Bi-Sn System Research Articles

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.

In-situ investigation of the time-temperature dependent lattice and microstructure of Sn-Bi alloys

Alloys based on the Sn-Bi system are widely considered as the most promising candidates for low temperature solders (LTS) in the electronics industry due to their low liquidus temperature, non-toxicity and relatively low cost. However, implementation of LTS is complicated as they exhibit different characteristics from conventional Pb-free solders. While the solid solubility of alloying additions in Sn is typically <1 wt% in the current generation of Pb-free solders, the solubility of Bi in Sn ranges from less than 3 wt% at room temperature to 21 wt% at the eutectic temperature of 139 °C. In consequence, the microstructure and properties of Sn-Bi solders change significantly under varying temperature during normal service, affecting reliability. Since these changes are time and temperature dependent, interpreting results obtained using ex-situ methods requires assumptions about the stability of the alloy during transitions from the test environment to the observation environment, leading to an incomplete understanding of these changes. To address this, in-situ synchrotron powder X-ray diffraction (PXRD) and in-situ scanning electron microscopy (SEM) techniques were developed to investigate the time-temperature dependent lattice and microstructure of hypo-eutectic Sn-37wt%Bi and near-eutectic Sn-57wt%Bi solder alloys. This made possible for the first time the observation of changes during temperature fluctuations. The study revealed that lattice and microstructure are highly sensitive to factors like temperature ramp rate and isothermal holding time, not previously considered. The results will impact the design of electronic circuitry and process parameters during electronics assembly to enhance reliability and performance.

Experimental investigation and thermodynamic calculation of the Co–Sn–Bi ternary system

Phase relations of the Co–Sn–Bi ternary system have been studied experimentally for the whole composition range at 250 and 450 °C by means of scanning electron microscopy (SEM) coupled with wave dispersive spectrometer (WDS) and x-ray diffraction (XRD). Five three-phase regions have been confirmed in this system at 250 °C, and three three-phase regions exist in 450 °C. No ternary compound can be observed at these two temperatures. Moreover, Bi has almost no solubility in the binary compounds αCo3Sn2, CoSn, CoSn2 and CoSn3. The primary solidification phases were also distinguished. Based on the experimental results, the thermodynamic calculations for Co–Bi and Co–Sn–Bi system were conducted by using the CALPHAD technique. A set of self-consistent thermodynamic parameters for the Co–Sn–Bi ternary system was obtained, and the experiment results are in good agreement with the calculated results.

The Effects of Temperature and Solute Diffusion on Volume Change in Sn-Bi Solder Alloys

The different rates of thermal expansion of the many materials that make up an electronic assembly combined with temperature fluctuations are the driver of the thermal fatigue failure of solder joints. A characteristic of the Sn-Bi system, which provided the basis for many of the low process temperature solder alloys that the electronics industry is now adopting, is the very temperature-sensitive solubility of Bi and Sn in the other phase. In this study, in situ synchrotron powder x-ray diffraction was used to characterize the temperature dependence of the lattice parameters of the βSn and Bi phases in Sn-57wt%Bi and Sn-37wt%Bi. The effects of temperature and solute were separated by comparing with the data from pure βSn and pure Bi and verified using density functional theory calculations. Furthermore, the coefficients of thermal expansion of βSn and Bi during heating were also derived to reveal the thermal expansion behavior.

Determination of bulk and surface properties of liquid Bi-Sn alloys using an improved quasi-lattice theory

Abstract The thermodynamic properties of Bi-Sn were studied at 600 and 900 K using a quasi-lattice theory. After successful fitting of Gibbs free energies of mixing and thermodynamic activities, the fitting parameters were used to investigate the enthalpy of mixing, the entropy of mixing, concentration fluctuations, Warren-Cowley short-range-order parameter, surface concentrations and surface tensions of the binary systems. Positive and symmetrically shaped enthalpies of mixing were observed in all composition range, while negative excess entropies of mixing were observed. Bi-Sn showed a slight preference for like-atoms as nearest neighbours in all composition range. The nature of atomic order in Bi-Sn at 600 and 900 K appeared similar. The highest tendency for homocoordination exists at composition where mole fraction of Bi is about 40%. It was also observed that Bi (whose surface tension is lower than that of Sn) has the highest surface enrichment in the Bi-Sn systems. Unlike many previous applications of the quasi-lattice theory where constant values were used to approximate coordination numbers, temperature and composition-dependent coordination numbers were applied in this work.

