Thermophysical Properties of Sn–Ag–Cu Based Pb-Free Solders

Lead–tin (Pb–Sn) alloys are the dominant solders used for electronic packaging because of their low cost and superior properties required for interconnecting electronic components. However, increasing environmental and health concerns over the toxicity of lead, combined with global legislation to limit the use of Pb in manufactured products, have led to extensive research and development studies of lead-free solders. The Sn–Ag–Cu ternary eutectic alloy is considered to be one of the promising alternatives. Except for thermal properties, much research on several properties of Sn–Ag–Cu alloy has been performed. In this study, five Sn–xAg–0.5Cu alloys with variations of Ag content x of 1.0 mass%, 2.5 mass%, 3.0 mass%, 3.5 mass%, and 4.0 mass% were prepared, and their thermal diffusivity and specific heat were measured from room temperature to 150 °C, and the thermal conductivity was calculated using the measured thermal diffusivity, specific heat, and density values. Also, the linear thermal expansion was measured from room temperature to 170 °C. The results show that Sn–3.5Ag–0.5Cu is the best candidate because it has a maximum thermal conductivity and a low thermal expansion, which are the ideal conditions to be a proper packaging alloy for effective cooling and thermostability.     

Non-reactive versus dissolutive wetting of Ag–Cu alloys on Cu substrates

The fundamental issues of equilibrium and non-equilibrium (dissolutive) wetting and spreading in the liquid Ag/solid Cu system are studied by the dispensed drop technique. To this end, wetting experiments of Cu, both mono- and poly-crystalline, with two types of Ag–Cu alloys, one non-saturated and the other saturated in copper, are performed at 900 °C in high vacuum by the dispensed drop technique. The results are compared with those given in the literature for the same or similar systems as well as with model predictions.     

Liquid-state and solid-state interfacial reactions between Sn–Ag–Cu–Fe composite solders and Cu substrate

The growth kinetics and morphology of the interfacial intermetallic compound (IMC) between Sn–3Ag–0.5Cu–xFe (x = 0, 0.5 wt%, 1 wt%) composite solders and Cu substrate were investigated in the present work. The Sn–Ag–Cu–Fe/Cu solder joint were prepared by reflowing for various durations at 250 °C and then aged at 150 °C. During soldering process, Fe particles quickly deposited in the vicinity of IMC, resulting in the formation of Fe-rich area. Isothermal equation of chemical reaction and phase diagrams were used to explain the effect of Fe on the growth kinetics of IMC during liquid-state interfacial reaction. It was shown that Fe could effectively retard the growth of interfacial Cu₆Sn₅ and Cu₃Sn layers during liquid-state reaction and reduce the size of Cu₆Sn₅ grains. Small cracks were observed in the Cu₆Sn₅ grains after reflowing for 2 min while they were found in the other composite solders reflowing for about 30 min. The Fe tended to suppress the growth of the Cu₃Sn layer during solid-state aging. However, the total thickness of IMCs (Cu₆Sn₅ + Cu₃Sn) for the composite solders with Fe particles was similar to that for SnAgCu without Fe particles.     

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.     

Structure of metastable precipitate in some Al–Cu–Mg–Ag alloys

Abstract

The high strength of some Al–Cu–Mg–Ag alloys has been attributed to very thin (~2·5 nm), but broad, hexagonal-shaped precipitates. Previous work has shown that the precipitates have a hexagonal unit cell, but different lattice parameters have been reported. In the present paper, the intensities of X-ray diffraction reflections from the precipitates have been measured on Buerger precession photographs, and it is shown that the crystal structure is monoclinic (space group P2/m) with the parameters a = b = 0·496 nm, c = 0·848 nm, γ = 120°. The special values of these parameters confer a hexagonal symmetry on the lattice. This unusual structure is a slightly distorted form of θ-CuAl₂, to which it appears to change after long aging times at 200°C.     

