The effect of cooling rate on the dendritic spacing and morphology of Ag3Sn intermetallic particles of a SnAg solder alloy

The size and morphology of intermetallic compounds of Sn–Ag solder alloys can have a significant influence on the mechanical strength of solder joints. The aim of the present study is to characterize the as-cast microstructure of a Sn–2 wt.% Ag solder alloy, and to correlate the resulting scale of the dendritic matrix and the morphology of the Ag₃Sn intermetallic compound (IMC) with the corresponding solidification cooling rate. Pre-heated low-carbon steel molds and a water-cooled solidification apparatus were used permitting a significant range of solidification cooling rates to be experimentally examined. It is shown that under very slow cooling conditions (0.02 °C/s) the microstructure of the sample is formed by a coarse dendritic matrix and a mixture of fiber and plate-like Ag₃Sn IMC in the interdendritic region with the fibers located along the board line separating the matrix. For cooling rates from 0.15 to 1.15 °C/s a mixture of spheroid and fiber-like IMC and secondary dendrite arm spacings between 15 and 40 μm, with the spheroids located in the center of the interdendritic region. At higher cooling rates, of about 8 °C/s only Ag₃Sn spheroids (of about 0.5 μm in diameter) prevail in the eutectic mixture.     

Synthesis of Sn–Ag binary alloy powders by mechanical alloying

Sn–Ag binary powders of 2–5 wt%Ag were synthesized by mechanical alloying. Structural evolutions, morphologies, particle size distributions and melting points of the milled Sn–Ag powders were studied. The results show that the milled Sn–Ag powders consist of a supersaturated solid solution of Ag in Sn, Sn(Ag), and Ag₃Sn. During ball milling, Sn, Ag particles in the Sn–3.5Ag powders are deformed, overlapped and cold-welded together to form the Sn/Ag composite particles with a lamellar structure, and then the composite particles are fractured into small spherical particles. When increasing the Ag content from 2 to 5 wt%, the average particle sizes of the 60 h milled Sn–Ag powders are changed from 2.2 to 5.7 μm, and the morphologies of them are changed from spherical shape to irregular shape, respectively. It indicates that the cold-welding and agglomeration of the Sn–Ag powders increases with the Ag content during MA. The melting point of the 60 h milled Sn–3.5Ag powders was detected to be 224.23 °C, near to the eutectic point of the Sn–Ag binary system (221 °C).     

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.     

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.     

In situ nanoparticulate-reinforced lead-free Sn–Ag composite prepared by rapid solidification

Rapid solidification technology has been successfully adopted for the preparation of in-situ nanoparticulate-reinforced Sn–Ag composite solder. The applied rapid solidification process promotes nucleation and suppresses the growth of intermetallic compounds (IMCs) of Ag₃Sn during the eutectic solidification, yielding fine Ag₃Sn nanoparticulates with spherical morphology in the solidified solder structures. Those spherical, homogeneously-distributed nanoparticulates IMCs are benefit to improve the surface area per unit volume and obstruct the dislocation lines passing through the solder. Hence, this in-situ nanoparticulate-reinforced Sn–Ag composite exhibits higher microhardness.     

Effects of microstructure and temperature on corrosion behavior of Sn–3.0Ag–0.5Cu lead-free solder

The heterogeneous microstructure of solder could be obtained when cooling rate of the solder joint was not even, which would affect the corrosion behavior of solder during service. The ambient temperature would also affect the corrosion behavior of solder joint. In this paper, the effects of microstructure and temperature on the corrosion behavior of Sn–3.0Ag–0.5Cu (SAC305) lead-free solder were investigated. The various microstructures of SAC305 lead-free solder were obtained by cooling specimens in air and furnace. Compared to the fine-fibrous Ag₃Sn phase inside the commercial SAC305 solder, platelet-like Ag₃Sn formed as cooling speed decreasing. The polarization behavior of SAC305 solders in 3.5 wt.% NaCl solution was not significantly affected by various microstructures, but sensitive to temperature.     

