Liquidus Projection and Solidification of the Sn-In-Cu Ternary Alloys

Sn-In alloys are promising low-melting-point Pb-free solders. Knowledge of the ternary Sn-In-Cu liquidus projection is important for Sn-In solder applications. Sn-In-Cu ternary alloys were prepared and their primary solidification phases and phase-transformation temperatures during heating were determined. The liquidus projection of the Sn-In-Cu ternary system was determined based on the primary solidification phase at different compositional regimes, the phase-transformation temperatures of the ternary alloys, the phase boundaries and reaction temperatures of the constituent binary systems, and the available ternary Sn-In-Cu data in the literature. No ternary compound was found in the as-cast alloys. The Sn-In-Cu liquidus projection has 11 primary solidification phase regions and seven ternary invariant reactions with the liquid phase, and η-(Cu₆Sn₅,Cu₂In) has a very large compositional regime as the primary solidification phase. A very interesting phenomenon that was also observed is that the solidification paths of some Sn-In-Cu alloys surpass the liquidus trough after their intersections.     

Phase equilibria of the Sn-Ag-Cu-Ni quaternary system at the sn-rich corner

Knowledge of phase equilibria of the Sn-Ag-Cu-Ni quaternary system at the Sn-rich corner is important for the understanding of the interfacial reactions at the Sn-Ag-Cu/Ni contacts, which are frequently encountered in recent microelectronic products. Various Sn-Ag-Cu-Ni alloys were prepared and equilibrated at 250°C. The alloys were then quenched and analyzed. The phases were determined by metallography, compositional analysis, and x-ray diffraction (XRD) analysis. No quaternary phases were found. The isoplethal sections at 60at.%Sn, 70at.%Sn, 80at.%Sn, and 90at.%Sn at 250°C are determined. The phase equilibrium relationship was proposed based on the quaternary experimental results and the 250°C isothermal sections of the four constituent ternary systems, Sn-Ag-Cu, Sn-Ag-Ni, Sn-Cu-Ni, and Cu-Ag-Ni. Because there are no ternary phases in all these three systems, all the compounds are in fact binary compounds with various solubilities of the other two elements.     

Liquidus Projections of Sn-Co-Ni and Sn-Rich Sn-Ag-Co-Ni Systems

Alloys based on Sn and Sn-Ag are commonly used as Pb-free solders, and Ni is frequently used in barrier layers. Co has been studied as a possible alloying element in both solders and barrier layers. Thus, the Sn-Co-Ni and Sn-Ag-Co-Ni alloy systems are important for electronic soldering. Forty-nine Sn-Co-Ni alloys and 24 Sn-rich Sn-Ag-Co-Ni alloys were prepared. The primary solidification phases of these as-cast alloys were determined, and based on these results and the available phase diagrams of the constituent systems, the liquidus projections of Sn-Co-Ni ternary and Sn-Ag-Co-Ni quaternary systems at 90 at.% and 95 at.% Sn were determined. In the Sn-Co-Ni system, no ternary compound was found; (Ni,Co)₃Sn₂ and (Ni,Co) are continuous solid solutions, and there are eight kinds of primary solidification phases: Sn, CoSn₃, CoSn₂, CoSn, (Ni,Co)₃Sn₂, (Ni,Co), Ni₃Sn, and Ni₃Sn₄. In the 90 at.% and 95 at.% Sn isoplethal sections of the Sn-Ag-Co-Ni liquidus projection, the primary solidification phases are CoSn₂, CoSn, Ni₃Sn₄, and Ag₃Sn.     

Equilibrium solidification of Sn-Ag-Sb thermal fatigue-resistant solder alloys

A survey of the liquidus surface and invariant reactions involving liquid has been made for solidification in the ternary Sn-Ag-Sb system. Differential thermal analysis and electron-beam microprobe analysis were used to measure liquidus temperatures and determine the composition of solid phases resulting from solidification. A liquidus projection and the composition of the phases coexisting at the two observed invariant reactions were determined. Ternary alloys based on the Sn-Ag-Sb system have been used as thermal fatigue-resistant solders where high heat loads must be dissipated. An analysis of the properties encountered from such solders is presented, based on the phase constitution resulting from the solidification behavior reported here. We wish to acknowledge the contribution of pure metals used in this research, as well as support and advice, by Cominco Electronic Materials of Spokane, Washington.     