Interfacial reactions in Sn–20In–2.8Ag/Cu couples

Interfacial reactions between Sn–20 wt.%In–2.8 wt.%Ag (Sn–20In–2.8Ag) Pb-free solder and Cu substrate at 250, 150, and 100 °C were investigated. A scallop-type η-Cu₆Sn₅ phase layer and a planar ε-Cu₃Sn phase layer formed at the interface at 250 °C. The indium content in the molten solder near the interface was increased with the formation of the η-Cu₆Sn₅ phase; and the η-Cu₆Sn₅, Ag₂In, Cu₂In₃Sn, and γ-InSn₄ phases formed from the solidification of the remaining solder. At 100 and 150 °C, only the η-Cu₆Sn₅ phase was found at the interface. However, unusual liquid/solid reaction-like interfacial morphologies, such as irregular elongated intermetallic layers and isolated intermetallic grains, were observed in the solid-state reactions. These η phase layers had less Sn content than the Sn–20In–2.8Ag alloy, resulting in an excess Sn-rich γ-InSn₄ phase accumulating at the interface and forming porous η layers on top of the initially formed dense η layers at 150 °C. At 100 °C, large elongated η grains were formed, whereas the interfacial layers remained almost unchanged after prolonged reaction. Based on the experimental evidence, the growth of the η phase was proposed to follow a diffusion-controlled mechanism at 250, 150 and 100 °C, while that of the ε phase was probably controlled by the reaction.     

Tensile creep behavior of Sn–Ag–Cu–Ni multicomponent lead-free solder alloy

In the process of electronic packaging, the dissolution of under bump metallizations, such as Cu and Ni, into liquid solder occurs during soldering, which can change the original solder to a multicomponent one. Under the trend of miniaturization, it is quite necessary to evaluate the properties of multicomponent solder with excessive Cu and Ni compositions. In this study, the tensile creep behavior of Sn–3.5Ag–2.0Cu–0.5Ni multicomponent lead-free solder alloy is investigated at three temperatures, i.e., 303, 348 and 393 K. The steady-rate creep rates are obtained in the range of 10^?4–10^?8 s^?1, when the normalized stress, σ/E, is in the range of 10^?4–10^?3. Based on the Dorn equation, the apparent stress exponent (n _a), threshold stress (σ _th), and activation energy of creep (Q _C) are calculated at the three temperatures. It is found that the Sn–3.5Ag–2.0Cu–0.5Ni solder alloy shows a better creep performance than pure tin and eutectic Sn–3.5Ag solder due to the strengthening effect of Ag₃Sn and (Cu,Ni)₆Sn₅ IMC precipitations. The true stress exponent for creep is identified to be 7, indicating that the creep behave is controlled by the dislocation-pipe diffusion in the tin matrix.     

Thermal shock reliability of micro–nano bimodal Cu–Ag sintered joints

In this study, we investigated the thermal shock reliability of die-attach technology using a micro–nano bimodal Cu–Ag paste, which can considerably reduce material costs compared with a nano-Ag paste. A reliability study of Cu-sintered joints can facilitate large-scale applications in the electric vehicle industry as only a few systematic studies have investigated the thermal shock reliability of low-cost Cu-sintered joints. To evaluate the thermomechanical stability and bond strength of the Cu–Ag sintered joints, a thermal shock test between ? 40 and 150 °C for 1000 cycles and die shear tests, respectively, were performed. The thermal shock test results clearly demonstrated that the micro–nano bimodal Cu–Ag sintered joints maintained a high strength (60 MPa) for 1000 cycles. The bimodal Cu–Ag paste die-attach is reliable because of stable microstructures that are free of cracks and interfacial debonding. The results showed that our bimodal Cu–Ag paste die-attach can be used in both Si and SiC power devices operating at high temperatures.

     

The morphology and kinetic evolution of intermetallic compounds at Sn–Ag–Cu solder/Cu and Sn–Ag–Cu-0.5Al2O3 composite solder/Cu interface during soldering reaction

In this work, Sn3.0Ag0.7Cu (SAC) composite solders were produced by mechanically intermixing 0.5 wt% Al₂O₃ nanoparticles into Sn3.0Ag0.7Cu solder. The formation and growth kinetics of the intermetallic compounds (IMC) formed during the liquid–solid reactions between SAC-0.5Al₂O₃ composite solder and Cu substrates at various temperatures ranging from 250 to 325 °C were investigated, and the results were compared to the SAC/Cu system. Scanning electron microscopy (SEM) was used to quantify the interfacial microstructure for each processing condition. The thickness of interfacial intermetallic layers was quantitatively evaluated from SEM micrographs using imaging software. Experimental results showed that IMC could be dramatically affected by a small amount of intermixing 0.5 wt% Al₂O₃ nanoparticles into Sn3.0Ag0.7Cu solder. A continuous elongated scallop-shaped overall IMC layer was found at SAC/Cu interfaces. However, after the addition of Al₂O₃ nanoparticles, a discontinuous rounded scallop-shaped overall IMC layer appeared at SAC-0.5Al₂O₃/Cu interfaces. Kinetics analyses showed that growth of the overall IMC layer in SAC/Cu and SAC-0.5Al₂O₃/Cu soldering was diffusion controlled. The activation energies calculated for the overall IMC layer were 44.2 kJ/mol of SAC/Cu and 59.3 kJ/mol for SAC-0.5Al₂O₃/Cu soldering, respectively. This indicates that the presence of a small amount of Al₂O₃ nanoparticles is effective in suppressing the growth of the overall IMC layer.     