Effects of nano-structured particles on microstructure and microhardness of Sn–Ag solder alloy

In this research, the typical nano-structured Polyhedral Oligomeric Silsesquioxane (POSS) particles were incorporated into the Sn–3.5Ag eutectic solder paste by mechanically mixing to form lead-free composite solder. The effects of nano-structured POSS additions on the microstructure and mechanical properties of as-fabricated composite solder alloys were systematically investigated. Experimental results indicated that the average size and spacing distance of Ag₃Sn intermetallic compounds (IMCs) in composite solder matrix decreased as compared to the Sn–3.5Ag eutectic solder. The 3 wt% addition of nano-structured POSS particles could enhance the microhardness of composite solder by 18.4% compared with the Sn–3.5Ag eutectic solder matrix. The average grain size and spacing distance of Ag₃Sn IMCs in Sn–Ag + 3 wt% POSS composite solder matrix reduced from 0.35 to 0.23 μm and from 0.54 to 0.32 μm, respectively. The refined Ag₃Sn IMCs, acting as a strengthening phase in the solder matrix, could enhance the microhardness of the composite solders.     

Effects of nano-Al2O3 additions on microstructure development and hardness of Sn3.5Ag0.5Cu solder

This work investigates the effects of nano-Al₂O₃ on the microstructure and microhardness of the Sn3.5Ag0.5Cu composite solder alloy. In comparison with solder without the addition of nano-Al₂O₃ particles, the formation of primary β-Sn phase, the Ag₃Sn phase average size, and the spacing lamellae decreased significantly in the composite solder matrix. In addition, the eutectic areas of the composite solder were wider than that of the Sn3.5Ag0.5Cu solder. This is attributed to the adsorption of nano-Al₂O₃ particles with high surface free energy on the grain surface during solidification. The wettability property was improved by 0.25–0.5 wt.% addition of nano-Al₂O₃ particles into the Sn3.5Ag0.5Cu solder. However, when the nano-Al₂O₃ particles concentration up to over 1.0 wt.% decreased the beneficial influence. Microhardness improved with the addition of nano-Al₂O₃ particles. This improved mechanical property was due to the composite microstructure, which is close to the theoretical prediction from dispersion strengthening theory.     

Microstructure features affecting mechanical properties and corrosion behavior of a hypoeutectic Al–Ni alloy

The aim of this article is to analyze the influence of microstructural parameters on the mechanical properties and corrosion behavior of a hypoeutectic Al–Ni alloy. Experimental results include secondary dendrite arm spacing, corrosion potential, current density, pitting potential, ultimate tensile strength and yield strength. It was found that cooling rates during solidification of about 0.6 °C/s and 8 °C/s can provide secondary dendritic spacings of 7 μm and 16 μm, respectively. Although the microstructure having their phases finely and homogeneously distributed was shown to induce better mechanical properties and higher pitting potential, its general corrosion resistance decreased when compared with the corresponding results of the coarser microstructure.     

The effects of third alloying elements on the bulk Ag<Subscript>3</Subscript>Sn formation in slowly cooled Sn–3.5Ag lead-free solder

The effects of third alloying elements (Cu, In, Zn) on the formation of bulk Ag₃Sn intermetallic compounds (IMCs) in slowly cooled Sn–3.5Ag lead-free solder were investigated by microstructural observation and thermal analysis technique. Microstructural observation shows that bulk Ag₃Sn IMCs existed in the microstructure of slowly cooled Sn–3.5Ag, Sn–3.5Ag–0.75Cu and Sn–3.5Ag–1.5In alloys, while no bulk Ag₃Sn IMCs formed in the slowly cooled Sn–3.5Ag–2.0Zn alloys. Thermal analysis results indicate that Ag preferably reacted with Zn to form Ag–Zn IMCs at high temperature rather than reacted with Sn to form Ag₃Sn plate.     

Characterization of Co-Sn intermetallic compounds in Sn-3.0Ag-0.5Cu-0.5Co lead-free solder alloy

The minor addition of Co into Sn-3.0Ag-0.5Cu lead-free solder alloy triggered the formation of Co-Sn intermetallic compounds. The Sn-3.0Ag-0.5Cu-0.5Co solder alloy was heated up to 300 °C or 400 °C and then cooled down to the room temperature at different rates. A new Co-Sn intermetallic phase, say, CoSn₃ containing small amount of Cu, were detected. Only CoSn₃ phase was formed in the solder alloy from 300 °C regardless of the cooling rate. However, during the solidification from 400 °C, the CoSn₂ + CoSn₃ cascade structures were illustrated after slow furnace cooling due to the peritectical reaction, i.e., CoSn₂ + L(Sn) → CoSn₃, while only CoSn₂ was observed after rapid quench. A novel DSC technique was employed herein to demonstrate the presence of this peritectical reaction. The mechanical properties of the individual phases of Co-Sn intermetallics were measured and compared with other sole phases in the solder alloy.     