Phase Equilibria in the Sn-Rich Corner of the Ni-Sb-Sn System

Phase equilibria in the ternary Ni-Sb-Sn system are of interest for high-temperature soldering, both considering ternary alloys as solder materials themselves or as the basis for understanding the reactions between Sb-Sn-based solders and Ni-based substrates. Therefore, the Sn-rich corner of the ternary Ni-Sb-Sn phase diagram with Sn content of more than 75 at.% was investigated by a combination of powder x-ray diffraction (XRD), differential thermal analysis (DTA), and electron probe microanalysis (EPMA). Ternary phase equilibria and phase compositions of the respective equilibrium phases were determined within the isothermal section at 200°C, and two isopleths were constructed for constant Sn contents of 80 at.% and 85 at.%. The experiments were supported by CALPHAD-type calculations of this ternary system to yield a consistent reaction scheme which shows four invariant ternary transition reactions in this composition range. A liquidus projection is presented, accompanied by the corresponding Scheil diagram.     

Phase Equilibria of the Sn-Ag-Cu-Ni Quaternary System at 210°C

Sn-Ag-Cu alloys are the most promising Pb-free solders, and Ni is a common barrier layer material for the under-bump metallurgy. Although the Sn-Ag-Cu-Ni quaternary system is important for industry, no solid-phase equilibrium information is available. This study determines the equilibrium phase relationship of the Sn-Ag-Cu-Ni system at 210°C. Quaternary alloys are prepared from the pure constituent elements and equilibrated at 210°C. The equilibrium phases formed in the quaternary alloys are determined using scanning electron microscopy, electron probe microanalysis, and x-ray diffraction (XRD) analysis. The quaternary phase relationships are constructed from the quaternary experimental results and the 210°C isothermal sections of the equilibrium phase diagrams of its constituent ternary systems. Two isoplethal sections, 90at.%Sn and 80at.%Sn, of the 210°C phase equilibria of the Sn-Ag-Cu-Ni system are determined. A Cu₆Sn₅ + Ni₃Sn₄ + Ag₃Sn + Sn four-phase region is observed in both isopleths, and no ternary and quaternary intermetallic compounds are found.     

Phase distribution and phase analysis in Cu6Sn5, Ni3Sn4, and the Sn-rich corner in the ternary Sn-Cu-Ni isotherm at 240°C

Metallurgical reactions between solders and under bump metallization (UBM) are key issues for the solder joint reliability in microelectronic packaging. A phase diagram consisting of solders and UBM materials are required to further understand the interfacial reactions and related phase transformation. In this study, series of ternary Sn-Cu-Ni alloys were designed, fabricated, and heat-treated at 240°C. Equilibrium phases of Sn, Ni₃Sn₄, and Cu₆Sn₅ were identified by XRD, and microstructure evidence in backscattered electron image (BEI) micrograph. Through detailed EPMA quantitative analysis, three acme compositions of the ternary region in the Sn-Cu-Ni isotherm near the Sn-rich corner were evaluated and determined. Furthermore, x-ray color mapping of tin, copper, and nickel were applied to study the phase distribution of the alloys with the aid of electron microprobe analysis (EPMA). According to the intensities of Sn, Cu, and Ni, collected by x-ray color mapping, special software was employed to map the corresponding concentrations on the Sn-Cu-Ni ternary isotherm. The degree of composition homogeneity and the phase distribution were further evaluated by phase-analysis techniques. Semiquantitative measurements by phase analysis can be extended to evaluate the phase boundaries with a statistical variation under 5% as compared to the quantitative analysis by EPMA. Finally, the isothermal section of the ternary Sn-Cu-Ni system near the Sn-rich corner at 240°C was constructed.     

