Phase equilibria of Sn-In based micro-soldering alloys

The phase equilibria of Sn-In-X (X=Ag, Bi, Sb, Zn), the most basic information necessary for the development of Pb-free micro-soldering alloys, were studied using the CALPHAD method. Thermodynamic analyses for describing the Gibbs energies of the constituent phases were made by optimizing the obtained data on the experimental phase diagrams, and such data in the literature, including data on thermochemical properties. The present results combined with the thermodynamic database which was recently developed by our group [I. Ohnuma et al., J. Electron. Mater. 28, 1164 (1999)] provide various information on phase equilibria such as liquidus and solidus surfaces, isothermal and vertical section diagrams, mole fractions of the phase constitutions, etc., and thermodynamic properties such as activity, heat of mixing, surface energy, viscosity, etc., in multi-component soldering alloy systems including the elements of Pb, Bi, Sn, Sb, Cu, Ag, Zn, and In. Typical examples for the phase diagrams and thermodynamic properties of Sn-In-X ternary systems are shown. The application of the database to the alloy design for Pb-free solders is also presented.     

Phase equilibria and the related properties of Sn-Ag-Cu based Pb-free solder alloys

We have recently developed a thermodynamic database for micro-soldering alloys which consists of the elements Pb, Bi, Sn, Sb, Cu, Ag, Zn, and In. In this paper, the phase equilibria and the related thermodynamic properties of the Sn-Ag-Cu base alloys are presented using this database, alloy systems being one of the promising candidates for Pb-free solders. The isothermal section diagrams of the Sn-Ag-Cu ternary system were experimentally determined by SEM-EDS, x-ray diffraction and metallographic techniques. Based on the present results as well as the previous data on phase boundaries and thermochemical properties, thermodynamic assessment of this system was carried out. The isothermal and vertical section diagrams, liquidus surface, mass fractions of the phase constitution, etc., were calculated. The predictions of surface energy and viscosity were also investigated. Moreover, a non-equilibrium solidification process using the Scheil model was simulated and compared with the equilibrium solidification behavior in some Sn-Ag-Cu base alloys. Calculated results based on the Scheil model were incorporated into a three-dimensional solidification simulation and the prediction of practical solidification procedures was performed.     

Thermodynamic database on microsolders and copper-based alloy systems

Recent progress on the thermodynamic databases of calculated phase diagrams in microsolders and Cu-based alloys is presented. A thermodynamic tool, Alloy Database for Microsolders (ADAMIS), is based on comprehensive experimental and thermodynamic data accumulated with the calculation of phase diagrams (CALPHAD) method and contains eight elements, namely, Ag, Bi, Cu, In, Sb, Sn, Zn, and Pb. It can handle all combinations of these elements and all composition ranges. The elements of Al and Au have also been added to ADAMIS within a limited range of compositions. Furthermore, a database of Cu-based alloys, including binary (Cu-X), ternary (Cu-Fe-X, Cu-Ni-X, and Cu-Cr-X), and multicomponent (Cu-Ni-Cr-Sn-Zn-Fe-Si) systems, has also been developed. Typical examples of the calculation and application of these data-bases are presented. These databases are expected to be a powerful tool for the development of Pb-free solders and Cu substrate materials as well as for promoting the understanding of the interfacial phenomena between them in electronic packaging technology.     

Experimental determination and thermodynamic calculation of the phase equilibria in the Cu-In-Sn system

The phase equilibria of the Cu-In-Sn system were investigated by means of the diffusion couple method, differential scanning calorimetry (DSC) and metallography. The isothermal sections at 110–900 C, as well as vertical sections at 10wt.%Cu–70wt.%Cu were determined. It was found that there are large solubilities of In in the ε(Cu₃Sn), δ(Cu₄₁Sn₁₁), and η phases in the Cu-Sn system, and large solubilities of Sn in the γ, η, and δ(Cu₇In₃) phases in the Cu-In system. The η phase was found to continuously form from the Cu-In side to the Cu-Sn side, and a ternary compound (Cu₂In₃Sn) was found to exist at 110 C. Thermodynamic assessment of the Cu-In-Sn system was also carried out based on experimental data of activity and phase equilibria using the CALPHAD method, in which the Gibbs energies of the liquid, fcc and bcc phases are described by the subregular solution model and that of compounds, including two ternary compounds, are represented by the sublattice model. The thermodynamic parameters for describing the phase equilibria were optimized, and agreement between the calculated and experimental results was obtained.     

