The measurement of interfacial energies for solid Sn solution in equilibrium with the Sn–Bi–Ag liquid

The equilibrated grain boundary groove shapes of solid Sn solution in equilibrium with Sn–Bi–Ag liquid were observed from a quenched sample by using a radial heat flow apparatus. The Gibbs–Thomson coefficient, solid–liquid interfacial energy, and grain boundary energy of the solid Sn solution were determined from the observed grain boundary groove shapes. The thermal conductivity of the solid phase for Sn-10 at.%Bi-2 at.%Ag alloy and the thermal conductivity ratio of the liquid phase to the solid phase for Sn-10 at.%Bi-2 at.%Ag alloy at the melting temperature were also measured with a radial heat flow apparatus and a Bridgman-type growth apparatus, respectively. A comparison of present results for solid Sn solution in the Sn–10 at.%Bi–2 at.%Ag alloy with the results obtained in previous works for similar solid Sn in equilibrium with different binary or ternary liquid was made. From the comparison, it can be concluded that for solid Sn solution in equilibrium with different liquid, the Gibbs–Thomson coefficient seems to be constant and does not depend on the composition of liquid but solid–liquid interfacial energy changes little bit with composition of liquid at a constant temperature.     

Direct observation of ternary solid state transformation in Bi–Cd–In alloy system

Abstract

Cast or solution treated specimens of a Bi–9·0Cd–26·7In (wt-%) alloy were observed to form a fine, three phase microstructure on aging at room temperature, replacing a single phase formed at a higher temperature. The three phases resulting from this solid state reaction were found to grow with a lamellar morphology into the high temperature phase, with a growth rate of 0·5–1·0 μm h^-1 at room temperature. The equilibrium temperature for the transformation was found to be ～25°C. Using a Hitachi S-4500 field emission SEM, the phase transformation was followed in progress at magnifications of 3000 and 10 000 times. It was noted that a volume change was associated with the transformation. It was concluded that the transformation is of the ternary eutectoid type.     

Investigation of Sn–Bi–In ternary solders with compositions varying from Sn–Bi eutectic point to 76 °C ternary eutectic

Journal of Materials Science: Materials in Electronics - The solder properties of Sn–Bi–In ternary solders of seven different compositions are examined. The compositions are...     

Liquid–solid interfacial reactions of Sn–Ag and Sn–Ag–In solders with Cu under bump metallization

Intermetallic compounds formed during the liquid–solid interfacial reaction of Sn–Ag and Sn–Ag–In solder bumps on Cu under bump metallization at temperatures ranging from 240 to 300 °C were investigated. Two types of intermetallic compounds layer, η Cu₆Sn₅ type and ε Cu₃Sn type, were formed between solder and Cu. It was found that indium addition was effective in suppressing the formation of large Ag₃Sn plate in Sn–Ag solder. During interfacial reaction, Cu consumption rate was mainly influenced by superheat of solder, contact area between solder and Cu and morphology of intermetallic compounds. The growth of η intermetallic compounds was governed by a kinetic relation: ΔX = tⁿ, where the exponent n values for Sn–Ag/Cu and Sn–Ag–In/Cu samples at 240 °C were 0.35 ± 0.01 and 0.34 ± 0.02, respectively. The n values increased with reaction temperature, and it was higher for Sn–Ag/Cu than that for Sn–Ag–In/Cu sample at the same temperature. After Cu was exhausted, ε intermetallic compound was converted to η intermetallic compound. The mechanisms for such growth of interfacial intermetallic compounds during the liquid–solid reaction were investigated.     

Formation of microstructure during solidification of Bi–Pb–Sn ternary eutectic

Abstract

To investigate the nature of the Bi–Pb–Sn ternary eutectic, specimens were solidified unidirectionally at very low speeds and quenched to form a representative solid/liquid interface for subsequent study. Specimens made using the generally accepted composition, as reported by Ho et al., did not form all three solid phases from the start of freezing. Specimens produced using the composition reported by Sakurai, i.e., 54 wt-%Bi, 28 wt-%Pb, and 18 wt-%Sn, did give all three phases from the beginning of freezing, indicating that it is the correct eutectic composition. It was found that this eutectic is of the faceted (Bi) non-faceted (X phase) non-faceted (Sn) type. Under the freezing conditions used, a double binary microstructure was formed, with one component consisting of Sn fibres in the X phase and the other of a Bi–Sn complex regular microstructure. While the occurrence of a double binary microstructure was predicted by C. S. Smith for a lamellar ternary eutectic, the current observation shows that it can also occur in a system with one fibrous phase.     