Phase equilibria and solidification properties of Sn-Cu-Ni alloys

Ternary Sn-Cu-Ni alloys were prepared and annealed at 240°C. The annealed alloys were metallographically examined and the equilibrium phases formed were identified on the basis of compositional determinations and x-ray diffraction (XRD) analysis. The isothermal section of the ternary Sn-Cu-Ni system at 240°C was proposed on the basis of experimental results of this study and related information on phase equilibrium available in the literature. The binary compounds, Cu₆Sn₅, Ni₃Sn₂, and Ni₃Sn₄, have very extensive ternary solubility. Continuous solid solutions form between Cu and Ni as well as between Cu₃Sn and Ni₃Sn. In addition to the isothermal section, the liquidus projection of the Sn-Cu-Ni system was determined based on results from the existing literature. Interfacial reactions between Sn-Cu alloys and Ni substrate and the primary solidification phases of various Sn-Cu-Ni alloys were also examined in this study.     

The thermal property of lead-free Sn-8.55Zn-1Ag-XAl solder alloys and their wetting interaction with Cu

The wetting behaviors between the quaternary Sn-8.55Zn-1Ag-XAl solder alloys and Cu have been investigated with the wetting balance method. The Al contents, x, of the quaternary solder alloys investigated were 0.01–0.45 wt.%. The results of differential scanning calorimeter (DSC) analysis indicate that the solders exhibit a solid-liquid coexisting range of about 7–10°C. The solidus temperature of the quaternary Sn-8.55Zn-1Ag-XAl solder alloys is about 198.2°C, while the liquidus temperatures are 205–207°C. The experimental results showed that the wettability of the Sn-8.55Zn-1Ag-XAl solder alloys is improved by the addition of Al. The mean maximum wetting force of the solders with Cu is within 0.75–1.18 mN and the mean wetting time is around 1.0–1.1 sec, better than the ∼1.3 sec of eutectic Sn-9Zn and Sn-8.55Zn-1Ag solder alloys. The addition of Al also depresses the formation of ε-Ag-Zn compounds at the interface between Sn-8.55Zn-1Ag-XAl solders and copper.     

The Effect of Bi Contamination on the Solidification Behavior of Sn-Pb Solders

This paper presents experimental results and theoretical calculations that evaluate the effects of Bi contamination on the solidification behavior of Sn-Pb alloys. The pasty (mushy) range, the type of solidification path, and the microstructure of the solidified alloys are described. The experimental results are obtained from thermal analysis and metallography, and the solidification calculations are performed using the lever rule and Scheil assumptions. The experimental results show that the solidification behavior of the contaminated solder at cooling rates of 5°C/min and 23°C/min is closer to the predictions of the lever rule calculations than those of the Scheil calculations. Although the freezing range of Bi-contaminated Sn-Pb solders is increased, formation of a ternary eutectic reaction at 95°C is not observed for contamination levels below the Bi mass fraction of 6%.     

An investigation of phase equilibria of the Bi-Sb-Sn system

The liquidus surface and invariant reactions involving the liquid phase are calculated to determine the solidification behavior of Bi-Sb-Sn alloys. Only two U-type invariant reactions Liq. + Sb₂Sn₃ = SbSn + (Sn) at 517.9Kand Liq. + SbSn = (Sn) + (Sb) at 411.8K are found. Thermodynamic calculation does not predict the presence of a ternary eutectic reaction in this system. The calculated solidification behavior of ternary alloys are found to be consistent with the experimental data as determined by differential scanning calorimetry. An isothermal section of the Bi-Sb-Sn system at 398K is also established. The equilibrium composition of the solid phases are determined by analytical electron microscopy. In contrast to an earlier report, the Bi-content in the SbSn phase is found to be about 5 wt.%.     

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.     

