Au-Sn Alloys Research Articles

Preparation of cyanide-free gold-tin alloys by ultrasonic-assisted pulse electrodeposition

This work adopted a novel method to prepare cyanide-free gold-tin alloys thin-film via using ultrasonic-assisted pulse electrodeposition. The effects of ultrasonic were studied and the optimal ultrasonic time was determined. It was demonstrated that ultrasonic wave can enhance the ion diffusion ability of the cyanide-free Au-Sn alloys electroplating solution and hinder the generation of bubbles on the surface of the cathode. As a result, a uniform and dense gold-tin alloys coating was obtained. Moreover, the ultrasonic facilitated the increase of deposition rate and the reduction of the content of Sn with the range of 30–60 at.% compared with stirring-pulse electrodeposition. The gold-tin alloys thin-film can be used in electronic packaging.

Aluminium, gold-tin and titanium-tungsten alloys for mid-infrared plasmonic gratings

The field of mid-infrared (MIR) plasmonics has shown great potential applications in spectroscopic sensing, infrared light sources and detectors. MIR plasmonic materials that are compatible with common fabrication processes may enable cost-effective and reliable plasmonic device platforms. In this work, we examined aluminium metal (Al), gold-tin (AuSn) and titanium-tungsten (TiW) alloys regarding their usability for surface plasmon polariton (SPP) excitation in the MIR regime using a grating configuration. The angle dependence and the influence of varying depths of gratings were numerically and experimentally studied for the chosen materials. The structures were fabricated on eight-inch silicon (Si) substrates and characterized with a free-beam reflection measurement setup in the MIR regime. The fabricated gratings show narrow resonance dips, which are in good agreement with the simulations, revealing that Al, AuSn and TiW alloys are reliable plasmonic materials for MIR plasmonic devices.

Direct Observation of Crystal Facet Transformation During Gold-Tin Alloy Electroplating

Gold-Tin alloys are commonly used as solder materials in semiconductor industries. In this work, Au-Sn alloys have been electroplated with different bath conditions. For the first time, Au-Sn alloy single crystals have been observed. There are two kinds of single crystals were grown with the compositions of Au4Sn and Au2Sn, different from literature-stated Au5Sn and AuSn. The effects of bath composition and plating factors on crsytal facet development of Au-Sn deposits have been investigated. SEM/EDX analysis showed that only Au4Sn and Au2Sn phases are single crystals among all plated deposits. The growth of either Au4Sn or Au2Sn phases showed preferred orientation with plating conditions. We report on quantitative SEM/EDX characterizations used to study the facet transformation behavior of Au-Sn alloy at the nanometer scale. We have obtained direct evidence that the single crystals and crystal facet transformation of Au4Sn and Au2Sn does exist in Au-Sn alloy, and high Sn content enhanced the crystal growth through a spiral-like growth mechanism.

The effect of naphthalene-based additives on tin electrodeposition on a gold electrode

In this work, we study the adsorption behavior of naphthalene derivatives: naphthalene (NPT), naphthalenesulfonate (NPTS), hydroxynaphthalenesulfonate (HNPTS) and ethoxylated α-napthalenesulfonic acid (ENSA, a commonly used additive in the tin electroplating industry), on gold electrodes and their effect on the tin electrodeposition process, by means of in situ and ex situ surface analysis techniques (cyclic voltammetry, in situ Surface Enhanced Raman Spectroscopy and ex situ Scanning Electron Microscopy). From experiments and density functional theory calculations, we conclude the formation of films of NPT, NPTS, HNPTS and ENSA, where NPT and NPTS lie flat on the gold surface and HNPTS and ENSA undergo oxidative polymerization. The nature and stability of the films are strongly dependent on the surface structure, interaction between the molecules, and applied potential. NPTS is observed to form a denser film than NPT due to attractive interactions between the adsorbed molecules on gold and tin. Tin electrodeposition is strongly affected by the presence of the NPT, NPTS, HNPTS and ENSA films. Tin bulk electrodeposition is inhibited in the presence of NPT and NPTS, but slightly promoted in the presence of HNPTS. Tin deposits grown in the presence of NPT and NTPS have the same morphology, and the characteristic size of the electrodeposited features is smaller than in their absence. The tin deposit grown in the presence of HNPTS exhibits markedly different and smaller features. Some amount of sulfur form is incorporated in the deposit as a result of the reductive desulphonation of NPTS, HNPTS and ENSA on the gold electrode. The effect of ENSA was compared to the results obtained with NPT, NPTS and HNPTS. ENSA exhibits a similar behavior to NPT, NPTS and HNPTS during tin deposition process in terms of the voltammetry of tin deposition, but it severely inhibits the bulk deposition. Naphthalene derivatives and ENSA have an effect on the tin bulk deposition process, but show no or little effect on the formation of AuSn (surface) alloys, which is ascribed to the slow nature of Sn UPD on gold and the SnAu alloying process.

