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.     

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.     

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.     

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 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.     

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.     

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.     

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.     

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%.     

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.     

An evaluation of the lap-shear test for Sn-rich solder/Cu couples: Experiments and simulation

The lap-shear technique is commonly used to evaluate the shear, creep, and thermal fatigue behavior of solder joints. We have conducted a parametric experimental and modeling study, on the effect of testing and geometrical parameters on solder/copper joint response in lap-shear. It was shown that the farfield applied strain is quite different from the actual solder strain (measured optically). Subtraction of the deformation of the Cu substrate provides a reasonable approximation of the solder strain in the elastic regime, but not in the plastic regime. Solder joint thickness has a profound effect on joint response. The solder response moves progressively closer to “true” shear response with increasing joint thickness. Numerical modeling using finite-element analyses were performed to rationalize the experimental findings. The same lap-shear configuration was used in the simulation. The input response for solder was based on the experimental tensile test result on bulk specimens. The calculated shear response, using both the commonly adopted far-field measure and the actual shear strain in solder, was found to be consistent with the trends observed in the lap-shear experiments. The geometric features were further explored to provide physical insight into the problem. Deformation of the substrate was found to greatly influence the shear behavior of the solder.     

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.     

Solderability testing of Sn-Ag-XCu Pb-free solders on copper and Au-Ni-plated kovar substrates

Solderability was evaluated for four Pb-free alloys: 95.5Sn-4.3Ag-0.2Cu (wt.%), 95.5Sn-4.0Ag-0.5Cu, 95.5Sn-3.9Ag-0.6Cu, and 95.5Sn-3.8Ag-0.7Cu on oxygen-free electronic grade (OFE) Cu and Au-Ni plated Kovar substrates. The solderability metric was the contact angle, θ_c, as determined by the meniscometer/wetting balance technique. Tests were performed at 230°C, 245°C, and 260°C using rosin-based, mildly activated (RMA) flux, a rosin-based (R) flux, and a low-solids (LS) flux. The Pb-free solders exhibited acceptable to poor solderability (35°<θ_c<60°) on Cu with the RMA flux. Nonwetting occurred in most tests using the R flux. Wetting was observed with the LS flux, but only at 245°C and 260°C and with high contact angles. The solderability of the Pb-free solders improved at all test temperatures on the Au-Ni plated Kovar substrate when using the RMA flux (30°<θ_c<50°). Wetting was observed with the R flux (35°<θ_c<60°) and LS flux (50°<θ_c<85°) for all temperatures. The Pb-free solders had generally lower wetting rates and longer wetting times on Cu than the 63Sn-37Pb solder. The wetting rate and wetting time data were superior on the Au-Ni plated Kovar substrates. In general, solderability, as measured by θ_c along with the wetting rate and wetting time, did not exhibit a consistent dependence on the composition of the Sn-Ag-XCu (X=0.2, 0.5, 0.6, and 0.7) alloys. The better performers were 95.5Sn-3.9Ag-0.6Cu alloy with the RMA flux (both Cu and Au-Ni plated Kovar) and 95.5Sn-3.8Ag-0.7Cu with the R and LS fluxes (Au-Ni-Kovar, only). The solder-flux interfacial tension, γ_LF, had a significant impact on the θ_c values. The magnitudes of the contact angle θ_c suggested that the four Pb-free solders would experience higher solderability defect counts at the printed wiring assembly level.     

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.     

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.     

Mechanism of fillet lifting in Sn-Bi alloys

The probability of fillet lifting in through-hole solder joints was determined for a series of seven binary Sn-Bi solders with Bi mass contents between 26% and 65%. A 20-pin ceramic dual inline package was soldered to printed wiring boards with three board thicknesses and two pad sizes. A laboratory scale drag soldering process designed to simulate wave soldering was employed. Composition has the largest effect on failure with a peak in lift-off probability near 8%Bi. The presence of intermetallic needles of Cu₆Sn₅ attached to the pad side of the separation surface, but pulled from the solder side, indicates that fillet lifting occurs during solidification. This conclusion is further supported by real time observation of fillet lifting. The solidification of these alloys is analyzed using a Scheil analysis and the tendency for fillet lifting is related to a hot tearing criterion originally developed for castings.     

激光熔覆层凝固特征与凝固组织控制研究

用5kW横流CWCO2激光器在1Cr18Ni9Ti与Cr18Ni12Mo3Ti阀门零件上熔覆NiCrFeBSi合金,得到了厚2～3.5mm,表面平整光滑,组织细密的合金熔覆层,并结合熔覆试验,分析探讨了激光熔覆层快速凝固过程中的凝固特征,凝固组织及组织控制方法。     

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.     

Microstructure and thermal behavior of Sn-Zn-Ag solders

The microstructure and thermal behavior of the Sn-Zn-Ag solder were investigated for 8.73–9% Zn and 0–3.0% Ag. The scanning electron microscopy (SEM) analysis shows the Ag-Zn compound when the solder contains 0.1% Ag. X-ray diffraction (XRD) analysis results indicate that Ag₅Zn₈ and AgZn₃ become prominent when the Ag content is 0.3% and above. Meanwhile, the Zn-rich phase is refined, and the Zn orientations gradually diminish upon increase in Ag content. The morphology of the Ag-Zn compound varies from nodular to dendrite structure when the Ag content increases. The growth of the Ag-Zn compounds is accompanied by the diminishing of the eutectic structure of the Sn-9Zn solder. Differential scanning calorimetry (DSC) investigation reveals that the solidus temperature of these solders exists at around 198°C. A single, sharp exothermic peak was found for the solders with Ag content less than 0.5%. Liquidus temperatures were identified with the DSC analysis to vary from 206°C to 215°C when the Ag content ranges from 1.0% to 3.0%