Thermodynamic modeling of the Au-In-Sb ternary system

The phase diagram of the Au-In-Sb ternary system is of importance in predicting the interface reaction between In-based solders and the Au substrate. Using the calculated phase diagram method and based on experimental data of thermodynamic properties and phase boundaries, the Au-Sb binary system was assessed, and the In-Sb binary system was revised. On the basis of the constituent binary systems, Au-Sb, In-Sb, and Au-In, the Au-In-Sb ternary system was modeled. Several isothermal and vertical sections were calculated, among which the one at 500 K reproduces the experimental diagram well.     

Use of multicomponent phase diagrams for predicting phase evolution in solder/conductor systems

Although the complete phase equilibrium is never reached in interconnection applications, the assumption of local equilibrium at the interfaces is generally valid in most systems composed of dissimilar materials. Therefore, the tie lines in ternary (or multicomponent) phase diagrams—together with the relevant stability diagrams and the mass balance requirements—can be used for predicting the phase sequences (i.e. diffusion paths) formed, for example, in solder/conductor joints. Generally, binary phase diagrams alone cannot provide sufficient information on the phase formation in solder/conductor systems because they do not bear any information on the relative stabilities between different binary phases in multicomponent systems. As examples, the formation of intermetallic compounds in several solder/conductor systems with Au- or Cu-metallization was studied with the help of ternary phase diagrams as well as experimentally. The phase diagrams were calculated using thermodynamic methods. The experimental results confirmed that the dependence of formation of intermetallic compounds on temperature and solder composition is clearly represented by the phase diagrams supplemented with the stability diagrams. Editor's Note: All compositions are given in wt.% throughout the paper unless otherwise stated.     

Experimental investigation and thermodynamic calculation of phase equilibria in the Sn-Au-Ni system

The phase equilibria of the Sn-Au-Ni system, including six isothermal section diagrams in the Sn-rich portion at 200–600°C, as well as three vertical sections at Au:Ni=1:1, 50at.%Sn, and 40at.%Sn, were investigated by means of differential scanning calorimetry (DSC), x-ray diffraction, and metallography. The experimental results indicated that (1) there exists a ternary compound Sn₄AuNi₂ that is stable up to about 400°C, (2) there are larger solubilities of Au in the Ni₃Sn₂ phase in the Sn-Ni system and Ni in the SnAu phase in the Sn-Au system, and (3) there is the two-phase equilibrium between Sn₂Ni₃ and SnAu compounds below 400°C, rather than the continuous phase region from the Sn₂Ni₃ to the SnAu phases reported previously. Thermodynamic assessment of the Sn-Au-Ni system was also carried out by using the calculation of phase diagrams (CALPHAD) method, in which the Gibbs energies of the liquid, fcc, and hcp phases are described by the subregular solution model and that of compounds, including a ternary compound, are represented by the sublattice model. The thermodynamic parameters for describing each phase were optimized and good agreement between the calculated and experimental results was obtained.     

热力学计算在无铅钎料合金设计中的应用

依据CALPHAD方法进行相平衡计算过程中热力学模型的建立及其求解作了简要介绍，对热力学数据库的特征作了描述。对基于CALPHAD方法而开发的Thermo Calc系统的数据库及几个主要模块的功能作了概括的介绍。并针对在无铅钎料研究领域，利用ThermoCalc进行无铅钎料多元合金相平衡计算、预测液态钎料的表面张力和粘度以及钎料与基底之间形成的金属间化合物的驱动力的研究进展进行了总结。着重介绍了日本Tohoku大学材料系的研究人员开发的钎料合金热力学数据库。     

Thermodynamic Assessment of Phase Equilibria in the Sn-Ag-Ni System with Key Experimental Verification

The phase transformations of the Sn-Ag-Ni system were investigated by means of differential scanning calorimetry (DSC). Based on the experimental data of phase equilibria and thermodynamic properties determined by the present work and previous literature, the thermodynamic assessments of the Sn-Ag-Ni system were carried out using the calculation of phase diagrams (CALPHAD) method. The thermodynamic parameters for describing the phase equilibria were optimized, and reasonable agreement between the calculations and experimental data was obtained in the Sn-Ag-Ni ternary system.     

