Effect of adding Ce on interfacial reactions between Sn–Ag solder and Cu

We investigated the effect of adding cerium (Ce) to low Ag content Sn–1.0wt.%Ag solder on the interfacial reactions between the Sn–1.0Ag solder and Cu substrate. The formation and growth of interfacial intermetallic compounds (IMCs) between the Sn–1.0Ag–0.3Ce solder and Cu substrate were studied and the results were compared to those obtained for the Ce-free Sn–1.0Ag/Cu and most promising Sn–3.0Ag–0.5Cu/Cu systems. The addition of Ce to the Sn–Ag solder significantly reduced the growth of the interfacial Cu–Sn IMCs, retarded the interfacial reactions between the solder and the substrate, and prevented the IMC from spalling from the interface. The Sn–1.0Ag–0.3Ce solder alloy had a good interfacial stability with the Cu substrate during solid-state isothermal aging in the viewpoint of IMC growth.     

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.     

Interfacial reactions with and without current stressing at Sn–Co/Ag and Sn–Co/Cu solder joints

Sn–Co alloys are promising Pb-free solders, while plating layers and substrates of Ag and Cu are commonly encountered in electronic products. This study examines the interfacial reactions between Sn–0.25 wt% Co/Ag and Sn–0.25 wt% Co/Cu at 180 and 210 °C, with and without current stressing. CoSn₃ precipitates are found in the solder matrix in the as-prepared condition. In Sn–0.25 wt% Co/Ag couples, a continuous Ag₃Sn reaction phase layer is observed at the interface and Ag₃Sn phase particles are dispersed in the matrix, with and without current stressing. When there is a 500 A/cm² electrical current, the growth rate of the Ag₃Sn phase is not affected at either the cathode side or the anode side. However, the passage of an electrical current leads to the formation of needle-like Ag₃Sn phase particles in the solder matrix. In Sn–0.25 wt% Co/Cu couples, both Cu₆Sn₅ and Cu₃Sn reaction phases are formed at the interface, with and without current stressing. Cu₆Sn₅ precipitates, with a higher Co content, are found in the matrix, mostly nucleated on CoSn₃ precipitates. When there is a 500 A/cm² electrical current stressing, all the reaction phase layers are thicker and the anode interfaces are nonplanar. It is observed that there is cracking and that there are discontinuous Cu₆Sn₅ layers at the interface and that a significant amount of Cu₆Sn₅ phase in the matrix accompanies 500 A/cm² electrical current stressing.     

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.     

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.     

Interfacial reaction of Sn–2.0Ag–2.5Zn solder on Cu and Ni–W substrates

The interfacial reactions of Sn–2.0Ag–2.5Zn solder on Cu and Ni–W substrates after soldering and subsequent aging have been investigated in this study. Ni–W alloy layers with tungsten content of 3.0 and 10.0 at.% were electrodeposited on copper substrate. The interfacial micrographs of solder joints prepared at 250 °C for 15 s and aged at 150 °C for 24, 96 and 216 h are shown. Double-layer IMC composed of Cu₅Zn₈ and Ag₃Sn was observed at the interface of Sn–2Ag–2.5Zn and Cu couple, which was compact and acted as a barrier layer to confine the further growth of Cu–Sn IMC. On Ni–W barrier layer, a thin Ni₃Sn₄ film appeared between the solder and Ni–W layer, whose thickness decreases with the increase of W content. During the aging process, a thin layer of the Ni–W substrate transforms into an amorphous bright layer, and the thickness of amorphous layer increased as aging time extended. Referring to the elemental line-distribution and the thickness of different layers at the interface, the formation of the bright layer is caused by the fast diffusion of Sn into Ni–W layer.     