Characterization of interfacial IMCs in low-Ag Sn–Ag–xCu–Bi–Ni solder joints

This research investigated the effects of Cu content on the interfacial IMCs in low-Ag Sn–0.7Ag–xCu–3.5Bi–0.05Ni (x = 0.3, 0.5, 0.7, and 1.5 wt%, respectively) solder joints by deep-etching method and SEM observation. Experimental results indicated that as Cu content increased in the solder, the grain size of the IMCs increased and the thickness of the IMCs decreased on Cu substrate. When the concentration of Cu in the solder was 0.3 wt%, the IMC on the soldering interface was (Cu, Ni)₆Sn₅. The concentration of Ni in (Cu, Ni)₆Sn₅ IMC was significantly suppressed by the increase of Cu content in the solder. As Cu content increased to 1.5 wt%, the concentration of Ni in the IMC decreased to 0 and the IMC transformed from (Cu, Ni)₆Sn₅ to Cu₆Sn₅. Due to the increase of Cu content, more and more (Cu, Ni)₆Sn₅ grains nucleated on Ni substrate, and the morphology of (Cu, Ni)₆Sn₅ transformed from polyhedrons to tiny prisms.     

Interfacial interaction of solid cobalt with liquid Pb-free Sn–Bi–In–Zn–Sb soldering alloys

Dissolution kinetics of cobalt in liquid 87.5%Sn–7.5%Bi–3%In–1%Zn–1%Sb and 80%Sn–15%Bi–3%In–1%Zn–1%Sb soldering alloys and phase formation at the cobalt–solder interface have been investigated in the temperature range of 250–450 °C. The temperature dependence of the cobalt solubility in soldering alloys was found to obey a relation of the Arrhenius type c _s = 4.06 × 10² exp (−46300/RT) mass% for the former alloy and c _s = 5.46 × 10² exp (−49200/RT) mass% for the latter, where R is in J mol⁻¹ K⁻¹ and T in K. For tin, the appropriate equation is c _s = 4.08 × 10² exp (−45200/RT) mass%. The dissolution rate constants are rather close for these soldering alloys and vary in the range (1–9) × 10⁻⁵ m s⁻¹ at disc rotational speeds of 6.45–82.4 rad s⁻¹. For both alloys, the CoSn₃ intermetallic layer is formed at the interface of cobalt and the saturated or undersaturated solder melt at 250 °C and dipping times up to 1800 s, whereas the CoSn₂ intermetallic layer occurs at higher temperatures of 300–450 °C. Formation of an additional intermetallic layer (around 1.5 μm thick) of the CoSn compound was only observed at 450 °C and a dipping time of 1800 s. A simple mathematical equation is proposed to evaluate the intermetallic-layer thickness in the case of undersaturated melts. The tensile strength of the cobalt-to-solder joints is 95–107 MPa, with the relative elongation being 2.0–2.6%.     

Sn–In–Ag phase equilibria and Sn–In–(Ag)/Ag interfacial reactions

Experimental verifications of the Sn–In and Sn–In–Ag phase equilibria have been conducted. The experimental measurements of phase equilibria and thermodynamic properties are used for thermodynamic modeling by the CALPHAD approach. The calculated results are in good agreement with experimental results. Interfacial reactions in the Sn–In–(Ag)/Ag couples have been examined. Both Ag₂In and AgIn₂ phases are formed in the Sn–51.0 wt%In/Ag couples reacted at 100 and 150 °C, and only the Ag₂In phase is formed when reacted at 25, 50 and 75 °C. Due to the different growth rates of different reaction phases, the reaction layer at 100 °C is thinner than those at 25 °C, 50 °C, and 75 °C. In the Sn–20.0 wt%In/Ag couples, the ζ phase is formed at 250 °C and ζ/AgIn₂ phases are formed at 125 °C. Compared with the Sn–20 wt%In/Ag couples, faster interfacial reactions are observed in the Sn–20.0 wt%In–2.8 wt%Ag/Ag couples, and minor Ag addition to Sn–20 wt%In solder increases the growth rates of the reaction phases.     

A model for the prediction of liquid–liquid interfacial energies

A model for the calculation of liquid–liquid interfacial energies is presented. It is based on the assumption that the interface can be treated as a separate thermodynamic phase. Its derivation has been performed in an analogous way as the derivation of the Butler equation for the surface tension of liquid alloys. It requires as input parameters the excess free energy and the compositions of the bulk phases as functions of temperature. In addition, it also requires the partial molar volumes of the components. Comparison with existing experimental data for Al–Pb, Al–In, and Cu–Co in a non-equilibrium state shows very good agreements. For Al–Bi, the experimental data are either over or underestimated by a factor of ≈1.7, depending on which of the two thermodynamic assessments is used. For the Al-based systems, the calculated Al-mole fraction in the interface layer is close to the arithmetic average of the Al-mole fractions of the bulk phases.     