Influence of Doping with Indium onto Contact Melting of Tin and Bismuth

This study is dedicated to investigating contact melting, including cases of electrotransport is a Sn-Bi system after introduction of a small admixture of indium. It has been shown that the dopant ambiguously influences the rate of the contact melting. The structure of obtained alloys is strongly non-uniform and abounds with dendritic forms. An attempt is made to explain the results.

Study of microstructure, hardness and thermal properties of Sn-Bi alloys

Lead-free solders have become a main focus of the electronic industry in recent years, because of the high toxicity of lead. Alloys based on the Sn-Bi system figure as potential replacements for Sn-Pb alloys in soldering due to favorable properties and low cost. One of the main advantages of these alloys are low melting temperatures, while additional advantages include good compatibility with substrates, low process temperature, high reliability, and potential applications in conjunction with reduced graphene oxide nanosheets as thermal interface materials. In this paper, characterization of microstructural and thermal properties as well as hardness measurements of seven alloys of different Sn-Bi compositions are performed. Structural properties of the samples were analyzed using optical microscopy and scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS). Thermal conductivity of the samples was investigated using the xenon-flash method, and phase transition temperatures were measured using the differential scanning calorimetry (DSC) analysis.

Band engineering of XBi (X = Si, Ge, Sn, and Pb) single layers via strain and surface chemical-modulation

Regulating orbital properties of two-dimensional (2D) materials has received tremendous interest. Here, we propose theoretically a new class of 2D materials XBi (X = Si, Ge, Sn, and Pb) with metal monochalogenides structure to host tunable orbital properties. As an example, the SnBi system is mainly discussed. It has found that spin-orbit coupling (SOC) transforms the electronic properties of SnBi from semiconductor into metal, and the external strain can lead to a novel Dirac electronic state. Surface chemical-decoration is confirmed to be an effective way to obtain Bi-pz orbital filtering effect and p-p orbital inversion, which may be realized through PbTe-based heterostructure and quantum-well in experiment. Particularly, the orbital regulation can give rise to fascinating Rashba spin-splitting in H-SnBi single-layer and PbTe/SnBi heterostructure. These findings provide a new class of 2D materials to explore orbital interaction.

Surface and transport properties of liquid Bi–Sn alloys

The new Bi–Sn surface tension experimental data have been obtained by the pinned drop method for temperatures ranging from 623 to 1123 K and were subsequently compared with the model predicted values as well as with the literature data. Taking into account endothermic mixing in liquid Bi–Sn alloys, the surface properties (surface tension and surface segregation) were described by the Quasi-Chemical Approximation (QCA) for regular solution and the Self Association Model (SAM) aiming to estimate the effects of demixing on these properties. In addition, the transport and structural properties of Bi–Sn melts were analysed by theoretical models developed in the framework of statistical mechanics in conjunction with the Quasi-Lattice Theory (QLT) and Faber-Ziman Theory. The abovementioned properties are involved in the development of complex solder materials based on the Bi–Sn system.

Microstructure and thermal properties of Bi-Sn eutectic alloy

AbstractAlloys based on Sn and Bi are considered to be one of the best soft lead-free solders due to their low melting temperature and low price. In addition, Bi-Sn alloy with a eutectic composition represents a promising candidate as a metallic phase change material (PCM) for use in the field of thermal energy storage (TES). The accurate knowledge of microstructural and thermophysical properties such as the latent heat of melting, thermal conductivity, specific heat capacity is of crucial importance for PCM selection. Thus, the aim of the current study has been to determine the microstructure, latent heat of melting and thermal conductivity of a eutectic alloy from the binary Bi-Sn system.