Interfacial reactions in the pb-free composite solders with indium layers

A Pb-free composite solder is prepared with a Pb-free solder substrate and a plated-indium layer. The indium layer melts during the soldering process, wets the substrates, and forms a sound solder joint. Since the melting temperature of indium is 156.6°C, lower than that of the eutectic Sn-Pb, which is at 183°C, the soldering process can be carried out at a temperature lower than that of the conventional soldering process. Composite solder joints with three different Pb-free solders, Sn, Sn-3.5 wt.% Ag, and Sn-3.5 wt.% Ag-0.5 wt.% Cu, and two substrates, Ni and Cu, are prepared. The interfaces between the indium layer, Pb-free solder, and Ni and Cu substrate are examined. A good solder joint is formed after a 2-min reflow at 170°C. A very thick reaction zone at the indium/Pb-free solder interface and a thin reaction layer at the indium/substrate interface are observed.     

Reliability issues in Pb-free solder joint miniaturization

As solder joints become increasingly miniaturized to meet the severe demands of future electronic packaging, it is vitally important to consider whether the solder joint size and geometry could become reliability issues and thereby affect implementation of the Pb-free solders. In this study, three bumping techniques, i.e., solder dipping, stencil printing followed by solder reflow, and electroplating of solders with subsequent reflow, were used to investigate the interfacial interactions of molten Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and pure Sn solders on a copper pad at 240°C. The resultant interfacial microstructures, coming from a variety of Cu pads, with sizes ranging from 1 mm to 25 μm, and representing different solder bump geometries, have been investigated. In addition, a two-dimensional thermodynamic/kinetic model has been developed to assist the understanding of the kinetics of interdiffusion and the formation of interfacial intermetallic compounds. Experimental results and theoretical predictions both suggest that the solder bump size and geometry can influence the as-soldered microstructure; therefore, this factor should be taken into consideration for the design of future reliable ultrafine Pb-free solder joints.     

Surface tension measurements of the Bi-Sn and Sn-Bi-Ag liquid alloys

The maximum bubble pressure method has been used to measure the surface tension of pure Bi, surface tension and density of liquid binary Bi-Sn alloys (X_Bi = 0.2, 0.4, 0.6, and 0.8 molar fractions) at the temperature range from about 500 K to 1150 K. Similarly, there were investigated ternary alloys adding to the eutectic (3.8/at.%Ag-Sn) 0.03, 0.06, 0.09, and 0.12 molar fractions of Bi. The linear dependencies of densities and surface tensions on temperature were observed and they were described by straight-line equation. It has been confirmed that the additions of Bi to liquid Sn and to the eutectic alloy (3.8at.%Ag-Sn) markedly reduce the surface tension. Experimental data of the surface tension of liquid Bi-Sn were compared with modeling based on Butler’s method and a reasonable agreement was observed.     

Characterization of electroplated bismuth-tin alloys for electrically conducting materials

Electrically conducting adhesive materials are promising alternatives for lead (Pb)-containing solders in microelectronic applications. However, most common silver-filled epoxy materials have various limitations to meet the requirements of the solder joints yet. To overcome these limitations, several new formulations have been developed recently. Among them, a new high conductivity Pb-free, conducting adhesive developed for low temperature applications has been previously reported. This conducting adhesive contains a conducting copper filler powder coated with a low melting point metal or alloy, such as Sn or BiSn alloys. The low melting point layer serves as a joining material among the filler particles as well as to the substrate. In this paper, characterization of electroplated BiSn alloys on a Cu substrate is reported for their microstructure, electrical properties, oxidation behavior, and others. The experimental results have provided a better understanding of the joining mechanism of the newly developed Pb-free conductive adhesive materials.     

Experimental determination and thermodynamic calculation of the phase equilibria and surface tension in the Sn-Ag-In system

The phase equilibria of the Sn-Ag-In system were investigated by means of differential scanning calorimetry (DSC) and metallography. The isothermal sections at 180–600°C, as well as some vertical sections, were determined. Thermodynamic assessment of this system was also carried out based on the experimental data of thermodynamic properties and phase equilibria using the calculation of phase diagram (CALPHAD) method, in which the Gibbs energies of the liquid, fcc, and hcp phases are described by the subregular solution model, and those of compounds are represented by the sublattice model. The thermodynamic parameters for describing the phase equilibria were optimized, and reasonable agreement between the calculated and experimental results was obtained. The maximum bubble-pressure method and dilatometric method have been used in measurements of the surface tension and density of the binary In-Sn and ternary (Sn-3.8Ag)_eut + In (5 at.% and 10 at.%) liquid alloys, respectively. The experiments were performed in the temperature range from 160–930°C. The experimental data of the surface tension were compared with those obtained by the thermodynamic calculation of Butler’s model.     