Thermal property,wettability and interfacial characterization of novel Sn–Zn–Bi–In alloys as low-temperature lead-free solders

The effect of indium (In) addition on thermal property, microstructure, wettability and interfacial reactions of Sn–8Zn–3Bi lead-free solder alloys has been investigated. Results showed that addition of In could lower both solidus and liquidus temperatures of the solder alloys with wettabilty significantly improved. The spreading area of Sn–8Zn–3Bi–1.0In was increased by 34% compared to that of Sn–8Zn–3Bi. With the increase of In content, Zn-rich precipitates were smaller in size and distributed more uniformly, which might be beneficial for mechanical properties and corrosion resistance of the solders. The intermetallic compounds (IMCs) formed between Sn–8Zn–3Bi–xIn solder/Cu substrate was identified as Cu–Zn with a scallop layer adjacent to the solder and a flat layer to the substrate. The addition of In slightly influenced the thickness of the IMCs. The newly developed Sn–Zn–Bi–In solder system has great potential to replace the Sn–Pb solders as low-temperature lead-free solders.     

Determination of impurity–point defect binding energies in alloys

Abstract

Modelling of radiation induced or thermal non-equilibrium segregation needs data on impurity–point defect binding energies. These data are generally unavailable although some attempts have been made to calculate them. In the present paper an approach to calculating impurity–interstitial (mixed dumbbell) binding energies is established on the basis of strain field arguments. Earlier work is slightly modified for more accurate calculation of oversized impurity–vacancy binding energies. The method is applied to predictions of various impurity–point defect binding energies in several transition metal matrixes. With the aid of these predictions, other experimental and theoretical results on impurity–point defect binding energies and radiation induced segregation are reasonably explained.

MST/3512     

Effect of addition of Al and Cu on the properties of Sn–20Bi solder alloy

This study investigates the effect of the composite addition of Al and Cu on the microstructure, physical properties, wettability, and corrosion properties of Sn–20Bi solder alloy. Scanning electron microscopy and X-ray diffraction were used to identify the microstructure morphology and composition. The spreading area and contact angle of the Sn–20Bi–x (x?=?0, 0.1 wt% Al, 0.5 wt% Cu, and 0.1 wt% Al–0.5 wt% Cu) alloys on Cu substrates were used to measure the wettability of solder alloys. The results indicate that the alloy with 0.1 wt% Al produces the largest dendrite and the composite addition of 0.1 wt% Al and 0.5 wt% Cu formed Cu₆Sn₅ and CuAl₂ intermetallic compounds in the alloy structure. And the electrical conductivity of Sn–20Bi–0.1Al is the best, which reaches 5.32 MS/m. The spread area of the solder alloy is reduced by the addition of 0.1 wt% Al and 0.5 wt% Cu, which is 80.7 mm². The corrosion products of Sn–20Bi–x solder alloys are mainly lamellar Sn₃O(OH)₂Cl₂ and the corrosion resistance of 0.1 wt% Al solder alloy alone is the best. The overall corrosion resistance of Sn–20Bi–0.1Al–0.5Cu is weakened and the corrosion of solder alloy is not uniform.

     

Origin of microstructure in Bi–Pb and Bi–Sn binary eutectics

Abstract

To study in detail the nature of the advancing solid/liquid interface, thin specimens of the binary eutectic alloys Bi-Pb2 Bi and Bi-Sn were solidified at various low speeds (5.7–115 nm S^?1) and quenched to form a representative interface for subsequent examination. Both eutectic alloys were found to be of the faceted-non-faceted type, with Bi the faceting phase. The solid/liquid interface was observed to be nearly planar and isothermal at the lowest growth rate used, but became increasingly irregular and non-isothermal with increasing growth rate. The maximum observed temperature difference between the most and least advanced parts of the interface was estimated to be ～ 1 K. Massive Bi particles were formed at the lowest growth rate. Although still recognisable in outline, these Bi regions became subdivided at higher growth rates. An extracted particle of subdivided Bi was shown to be a single crystal. In the case of the Bi-Sn eutectic alloy, at the higher growth rate used, two phase projections were sometimes observed to form on the interface, indicating the presence of a layer of liquid undercooled with respect to both phases. An explanation is proposed to account for the formation of this layer.     