Microstructure, Thermal and Wetting Properties of Sn-Bi-Zn Lead-Free Solder

The microstructures, phase transformations, and wettability of Sn-Bi-Zn solder alloys were investigated by scanning electron microscopy, x-ray diffraction, and differential scanning calorimetry (DSC). The results show that the alloys are composed of primary Sn-rich phase or Zn-rich phase, (Sn + Zn) eutectic structure, and (Sn + Bi + Zn) ternary eutectic structure. The microstructural characterization of Sn-xBi-Zn alloys indicates that, with increasing Bi content, more of the eutectic (Sn-Bi-Zn) structures is formed. DSC profiles reveal that the eutectic peak of the samples did not differ very much, but the reaction temperature of the alloys decreases with increased Bi content. The spreading rates of solders increased with the addition of Zn, which affects the interfacial reactions between the solders and copper.     

Experimental Determination and Thermodynamic Modeling of the Sn-Rich Corner of the Ternary Ni-Pd-Sn Phase Diagram at 250°C

The Sn-rich portion of the phase diagram for the Ni-Pd-Sn ternary system was preliminarily obtained by interpolation of the three constituent binary systems using the Muggianu method. Based on this proposition, 23 Ni-Pd-Sn alloys were prepared and annealed at 250°C. The annealed alloys were analyzed by scanning electron microscopy, electron probe microanalysis, electron backscatter diffraction, and x-ray diffraction. All the binaries adjacent to the Sn-rich corner (i.e., PdSn₄, PdSn₃, PdSn₂, and Ni₃Sn₄) were found to have remarkable ternary solubility. The experimental results presented herein, together with a thermodynamic interpolation of the ternary system based on the results from the binary systems, were used to calculate the ternary phase diagram using the calculation of phase diagrams (CALPHAD) method. A substitution model was used to describe the Gibbs free energies of the liquid and solid solution phases, and a sublattice model was used to describe intermetallic compounds. A consistent set of thermodynamic parameters was obtained, ultimately leading to a better fit between the calculated results and the experimental data for this system.     

Experimental Studies and Thermodynamic Assessment of Ternary Cu-Pd-Sn Phase Relations Focusing on the Sn-Rich Alloys

Recently, Pd surface finishes over Cu substrates have been widely used in microelectronic packaging to prevent pad oxidation before soldering and to improve the bondability with Cu wires. The interfacial reactions between these newly developed Pd-based substrates and Pb-free solders, particularly Sn-based solders, are of practical importance. To completely understand interfacial reactions and phase transformation phenomena, phase equilibrium information on solders, intermetallics, and substrate materials are required. In this study, the phase equilibria of the Cu-Pd-Sn ternary system at 200°C was investigated by scanning electron microscopy, electron probe microanalysis, and electron backscatter diffraction. Particular emphasis was placed on the Sn-rich corner of the isothermal section. To analyze the experimental results, a thermodynamic assessment was performed using the computer coupling of phase diagrams and thermochemistry method.     

Intermetallic Formation and Fluidity in Sn-Rich Sn-Cu-Ni Alloys

This paper investigates the phase equilibria and solidification behavior of Sn-Cu-Ni alloys with compositions in the range of 0 wt.% to 1.5 wt.% Cu and 0 wt.% to 0.3 wt.% Ni. The isothermal section at 268°C in the Sn-rich corner was determined. No evidence for a ternary phase was found, and the section is in good agreement with past experimental studies that report wide solubility ranges for (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄. The vacuum fluidity test was applied to compositions that are liquid at 268°C to map the variation in microstructure and flow behavior with composition in this system. Significant variations in fluidity length were measured among the Sn-Cu-Ni alloys, and the variations correlate with the microstructure that develops during solidification. The generated fluidity map enables the selection of Sn-Cu-Ni solder compositions that exhibit good fluidity behavior during solidification and form near-eutectic microstructures.     

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.     

On the Sn-Bi-Ag ternary phase diagram

The selection and evaluation of Pb-free solders requires information that is best determined through a knowledge of ternary and higher order phase diagrams. As part of an ongoing program on Pb-free solder phase diagrams at the National Institute of Standards and Technology, a thermodynamic model is formulated for the Sn-Bi-Ag phase diagram. Thermodynamic functions for the various phases obtained by fitting measured data for the three constituent binary systems are extrapolated to the ternary system using the method of Muggianu. Modeling results are compared to preliminary experimental data for the ternary system and are applied in the calculation of the solidification path.