Boosting electrochemical reactivity of tin as an anode for Mg ion batteries through introduction of second phase

Tin (Sn)-based anodes attract extensive attention for magnesium ion batteries however challenges and issues exist, such as relatively difficult alloying reaction, sluggish diffusion kinetics and rapid capacity decay. Herein, we introduce a second phase of bismuth (Bi) with varying contents into Sn via magnetron co-sputtering and get insight into how the Bi introduction boosts the electrochemical reactivity of Sn through experimental and density functional theory (DFT) calculations. The experimental results reveal that the introduction of Bi can effectively trigger the alloying reaction of Sn with Mg and the further increasing of Bi significantly improves the electrochemical performance of Sn–Bi electrodes. Moreover, the activated function of introducing the Bi phase into Sn is found in the bulk rolled Sn–Bi system. The DFT calculations demonstrate that the introduction of Bi into Sn obviously lowers the defect formation energy of Mg insertion in Sn, which further rationalizes the experimental results. Additionally, operando X-ray diffraction is performed to probe the activation process and magnesiation/demagnesiation mechanisms of the Sn–Bi electrode. The activation mechanism of Bi on Sn alloying with Mg is discussed, considering the function of second phase, the influence of Bi solid solution, as well as the size effect.

Strain Rate Sensitivity of Mixed SAC-SnBi Solder Joints

Abstract As the electronics industry continues to evolve a concerted effort has developed to implement lower melting point solders. The ability to minimize the thermal exposure that an assembly is subjected to affords significant benefits with respect to both the reliability and the materials that can be used. One of the most popular low melt solder alloys currently being investigated by the industry is the Bi-Sn eutectic system, which has a melting point of 139°C. The BiSn system itself is not particularly novel as it was posited as a SAC alternative during the initial shift from Pb based solders. While a body of knowledge currently exists regarding this system, and the near eutectic variant BiSnAg, there are still concerns regarding its ductility, especially as a function of thermal exposure and strain rate. Bismuth is widely acknowledged as a brittle element and its presence in such quantities raises concerns of not just Cu6Sn5 embrittlement but also solder fragility in high strain rate types of environments. A challenge with regards to near term implementation is that most packages are not available with BiSn solder bumps. Therefore, it will be necessary to use components already balled with SAC 305 solder. This means that the resulting solder interconnect, reflowed below conventional SAC reflow temperatures, will form a type of mixed hybrid microstructure. This non-equilibrium microstructure will be composed of two regions, one Bi-rich region which is well past saturation and a second region which is Bi-deficient. It is of specific industrial interest then to not just investigate the BiSn solder system but also within the context of a realistic mixed interconnect. Recent work by several researchers has shown that this hybrid microstructure is unstable and quite active with respect to the movement and localized concentration of the Bismuth. The degree of mixing of these two regions has been shown to be highly dependent upon reflow temperature and the paste to ball volume ratio. Mixed SAC-BiSn solder joints were formed by placing SAC 305 spheres on BiSn paste deposits for a paste to ball volume ratio of .18. These samples were then reflowed at either 175°C or 200°C. SAC 305 control samples were also made using a conventional Pb-free reflow profile with a peak temperature of 247°C. A 22 mil Cu-OSP pad on a 1.0 mm thick FR4 substrate was used for all samples. A selection of the solder joints were then isothermally aged at 90°C for 200 hours. Using a joint level micromechanical tester, ball shear tests were conducted at a range of strain rates for samples in the as-reflowed and aged state. Using this information, the strain rate sensitivity of the interconnects was mapped and correlated with the observed failure modes. Investigations into the fracture mechanisms were conducted by examining the shear fracture surface with optical and scanning electron microscopy. Additionally, the evolution of the microstructure was characterized. Results showed a clear transition from ductile solder failure to a brittle separation failure at the higher strain rates.