Potentiostatic Electrodeposition of Au-Sn Alloys from a Non-Cyanide Bath for Soldering: Influence of Reagents Concentrations

This work presents an extensive study of a non-cyanide Au-Sn bath to obtain potentiostatically deposited Au-Sn alloys. In addition, Au-Sn bath speciation was studied through a program for chemical equilibrium simulation (CHEAQS Pro) and the electrochemical mechanisms were investigated using Cyclic Voltammetry (CV). Also, Rutherford Backscattering Spectroscopy (RBS) was used to determine the deposits characteristics and composition. Some recipes using different concentrations of ammonium citrate, SnCl2 and Na2SO3, and L-ascorbic acid presented the formation of colloidal gold during the electrodeposition processes. The incorporation of tin in the deposits obtained varied from 2 to 16 atomic % (at%).

The effect of the deposition rate on morphology, opto-electronic properties and formation intermetallic compounds of Au–Sn alloys

Microstructure and opto-electronic properties of thin films strongly depend on the deposition conditions. The 50 nm gold-tin alloy (Au-50%at.Sn and Au-67%at.Sn) layers were prepared using the physical vapour deposition technique. The tin and gold layers were deposited on the SiO2 - coated Si substrates in two systems Au∖Sn and Sn∖Au at four different deposition rates in the range from 0.05 Å/s up to 2.50 Å/s. The influence of the deposition rate on the sample morphology was investigated using atomic force microscope and X-ray diffraction. Changes in the microstructure of the Au–Sn layers cause variations in their opto-electronic properties. These features were examined using spectroscopic ellipsometry measurements. We found that Au-50%at.Sn alloy layers exhibit significantly lower values of optical resistivity than those obtained for Au-67%at.Sn alloy films, whereas the lowest value (29 μΩcm) was obtained for Au-50%at.Sn deposited at 0.15 Å/s and 0.50 Å/s.

Thermodynamic Prediction of the Free Energy of Mixing and Activities of some Goldbased Binary Liquid Alloys

The free energy of mixing and activities of gold in selected six Au-based binary liquid Au-Cu, Au-Zn, Au-Pb, Au-Ni, Au-Sn and Al-Au alloys at different working temperatures have been studied using the quasi-chemical approximation model (QCAM). The predicted free enegry of mixing and activities results were compared with available experimental values. The predicted results are in reasonable agreement with reported experimental data and confirms that the model is reliable and, thus should serve as an alternative for the prediction of thermodynamic properties of binary liquid alloys.
 Keywords: free energy of mixing, activities, Au, Binary liquid alloys, concentration

Complexation Behavior and Co-Electrodeposition Mechanism of Au-Sn Alloy in Highly Stable Non-Cyanide Bath

The complexation behavior and co-electrodeposition mechanism of Au-Sn alloy in a highly stable non-cyanide bath were studied by electrochemical analysis and quantum chemical calculation based on density functional theory (DFT). The interactions between metal ions and multiple complexing agents were revealed, and the mechanism on the high stability of the Au-Sn bath was clarified. The complexing agents potassium pyrophosphate (K4P2O7) and ethylene diamine tetraacetic acid (EDTA) have a synergetic complexation to Sn ions, which exist in one valence state of Sn2+ to form three complex ions, i.e., [Sn(P2O7)]2−, [Sn(P2O7)2]6− and Sn-EDTA; while the complexing agents of 5,5-dimethylhydantoin (DMH) and sodium sulfite (Na2SO3) have a complexation to Au ions, which exist in two different valence states (i.e., Au3+ and Au+) to form two complex ions of [Au(DMH)4]− and Au[(SO3)2]3−. Moreover, the existence of [Au(DMH)4]− inhibits the decomposition of [Au(SO3)2]3−. Meanwhile, EDTA participates in some form of coordination with Au+, inhibites the decomposition of [Au(SO3)2]3− and facilitates the reduction of [Au(SO3)2]3−. The co-electrodeposition of Au-Sn alloy is a diffusion-controlled process, and the nucleation of Au-Sn alloy is a progressive nucleation process. The antioxidant catechol significantly increases the overpotential of Au-Sn co-electrodeposition and improves the brightness of electrodeposited films of Au-Sn alloys.