Annealing effect to constitutive behavior of Sn–3.0Ag–0.5Cu solder

Sn–Ag–Cu based solder alloys are replacing Sn–Pb solders in electronic packaging structures of commercial electric devices. In order to evaluate the structural reliability, the mechanical property of solder material is critical to the numerical simulations. Annealing process has been found to stabilize material properties of Sn–37Pb solder material. In the current study, the annealing effect on tensile behaviour of Sn–3.0Ag–0.5Cu (SAC305) solder material is investigated and compared with Sn–37Pb solder. It is found that the tensile strength for both materials are more stabilized and consistent after the annealing process, nevertheless, the annealing process will improve the plasticity of SAC305 solder dominated by dislocation motion, and impede the occurrence of hardening deformation in Sn–37Pb solder dominated by grain-boundary sliding mechanism. Furthermore, the annealing effect is quantified in the proposed constitutive model based on unified creep–plasticity theory. The parameters are calibrated against the measured stress–strain relationships at the tensile strain rates ranging from 1?×?10^?4 to 1?×?10^?3 s^?1. The numerical regressions for dominant parameters in the proposed model reveal the intrinsic differences between SAC305 and Sn–37Pb solders under annealing treatment.     

Tie line compositions of ternary Cu–Ag–Au alloys within miscibility gap

Abstract

Tie line coordinates of ternary Cu–Ag–Au alloys in the solid immiscible region of the equilibrium phase diagram were determined experimentally by electron probe microanalysis. The two solid phases were analysed in samples which had been thermally treated from the as–cast condition to coarsen the phases. Tie line coordinates were also calculated theoretically using a thermodynamic model which is an extension of the regular solution model. The theoretical calculations agree well with the experimental analysis of the copper–rich phase as well as with a corrected analysis of the silver–rich phase. The necessity for the correction was attributed to the thermal coarsening treatment of the samples.

MST/191     

Cellular growth of lamellar eutectics in undercooled Ag–Cu alloy

Ag–Cu eutectic alloy was undercooled by the glass flux method and the solidification structure was investigated. It is revealed that when undercooling is not more than 70 K, the large difference in composition between two eutectic phases and very large thermal diffusion coefficient of the liquid result in cellular growth of the lamellar eutectics from the nucleation site. The variation in interface temperature during rapid solidification gives rise to a systematic change in microstructure within the sample. With the distance along the growth direction increasing, the finest lamellar spacing across the cellular eutectic rises, which indicates a gradually decreasing growth velocity of the primary eutectics. The primary lamellar eutectics near the nucleation site solidify under conditions far from equilibrium, and therefore are supersaturated with solute, and then partially remelted and ripened into anomalous eutectics. As undercooling increases, the area of the anomalous eutectics enlarges.     

Development of a Sn–Ag–Cu solder reinforced with Ni-coated carbon nanotubes

In this study, Ni-coated carbon nanotubes (Ni-CNTs) were incorporated into the 95.8Sn-3.5Ag-0.7Cu solder alloy using the powder metallurgy route. Up to 0.3 wt% of Ni-CNTs were successfully incorporated. The effects of Ni-CNTs on the physical, thermal and mechanical properties of Sn–Ag–Cu solder alloy were investigated. With the addition of increasing weight percentages of Ni-CNTs, the composite solders showed a corresponding decrease in density values and improved wetting properties. The thermomechanical property results showed an improvement in thermal stability for the composite solders. Mechanical characterization revealed an improvement in ultimate tensile strength (up to 12%) and 0.2% yield strength (up to 8%) with the addition of 0.05 wt% Ni-CNTs in the solder.     

Thermophysical Properties of Ag and Ag–Cu Liquid Alloys at 1098K to 1573K

The surface tension and density of liquid Ag and Ag–Cu alloys were measured with the sessile drop method. The sessile drop tests were carried out at temperatures from 1098K to 1573 K, on cooling (temperature decreasing stepwise) under a protective atmosphere of high purity Ar (6N). The density of liquid Ag and Ag–Cu alloys decreases linearly with increasing temperature, and an increase in concentration of copper results in a lower density. The surface tension dependence on temperature can be described by linear equations, and the surface tension increases with increasing Cu content. The results of the measurements show good agreement with existing literature data and with thermodynamic calculations made using the Butler equation.     