Electrochemical reduction of CO2 on Pb–Bi–Sn metal mixtures: importance of eutectics

Pure phases of Pb, Sn and Pb70Bi30, their combinations and the eutectics (Pb25Sn75, Pb44Bi56 and Pb29Bi46Sn25) of the Pb–Bi–Sn system were prepared and evaluated for electrochemical reduction of CO2 at − 2.0 V (vs Hg/HgO). The bulk composition, the phase structure and the surface composition were characterized by inductively ICP-AES, XRD and XPS respectively. The phases obtained from XRD data matches well with the ternary phase diagram of Pb, Bi and Sn. The electrochemical characterization was done by CV and LSV. The formation rate of formate ions was compared for all the alloy catalysts at − 2.0 V (vs Hg/HgO) in CO2 saturated 0.5 M NaHCO3. It was observed that addition of both Bi and Sn individually or both concurrently to Pb increases the rate of formate ion production for Pb–Bi, Pb–Sn binary systems and the Pb–Bi–Sn ternary system respectively. Interestingly, it was found out that the eutectic compositions of each alloy system (both binary and ternary) were electrocatalytically superior (Pb–Bi–Sn eutectic (Pb29Bi46Sn25) > Pb–Sn eutectic (Pb25Sn75) > Pb–Bi eutectic (Pb44Bi56)). Further, on these high performing eutectics, the hydrogen evolution is greatly suppressed.

Studies of the solidification characteristics in Sn-Ag-Cu-Bi solder alloys.

Because of the environmental health implications of Pb, and legislation backing Restriction on Hazardous Substances (RoHS) in electronic devices, Sn-based lead-free solders are developed. Near eutectic Sn-Ag-Cu (SAC) compositions is the most widely used Pb-free solder in the electronic industries. However, its high melting temperatures (>217 ℃) remains a concern. The addition of Bi to a Sn-based alloy can be used to lower melting temperature because of the presence of a deep valley eutectic in the Bi-Sn system. The effects of such additions in SAC on solidification path are rarely considered. The effects of Bi substitution for Sn on solidification of near eutectic SAC alloy are followed with differential thermal analysis (DSC). It is found that the solidus temperature of SAC-Bi alloy mixes is considerably lowered. For low concentrations of Bi, melting range (ΔTr) were generally less than 4 ℃. However, (ΔTr) considerably widens when the Bi content exceeds 2 wt. %. The DSC and X-ray diffraction (XRD) studies suggested that a higher concentration of Bi in SAC would lead to far eutectic solidification that is attended by a formation of primary Bi phase. The existence of primary Bi phase in a solidified solder would lead to embrittlement and thus would affect the integrity of soldered joints. This work suggests that the use of Bi in SAC solders should be restricted to less than 2 wt. % (by varying %Wt of Bi).

High-Performance Antimony–Bismuth–Tin Positive Electrode for Liquid Metal Battery

The liquid metal battery (LMB) is an attractive chemistry for grid-scale energy-storage applications. The full-liquid feature significantly reduces the interface resistance between electrode and electrolyte, endowing LMB with attractive kinetics and transport properties. Achieving a high energy density still remains a big challenge. Herein, we report a low-melting-point antimony–bismuth-tin positive electrode for LMB with high energy density and excellent rate performance for the first time. The electromotive force of Li||Sb–Bi–Sn system is determined by Li||Sb and Li||Bi chemistries. The Sn component plays a bifunctional role in the chemistry, decreasing the melting point of Sb–Bi–Sn alloy and providing rapid lithium diffusion paths for the electrode reaction. The ternary feature of the positive electrode ensures a low Sn concentration for the eutectic requirement of the positive electrode working at moderate temperature (500 °C). The stepwise reaction characteristics of Sb and Bi provides a dynamic micr...