Structural, mechanical, hardness indentation measurements of Sn65−x Ag25Sb10Cu x solder alloys rapidly solidified from melt at cooling rate 1.1×105 K s−1

Structural, mechanical properties, and hardness indentation measurements of Sn_65–x Ag₂₅Sb₁₀Cu _x (x=0, 0.5, 1.0, 1.5, 2.0, and 2.5 wt %) solder alloys have been studied and analyzed. The alloy exhibits mechanical properties superior to those in both the Sn–Ag₂₅ binary and Sn–Ag₂₅Sb₁₀ ternary solder alloys. The addition of small amounts of Cu is found to refine the effective grain size, while retaining the uniform distribution of Ag₃Sn, SnSb, and Cu₁₀Sn₃ precipitates in the solidification microstructure, thus significantly improving the ductility and strength.     

Effect of addition of TiO2 nanoparticles on the microstructure,microhardness and interfacial reactions of Sn3.5AgXCu solder

In this work, TiO₂ nanoparticles were successfully incorporated into Sn3.5Ag and Sn3.5Ag0.7Cu solder, to synthesize novel lead-free composite solders. Effects of the TiO₂ nanoparticle addition on the microstructure, melting property, microhardness, and the interfacial reactions between Sn3.5AgXCu and Cu have been investigated. Experimental results revealed that the addition of 0.5 wt.% TiO₂ nanoparticles in Sn3.5AgXCu composite solders resulted in a finely dispersed submicro Ag₃Sn phase. This apparently provides classical dispersion strengthening and thereby enhances the shear strength of composite solder joints. After soldering, the interfacial overall intermetallic compounds (IMC) layer of the Sn3.5AgXCu lead-free solder joint was observed to have grown more significantly than that of the Sn3.5AgXCu composite solder joints, indicating that the Sn3.5AgXCu composite solder joints had a lower diffusion coefficient. This signified that the presence of TiO₂ nanoparticles was effective in retarding the growth of the overall IMC layer.     

Investigation of the fracture morphologies of Sn3.8Ag0.7Cu joints under high-velocity conditions

The lab-designed drop tests and the pendulum-type impact tests were used to study the effect of high-velocity impact on the fracture morphologies of the Sn3.8Ag0.7Cu (wt. %) solder joints. The U-notch butted-type specimen was selected. The typical fracture morphologies created by the high-velocity tests were compared. It is believed that the large Ag₃Sn platelets precipitated at the Cu/solder interfaces after air cooling provided the initiation site for cracking and caused the brittle fracture. Using SEM, the fracture morphologies along the interfacial Ag₃Sn intermetallic compounds (IMCs) were observed on the fracture surfaces. The results showed that the fracture morphologies were a function of the stress state and the orientation of the Ag₃Sn platelets within the solder joints strongly influenced the fracture morphology.     

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.     

Electrochemical behaviour of aluminium alloys containing indium and tin in NaCl solution

The electrochemical behaviour of Al, Al–In, Al–Sn and Al–Sn–In alloys in 2 M NaCl solution has been studied using an open circuit potential, potentiodynamic polarization and ac impedance measurements as well as by optical microscopy examination. The addition of alloying components to aluminium produced in all cases a considerable activation of aluminium. The activation is manifested by shifting the open corrosion potential and the pitting potential in the negative direction (for about 0.6 V) and significant reducing of the passive potential region. The degree of activation depended on alloying element and it was found that there is an increase in the order: Al < Al–In < Al–Sn ≈ Al–Sn–In. The anodic dissolution of the Al–Sn and Al–Sn–In alloys started at open circuit potential which is only 0.45 V more positive than the thermodynamic Al³⁺/Al potential. The ac impedance measurements performed at different potentials in wide potential range (corresponding to passive and active state of each examined samples) confirmed the great activity of Al–Sn and Al–Sn–In alloys compared to aluminium.     