Substrate Dissolution and Shear Properties of the Joints between Bi-Ag Alloys and Cu Substrates for High-Temperature Soldering Applications

The present study investigated interfacial reactions between Cu substrates and Bi-Ag alloys during soldering. Without forming intermetallic compounds (IMCs), the molten solder grooved and further penetrated along the grain boundaries (GBs) of the Cu substrate. An increase in Ag content enhanced GB grooving, raised the dissolution rate and also the amount of dissolved Cu in molten Bi. A stoichiometric Cu-Bi phase formed isothermally in liquid solders and considerably affected the Cu dissolution kinetics. The results also show that Bi-Ag/Cu joints possessed a better shear strength than the Pb-Sn/Cu, which implies that mechanical bonding by grain-boundary grooves was strong enough to withstand shear deformation.     

Studies of the Ag-In phase diagram and surface tension measurements

The phase boundaries of the Ag-In binary system were determined by the diffusion couple method, differential scanning calorimetry (DSC) and metallographic techniques. The results show that the region of the (hcp) phase is narrower than that reported previously. Thermodynamic calculation of the Ag-In system is presented by taking into account the experimental results obtained by the present and previous works, including the data on the phase equilibria and thermochemical properties. The Gibbs energies of liquid and solid solution phases are described on the basis of the sub-regular solution model, and that of the intermetallic compounds are based on the two-sublattices model. A consistent set of thermodynamic parameters has been optimized for describing the Gibbs energy of each phase, which leads to a good fit between calculated and experimental results. The maximum bubble pressure method has been used to measure the surface tension and densities of liquid In, Ag, and five binary alloys in the temperature range from 227°C to about 1170°C. ON the basis of the thermodynamic parameters of the liquid phase obtained by the present optimization, the surface tensions are calculated using Butler’s model. It is shown that the calculated values of the surface tensions are in fair agreement with the experimental data.     

The solid solubility of Ag and Cu in the Sn phase of eutectic and near-eutectic Sn-Ag-Cu solder alloys

Early studies of Ag-Sn and Cu-Sn binary alloys showed very low values, 0.04 wt.% for Ag and 0.0063 wt.% for Cu, for the solid solubility of these elements in Sn at the eutectic temperature. In recent work on “as-cast” Sn-Ag-Cu solder alloys, much higher values have been reported for the Ag and Cu content of the Sn phase. In the present study, wavelength dispersive x-ray microprobe measurements made on a near-equilibrium sample confirmed the earlier solubility values. It was concluded that higher values, some of which are reported in the current paper, represent nonequilibrium, supersaturated solid solutions.     

Zn-Al-Mg-Ga alloys as Pb-free solder for die-attaching use

Zn-based alloys have been investigated to replace Pb-5%Sn solder for die-attaching use. We have found that a Zn-4%Al-3%Mg-3%Ga alloy has a 309°C solidus and a 347°C liquidus. A die-attaching test was done with preforms of this alloy, Ag-plated lead-frames, and Au-plated dummy dies. Good die-attaching with a small amount of voids can be achieved at 320°C or higher. In subsequent reliability tests, no failure was observed until 1000 cycles between −65°C and 150°C or until 1000 h at 85°C and 85% humidity. Although the poor workability and poor ability of stress relaxation at room temperature of this alloy may somewhat limit its application areas, this solder is the first Pb-free solder for die-attaching use to our knowledge.     

基于网络Voronoi图的道路网络连续k近邻查询

道路网络中的连续查询是查找在一条路径上满足查询条件的对象。它是空间网络数据库中的一种重要查询类型。现提出了道路网络中基于k阶Voronoi图的连续k近邻查询方法，该查询方法用分枝限界的思想动态地创建局部Voronoi图，降低了查询代价。     

Phase equilibria in InAsSbP quaternary alloys grown by liquid phase epitaxy

The quaternary alloy InAs_1−x−ySb_xP_y, lattice-matched to InAs, is a promising material for the production of infrared light sources for the detection of gases in the 2–4 μm region of the spectrum. In this work, thermodynamic phase equilibrium calculations have been carried out to determine the compositions required for liquid phase epitaxial growth and the extent of the miscibility gap in the solid material. For high band gap materials, the desired growth temperature is found to be intermediate between a low temperature required to grow P-rich solids and higher temperatures required to avoidspinodal decomposition. Conventional LPE growth at an intermediate temperature of 583°C is found to produce good material with high luminescence efficiency and excellent optical characteristics. Problems with phosphorus loss from the melt are also discussed and lower growth temperatures are found to considerably reduce this problem. Growth in the metastable region between the binodal and spinodal lines has been achieved with the production of phosphorus-rich solids with concentrations up to y = 0.445.