Self-diffusion Measurements of Liquid Sn Using the Shear Cell Technique and Stable Density Layering

By utilizing the shear cell technique and achieving stable density layering with the addition of an alloying element Bi, the self-diffusion coefficients of liquid Sn were measured on the ground at 573 K, 773 K, and 973 K (300 °C, 500 °C, and 700 °C). Moreover, the impurity diffusion coefficients of Bi in the liquid Sn were simultaneously measured, to confirm the suppression of natural convection. From the experimental results, natural convection was confirmed to be suppressed, given that the impurity diffusion coefficients of Bi were in good agreement with the microgravity reference data. Upon changing the amounts of added Bi within 5 at. pct Bi, the self-diffusion coefficients of liquid Sn did not vary significantly. Moreover, the SnBi system within 5 at. pct Bi can be regarded as a dilute solution by calculating the activity of Sn in the SnBi system beforehand. The self-diffusion coefficients of the liquid Sn were in good agreement with the power law of temperature dependence from the microgravity reference data. After confirming the suppression of natural convection and that the SnBi system is a dilute solution, the self-diffusion coefficient of liquid Sn was determined as 5.00 ± 0.16 × 10−9 m2 s−1 at 773 K (500 °C). The effectiveness of the experimental method used in this study for the measurement of the self-diffusion coefficients was confirmed, under the condition that the liquid system could be regarded as a dilute solution.

Thermodynamic accessment experimentally on Bi-Sn System by Calorimeter

In this study experimentally measurement of thermodynamic properties as integral enthalpies of mixing on Bi-Sn Binary materials are determined. It has done with calorimeter at 767,813 and 855 K by varying the entire composition of Sn. The four pieces of α-Al2O3 at has been dropped at the dropping temperature for calibration of the calorimeter. The enthalpies of mixing are plotted against tin composition and found endothermic in nature for all the compositions. The maximum enthalpies of mixing are measured at xSn∼0.60 for 767,813 and 855 K. A slightly temperature dependency has been observed for entire composition of Sn. The experimental value has been treated with Redlich-Kister polynomial for substitutional. The experimental values are compared with the value obtained by Redlich-Kister polynomial and found a close agreement with the experimental values

Effect of electrotransfer and indium additions on contact melting and phase formation in bismuth–tin systems

Contact melting with and without electrotransfer is studied in Sn–Bi systems with small additions of indium to each component. The effect admixtures have on the rate of contact melting is shown to be unclear. The structures of the obtained alloys are highly inhomogenous with multiple inclusions of dendrites. An X-ray analysis of the transformed interlayers is performed. A number of intermetallides are observed, including ones not found on the phase diagrams of binary systems. An attempt is made to explain these findings.

Solderability of SnBi-nano Cu solder pastes and microstructure of the solder joints

The solderability of the Sn58Bi (SnBi)-nano Cu solder pastes and the microstructure of the solder joints were investigated. Experimental results indicated that the addition of the nano Cu particles in the SnBi solder paste shows limited effect on the solidus. The liquidus of the SnBi-3nano Cu solder paste was 1 °C higher than the SnBi solder paste. Solid Cu6Sn5 intermetallic particles formed in the SnBi-3nano Cu solder paste during the heating process. The Cu6Sn5 intermetallic particles decreased the mobility and wettability of the molten solder. Meanwhile, the Cu6Sn5 nano particles worked as nucleation sites for the formation of Bi grains and Sn–Bi eutectic phase during the cooling process and led to the grain refinement of the solder bulk. The SnBi-1nano Cu solder paste showed the smallest grain size in this research. Additionally, the SnBi-3nano Cu/Cu solder joint showed a eutectic microstructure of Sn–Bi system at the center of the solder bulk but a hypereutectic microstructure with polygon Bi grains near the margin in the solder bulk.

Planning Technology for Preparing High-Entropy Alloys (Solders) of the Cu–Ga–Pb–Sn–Bi System

Viscosity of alloy of the Cu–Ga–Pb–Sn–Bi system of equiatomic composition in a specimen heating regime followed by cooling is determined with the aim of planning technology for preparing ingots of high-entropy solders. Temperature and time dependences are studied for wetting angle and spot diameter of a steel St3 wetted surface under conditions of combined solder and substrate heating. Ingot and bimetal specimen microstructure obtained as a result of spreading tests is studied metallographically