Interfacial microstructure and mechanical properties of In–Bi–Sn lead-free solder

The interfacial microstructure and mechanical properties of a low melting temperature lead-free solder of In-18.75Bi-22.15Sn (in at.%) (In–Bi–Sn) were investigated. The microstructure analysis of bulk In–Bi–Sn revealed that irregular lamellar γ-Sn phases distributed in the In₂Bi matrix. There was only a single endothermic peak with an onset temperature of 62 °C on the DSC curve, indicating that In–Bi–Sn is close to a ternary eutectic solder. The ultimate tensile strength of the bulk In–Bi–Sn was 21.76 MP at a strain rate of 10^?2s^?1 at 25 °C. The elongation of the bulk In–Bi–Sn solder reached 87 %, indicating an excellent ductility of the In–Bi–Sn solder. Two intermetallic compounds (IMCs), needle-like Cu(In,Sn)₂ and laminar Cu₆(In,Sn)₅, formed at the In–Bi–Sn/Cu interface. An IMC layer of polyhedral crystallites of InNi formed at the In–Bi–Sn/Ni interface. The shear strength of Cu/In–Bi–Sn/Cu solder joints was 21.15 MP, and the shear fractograph showed that the ductile fracture with dimples appearance occurred in the solder.     

Microstructure of vapour quenched Ti–29 wt% Mg alloy solid solution

A Ti– wt% Mg (Ti–45 at% Mg) alloy produced by vapour quenching has been studied using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, imaging parallel electron energy-loss spectroscopy and electron diffraction. The alloy is shown to be composed of highly supersaturated solid solutions between two elements hitherto believed to be immiscible. It has a microstructure consisting of columnar units, columnar grains, columnar subgrains, herring-bone patterns and parallel stripes ranging in size from 15 m to 2 nm. Superimposed on the microstructure are periodic compositional bands with a period of 0.5 m. The origins of the periodic compositional bands and the herring-bone patterns are discussed.     

Formation of microstructure resulting from quasi-peritectic reactions during freezing of Bi–Pb–Sn and Cu–Ni–Sn ternary alloys

Abstract

The quasi-peritectic reaction, frequently observed in ternary alloy systems, takes the form L + α?β + γ, where L indicates the liquid phase, and α, β and γ indicate solid phases. The formation of microstructure resulting from the kinetics of this reaction is considered from a theoretical point of view and compared with experimental observations of microstructures formed by quasi-peritectic reactions in two ternary alloy systems, Bi–Pb–Sn and Cu–Ni–Sn. Based on these considerations, an explanation is proposed for experimental observations previously reported in literature concerning phase transformations from the liquid in multicomponent ferrous alloys.     

Wetting transition of grain boundaries in the Sn-rich part of the Sn–Bi phase diagram

The microstructural evolution of tin-rich Sn–Bi alloys after the grain boundary wetting phase transition in the (liquid + β-Sn) two-phase region of the Sn–Bi phase diagram was investigated. Three Sn–Bi alloys with 30.6, 23, and 10 wt% Bi were annealed between 139 and 215 °C for 24 h. The micrographs of Sn–Bi alloys reveal that the small amount of liquid phase prevented the grain boundary wetting transition to occur during annealing close to the solidus line. The melted area of the grain boundary triple junctions and grain boundaries increased with increasing the annealing temperature. When the amount of liquid phase exceeded 34 wt% during annealing, increasing temperature has not affected the wetting behavior of grain boundaries noticeably and led only to the increase of the amount of liquid phase among solid grains in the microstructure. The XRD results show that the phase structure and crystallinity remained unchanged after quenching from various annealing temperatures.     

Investigation of microhardness and thermo-electrical properties in the Sn–Cu hypereutectic alloy

Sn–3 wt% Cu hypereutectic alloy was directionally solidified upward with different growth rates (2.24–133.33 μm/s) at a constant temperature gradient (4.24 K/mm) and with different temperature gradients (4.24–8.09 K/mm) at a constant growth rate (7.64 μm/s) in the Bridgman-type growth apparatus. The measurements of microhardness of directionally solidified samples were obtained by using a microhardness test device. The dependence of microhardness HV on the growth rate (V) and temperature gradient (G) were analyzed. According to these results, it has been found that with the increasing the values of V and G the value of HV increases. Variations of electrical resistivity (ρ) and electrical conductivity (σ) for casting samples with the temperature in the range of 300–500 K were also measured by using a standard dc four-point probe technique. The variation of Lorenz coefficient with the temperature for Sn–3 wt% Cu hypereutectic alloy was determined by using the measured values of electrical and thermal conductivities. The enthalpy of fusion for same alloy was determined by means of differential scanning calorimeter from heating trace during the transformation from eutectic liquid to eutectic solid.