Transition edge sensors for quench localization in SRF cavity tests

Transition Edge Sensors (TES) are bolometers based on the gradual superconducting transition of a thin film alloy. In the frame of improvement of non-contact thermal mapping for quench localisation in SRF cavity tests, TES have been developed in-house at CERN. Based on modern photolithography techniques, a fabrication method has been established and used to produce TES from Au-Sn alloys. The fabricated sensors superconducting transitions were characterised. The sensitive temperature range of the sensors spreads over 100 mK to 200 mK and its centre can be shifted by the bias current applied between 1.5 K and 2.1 K. Maximum sensitivity being in the range of 0.5 mV/mK, it is possible to detect fast temperature variations (in the 50 μs range) below 1 mK. All these characteristics are an asset for the detection of second sound. Second sound was produced by heaters and the TES were able to distinctively detect it. The value of the speed of second sound was determined and corresponds remarkably with literature values. Furthermore, there is a clear correlation between intensity of the signal and distance, opening possibilities for a more precise signal interpretation in quench localisation.

Fast scanning calorimetric study of nucleation rates and nucleation transitions of Au-Sn alloys

Fast scanning (or chip-) calorimetry has been applied to systematically investigate the nucleation kinetics of two near eutectic Au-Sn compositions. The determined nucleation rates show a maximum at finite undercooling. A comparison of the experimental data with model calculations indicates that the observed maxima of the nucleation rates are due to a transition of the nucleation mechanism in the deeply undercooled liquid.

Alloy oxidation as a route to chemically active nanocomposites of gold atoms in a reducible oxide matrix.

While nanoparticles are being pursued actively for a number of applications, dispersed atomic species have been explored far less in functional materials architectures, primarily because composites comprising dispersed atoms are challenging to synthesize and difficult to stabilize against sintering or coarsening. Here we show that room temperature oxidation of Au-Sn alloys produces nanostructures whose surface is terminated by a reducible amorphous oxide that contains atomically dispersed Au. Analysis of the oxidation process shows that the dispersal of Au in the oxide can be explained by predominant oxygen anion diffusion and kinetically limited metal mass transport, which restrict phase separation due to a preferential oxidation of Sn. Nanostructures prepared by oxidation of nanoscale Au-Sn alloys with intermediate Au content (30-50%) show high activity in a CO-oxidation probe reaction due to a cooperative mechanism involving Au atoms as sites for CO adsorption and reaction to CO2 embedded in a reducible oxide that serves as a renewable oxygen reservoir. Our results demonstrate a reliable approach toward nanocomposites involving oxide-embedded, atomically dispersed noble metal species.

A strategy for fabricating nanoporous gold films through chemical dealloying of electrochemically deposited Au-Sn alloys

We report a novel strategy for the fabrication of nanoporous gold (NPG) films. The fabrication process involves the electrodeposition of a gold–tin alloy, followed by subsequent chemical dealloying of tin. Scanning electron microscopy (SEM) images show a bicontinuous nanoporous structure formed on the substrates after chemical dealloying. Energy dispersive x-ray (EDX) analysis indicates that there are no impurities in the Au–Sn alloy film with an average composition of 58 at. % Au and 42 at. % Sn. After dealloying, only gold remains in the NPG film indicating the effectiveness of this technique. X-ray diffraction (XRD) results reveal that the as-prepared Au–Sn alloy film is composed of two phases (Au5Sn and AuSn), while the NPG film is composed of a single phase (Au). We demonstrate that this approach enables the fabrication of NPG films, either freestanding or supported on various conductive substrates such as copper foil, stainless steel sheet and nickel foam. The resulting NPG electrode exhibits enhanced electrocatalytic activity toward both H2O2 reduction and methanol oxidation compared to the polished Au disc electrode. Our strategy provides a general method to fabricate high quality NPG films on conductive substrates, which will broaden the application potential of NPG or NPG-based materials in various fields such as catalysis, optics and sensor technology.

Composition control of co-electroplating Au–Sn deposits using experimental strategies

The optimal electroplating parameters for a pulse-current co-electroplating system of Au–Sn deposits in a non-cyanide electrolyte were investigated using experimental strategies, including fractional factorial design (FFD) and central composite design (CCD) coupled with the response surface methodology. pH value, ethylene diamine tetraacetic acid (EDTA) concentration, catechol concentration and metallic ions molar ratio (i.e., [Au]/[Sn]) were identified as the key factors affecting the composition of Au–Sn deposits in the FFD study. A reliable model between the response variable and the key factors of pH value, EDTA concentration and catechol concentration was established for the composition control of Au–Sn alloys in the CCD study. The standard deviation of the response variable (tin content) was set at the minimum level to determine the optimal co-electroplating parameters for the predicted composition value of Au–Sn deposits. Pair T test was conducted to validate both predicted and observed composition values under the optimal electroplating parameters, and the composition of Au–Sn deposits can be precisely controlled based on the established model. Scanning electron microscope observation and X-ray diffractometer analysis revealed that the morphology and crystalline of the Au–Sn deposits were composition-dependent.

Using impedance to study surface segregation on restored electrodes of Ag-Sn and Au-Sn alloys

Results from investigating time dependences for the capacitance of double electric layers, observed from the time of the mechanical restoration of electrodes of Ag-Sn, Au-Sn binary alloys with low contents of the second component in solutions of surface-inactive electrolytes, are analyzed. It is shown that the observed time dependences are due to surface segregation of the second component of alloys at the boundary with a solution.