Wetting properties and interfacial microstructures of Sn–Zn–xGa solders on Cu substrate

The wetting properties and interfacial microstructures of Sn–9Zn–xGa lead-free solders with Cu substrate were investigated. The wetting property is improved remarkably with the increase of Ga content in the Sn–9Zn lead-free solder. The lower surface tension, which results from the decrease of the oxidation of the Zn atoms owing to the formation of the Ga-rich protective film covered on the liquid solder, is the key reason for the better wettability. During soldering, the Cu₅Zn₈ compounds layer form at the interface of Sn–9Zn/Cu and the IMCs formed at the solder/Cu surface become much thicker when the Ga content is from 0.1 wt.% to 3 wt.%. However, neither Cu–Sn compounds nor Ga-rich phases are observed at the solder/Cu surface.     

Effect of Al content on the formation of intermetallic compounds in Sn–Ag–Zn lead-free solder

The effect of addition of Al, up to 1 wt.%, on the formation of intermetallic compounds in the microstructure of Sn–3.7%Ag–0.9%Zn lead-free solder was investigated. The typical microstructure of Sn–Ag–Zn solder is composed of the β-Sn phase and mixed granules which contain the intermetallic compounds (IMCs) of Ag₃Sn and AgZn. After alloying with 0.5 wt.% Al, the microstructure of the explored solder evolves into a mixture of the bulk Agl IMCs and β-Sn phase, and most of the bulk Ag₂Al IMCs distribute on and around the grain boundaries. The addition of 1 wt.% Al into the Sn–Ag–Zn solder brings many granules and bars of the Ag₂Al IMCs, while the amount of the bulk Ag₂Al IMCs decrease greatly. The above observation suggests that the bulk Ag₂Al IMCs is replaced by the granule-like Ag₂Al IMCs appearing along the grain boundaries. Since the grain size of the solder alloyed with 0.5%Al is relatively small as compared to the one alloyed with 1 wt.% Al, the growth of the Ag₂Al IMCs was prompted through the feasible diffusion channels along the grain boundaries. Thus, the addition of Al plays an important role on the morphology of the Ag₂Al IMCs in the final microstructure of the explored Sn–Ag–Zn solder.     

Development of solidification microstructure and tensile mechanical properties of Sn-0.7Cu and Sn-0.7Cu-2.0Ag solders

Non modified and Ag-modified eutectic Sn-0.7Cu solder alloys were directionally solidified under transient heat flow conditions. The microstructure of the Sn-0.7Cu alloy has been characterized and the present experimental results include the cell/primary dendrite arm spacing (λ₁) and its correlation with: the tip cooling rate ( $\mathop T\limits^{ \bullet }$ ) during solidification, ultimate tensile strength (σ_u) and elongation to fracture (δ). Distinct morphologies of intermetallic compounds have been associated with the solidification cooling rate for both alloys examined. For the Sn-0.7Cu alloy, cellular regions were observed to occur for cooling rates lower than 0.9 K/s, being characterized by aligned eutectic colonies. On the other hand, the alloy containing 2.0 wt %Ag enabled the launch of tertiary branches within the dendritic arrangement. The comparison of results allows stating that finer solder microstructures are shown to be associated with higher ultimate tensile strengths (σ_u) for both alloys although a more complex microstructure was found for the SAC alloy. In contrast the elongation (δ) exhibited opposite tendencies. The growth of coarse Ag₃Sn fibers and platelets within interdendritic regions seems to contribute for the reduction on ductility observed for the SAC alloy.     

Influence of minor Ag nano-particles additions on the microstructure of Sn30Bi0.5Cu solder reacted with a Cu substrate

The objective of this paper is to study the influence of minor Ag nano-particles additions on the microstructural evolution of Sn30Bi0.5Cu lead-free solder matrices and the interfacial reactions. Ag nano-particle reinforced SnBiCu–xAg (x = 1, 2, 5) composite solder pastes were prepared by a mechanical mixing method and then were reflowed on Cu substrates. Three cooling methods (furnace cooling, air cooling and water cooling) were adopted in this study. Microstructural evolution of composite solder matrices and the intermetallic compound (IMC) layers formed in solder joints were investigated by microstructural observation and phase analysis. The addition of Ag nano-particles did not change the microstructure of solder matrices apparently both in the furnace cooled and water cooled samples. In the air cooled samples, Bi-rich grains were refined in the composite solder matrices due to the adsorption of Ag₃Sn micro-particles on them. The growth of Cu₆Sn₅ IMC layers in air cooled SnBiCu–xAg/Cu solder joints was influenced by the adsorption of Ag₃Sn micro-particles both on the surface of Cu₆Sn₅ layers and on the surface of the Bi-rich grains.