Combined effects of Ag content and cooling rate on microstructure and mechanical behavior of Sn–Ag–Cu solders

Sn–0.7 wt%Cu–1.0 wt%Ag and Sn–0.7 wt%Cu–2.0 wt%Ag alloys were directionally solidified under transient conditions undergoing cooling rates varying from 0.1 to 25 K/s. The microstructure was characterized along the castings lengths and the present experimental results include the secondary dendrite arm spacing (λ₂) and its correlation with: the tip cooling rate ($\dot{T}$) during solidification and microhardness (HV), yield tensile strength (σ_y), ultimate tensile strength (σ_u) and elongation to fracture (δ). The aim is to examine the effects of Ag content and tip cooling rate on both the microstructure and mechanical properties. The initiation of tertiary branches within the dendritic arrangement, as well as the distinct morphologies of the intermetallic compounds (IMC) related to the solidification cooling rate was also assessed for both examined alloys. While the Cu₆Sn₅ phase appeared as large faceted crystals along the entire casting length, very fine Ag₃Sn spheroids prevailed at higher cooling rates (>7.5 K/s and > 4.0 K/s for 1.0 wt%Ag and 2.0 wt%Ag alloying, respectively) with a mixture of Ag₃Sn coarser spheroids and fibers predominating at lower cooling rates. The Sn–0.7 wt%Cu–2.0 wt%Ag alloy exhibited smaller dendritic spacings and HV of about two times higher than the corresponding values of the Sn–0.7 wt%Cu–1.0 wt%Ag alloy. A single Hall–Petch equation is proposed relating δ to λ₂ for both alloys, which means that the increase in Ag content from 1.0 to 2.0 wt% does not affect the elongation. It is shown that δ decreases with the increase in λ₂.     

Influence of POSS nano-particles on Sn–3.0Ag–0.5Cu–xPOSS/Cu composite solder joints during isothermal aging

Polyhedral oligomeric silsesquioxanes (POSS) nano-particles reinforced Sn–3.0Ag–0.5Cu–xPOSS (x = 1, 3, 5) composite solders were prepared and were reflowed on Cu substrates at 543 K. Then, the solder joints were isothermal aged at 393 K for 24 and 48 h, respectively. Microstructural evolution of the solder joints were observed by scanning electron microscopy and the influence of POSS nano-particles on the solder joint were investigated. The results showed that β-Sn primary phase was refined and the number of grain boundary increased with the addition of POSS nano-particles. The growth rates of intermetallic compounds (IMCs) layer were suppressed by the adsorption affection of POSS nano-particles to the IMCs layer during isothermal aging. Moreover, the dissolution process of the IMCs layer, which was accompanied by with the growth of the IMCs layer, changed the morphology of the IMCs layer. The growth rate and the dissolution rate of the IMCs layer in Sn–3.0Ag–0.5Cu–3POSS/Cu composite solder joint were the lowest.     

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.     

Ag—Cu复合材料研究

一、前言 Ag—Cu复合材料的良好综合性能赢得了电子、计算、遥感等仪器仪表的广泛应用。随着现代技术和工业发展,不仅对材料提出了更高的综合要求,而且对白银的用量日益增多,量大面广。但白银资源有限。近年来全世界每年耗银量约一万三千吨,而产量只有一万吨。我国也是如此。所以白银短缺,供不应求是个世界性问题。为了解决这个矛盾和适应综合性能的要求,发达国家广泛采用了固相轧制连续复合、热轧、热压复合、自动连续焊、电镀、     

DSC and TEM characterizations of thermal stability of an Al–Cu–Mg–Ag alloy

Assessment of long-term stability of an aluminium alloy exposed to elevated temperatures is important in the design of lightweight aerospace structures. The manner in which differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) are used together in monitoring microstructural evolution, and thereby assess phase stabilities in an Al–5.1Cu–0.8 Mg–0.5 Ag–0.7 Mn–0.13 Zr (wt%) alloy, are described. DSC thermograms of the alloy, spanning room temperature to 400°C, revealed the presence of two endotherms and an exotherm. TEM investigation has identified these thermal events to be associated with , S, and precipitates. Quantitative TEM was used to measure diameter, thickness, number density, and volume fraction of the precipitates in the alloy exposed at 135°C for times as long as 3000 h. The quantitative TEM results are correlated with the DSC signatures relating to precipitation, dissolution, and coarsening reactions affecting the , S, and precipitates in the exposed alloy.