AuSn20 Eutectic Electrodeposition through Alternative Complexing of Pyrophosphoric Acid: Insights from Electrochemical and DFT Methods

Eutectic AuSn20 solder is an important material for electronic packaging technology due to its superior mechanical and thermal conductive properties. In this work, AuSn20 alloy films are prepared via the electrodeposition method for the first time. The electrodeposition is cost-effective with improved control over the alloy content when compared to traditional powdered metallurgy methods. Pyrophosphoric acid was found to be an effective complexing agent to minimize the difference of the deposition potentials between Au and Sn, making the codeposition of AuSn alloys possible. Importantly, electrochemical characterization was combined with density functional theory (DFT) calculations to provide insight into the mechanism of the alloy codeposition when pyrophosphoric acid was used as the complexing agent. In particular, natural bond orbital (NBO) charge distribution and the lowest unoccupied molecular orbital (LUMO) characteristics of [P2O7]4–-Sn(II) and [P2O7]4–-Au(I) complexes are calculated, suggesting th...

Electrodeposition of Sn and Au–Sn alloys from a single non-cyanide electrolyte

In this paper, it is demonstrated that Sn and Au–Sn alloys can be electrodeposited from a single non-cyanide electrolyte. This electrolyte consists of KAuCl4 · xH2O, SnCl2 · 2H2O, sodium sulfite (Na2SO3), tri-ammonium citrate and Triton X-100. The Sn is electrodeposited at a current density of 4 mA/cm2 or higher, while the Au-rich intermetallic (Au5Sn) is electrodeposited at lower current densities (0.8–1.5 mA/cm2). Microstructural analysis shows that the deposits are uniform and smooth. If the deposits are plated at current densities from 2 to 4 mA/cm2, which corresponds to a composition range of 30–80 at% Sn, multiple phases co-exist in the deposits.

Electrodeposition of Au-Sn Alloys from a Modified Non-Cyanide Bath

This work presents improved Au-Sn baths to obtain a potentiostatically deposited Au-Sn alloys, by preventing bath degradation during the electrochemical process. Au-Sn bath speciation was studied through a program for chemical equilibrium simulation (CHEAQS Pro). The electrochemical reactions were investigated using Cyclic Voltammetry (CV). Rutherford Backscattering Spectroscopy (RBS) was used to determine the deposits composition. The results indicated that the best bath composition was KAuCl4 (5 g l-1), SnCl2×2 H2O (5 g l-1), ammonium citrate (150 g l-1), l-ascorbic acid (7.5 g l-1) and sodium sulfite (60 g l-1) to obtain an alloy with 16 at.% Sn. The best potential was defined as -600 mV, due to the largest thickness obtained.

Study on the two dealloying modes in the electrooxidation of Au–Sn alloys by in situ Raman spectroscopy

The electrochemical processes in dealloying of Au–Sn alloys in a solution of 2 mol dm −3 HCl have been first investigated in detail by means of in situ potential-dependent and time-resolved Raman spectra. Two dealloying modes were found occurring within different potential regions in the electrooxidation of Au–Sn alloys. One is the mode known as classical dealloying, where Sn is selectively dissolved; and the other a so-called quasi-dealloying mode found here, in which Au re-deposits automatically after simultaneous dissolution with Sn. Meanwhile, nanoporous gold, thin layers of gold nanoparticles stacked on the surface, and colloidal gold in the solution can be prepared from the Au–Sn alloys simply by an electrochemical control of potential. Moreover, the quasi-dealloying manner of Au–Sn alloys has also been grafted onto a pure Au electrode with a tin overlayer by electrodeposition to construct the SERS substrate conveniently.

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.

Bulk and surface properties of liquid Al–Mg, Au–Sn, and Mg–Tl compound forming alloys

We have used phenomenological models based on statistical mechanics to study the bulk and surface properties of Au–Sn, Al–Mg, and Mg–Tl binary liquid alloys. Our study reveals that the three alloys are weakly ordered systems with the most stable intermetallic complexes at temperatures of 823 K, 1073 K and 923 K been AuSn, Al 3Mg 2 and Mg 4Tl, respectively. An analysis of Warren–Cowley short-range order parameter indicates that the weakest intermetallic compound is Al–Mg while Au–Sn is observed to be more chemically ordered than Mg–Tl. Furthermore, our surface properties calculations shows that Mg-atom and Sn-atom segregate to the surface over the whole concentration range of Al–Mg and Au–Sn alloys, respectively. In Mg–Tl, there is a competing effect between Tl-atom being drawn into the bulk and at the same time Tl-atom wanting to segregate to the surface over the whole concentration range.