζ and eutectic specimens but not for the δ specimen.