Interfacial reaction between Au–Sn solder and Au/Ni-metallized Kovar

Gold-tin (Au–Sn) solder and Kovar alloy have been widely used in many fields such as mechanical engineering, atomic energy industry, aerospace facility, and electronic devices. Solder bonds strongly to the metallized substrate by forming intermetallic compounds (IMCs) at the interface. The IMC layer may adversely affect the reliability of the joints due to excessive growth and thermal fatigue during storage and service. Therefore, knowledge of the interfacial reactions between the Au–Sn solder and Au/Ni-metallized Kovar in microelectronic and optoelectronic packaging is essential. In this study, the microstructural evolution and interfacial reactions between the Au–Sn solder and Au/Ni-plated Kovar substrate were studied during aging at 180 and 250 °C for up to 1,000 h. The microstructure of the Au–Sn/Ni/Kovar joint was stable during aging at 180 °C. The solid-state interfacial reaction was much faster at 250 °C than at 180 °C. The joints aged at 250 °C fractured along the interface, thereby demonstrating brittle failure possibly because of the brittle IMC layer at the interface. The complete consumption of the thin Ni layer significantly weakened the joint interface during aging at 250 °C and clearly demonstrated the need for a thicker Ni layer in order to ensure the high temperature reliability of the Au–Sn/Ni/Kovar joint above 250 °C.     

Corrosion behavior of Sn–3.0Ag–0.5Cu solder under high-temperature and high-humidity condition

The aim of this study is to evaluate the corrosion behavior of Sn–3.0Ag–0.5Cu (SAC305) solder alloy under high-temperature and high-humidity condition. The corrosion of SAC305 alloy was attributed to the oxidation of Sn, which formed SnO₂ and SnO, and SnO₂ existed on the outer layer of the oxide film. After a period exposure, a stable and dense protective oxide film formed on the specimen surfaces, and the specimen which exposed at 75 °C had the thickset oxide film.     

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.     

A study of Ag additive methods by comparing mechanical properties between Sn57.6Bi0.4Ag and 0.4 wt% nano-Ag-doped Sn58Bi BGA solder joints

Sn–Bi solder was proposed as one of the most promising substitutes for lead solder due to its lower melting temperature, good wettability, good yield strength and cost efficiency. With Ag elements added, the mechanical properties of Sn–Bi solder were improved obviously. There are two ways that are commonly used to add the reinforced particles into the solder. The first way (Way I) is to blend the reinforced particles with solder powders together, and then followed by pressure forming, sintering, cooling, crystallization and serious machining methods under inert atmosphere to make the solder paste. Another way (Way II) is to directly add the reinforced nano or micro particles into the solder paste by sufficient mechanical-stirring. In this research we would like to get fully understanding on the effects of these two ways of Ag addition on the mechanical properties of Sn–Bi–Ag solder joints during aging. Sn57.6Bi0.4Ag solder stands for the Way I and the doped Sn58Bi + 0.4Ag solder stands for the Way II. These two kinds of joints were compared via micromorphology observation, thermal failure analyses as well as balls shear strength measurement after different aging time (under 100 °C, from 0 to 800 h). The mechanical properties of Sn57.6Bi0.4Ag and the doped Sn58Bi + 0.4Ag solder joints during aging were shown to be associated with the changes of micromorphology, the dissolution of IMCs, as well as the flatness of the joints’ interface. Before long-time aging, the doped Sn58Bi + 0.4Ag solder joints showed better mechanical performance than Sn57.6Bi0.4Ag solder joints. During aging, Sn56.7Bi0.4Ag solder joints had better performance in preventing the dissolution of Ni–Sn IMCs into the solder side, having smoother interfaces, comparing with Sn58Bi + 0.4Ag solder joints. The degenerated phenomenon of Ag nanoparticle reinforcement seriously happened in the doped Sn58Bi + 0.4Ag solder joints. After longtime aging, Sn57.6Bi0.4Ag solder joints had better mechanical properties than the doped Sn58Bi + 0.4Ag solder joints.     

Effect of microstructure and Ag3Sn intermetallic compounds on corrosion behavior of Sn–3.0Ag–0.5Cu lead-free solder

In this paper, the effects of microstructure on the corrosion behavior of Sn–3.0Ag–0.5Cu (SAC305) lead-free solder were investigated by potentiodynamic polarization and atmospheric corrosion test. Scanning electron microscopy and X-ray diffraction were used to characterize the samples after the electrochemical and atmospheric corrosion tests. Results showed that commercial SAC305 solder exhibits better corrosion resistance than air-cooled and furnace-cooled SAC305 solders both in 3.5 wt% NaCl solution and at 60 °C/100 % relative humidity condition.     

Effect of the soldering time on the formation of interfacial structure between Sn–Ag–Zn lead-free solder and Cu substrate

The evolution of interfacial structure between the Sn–3.7%Ag–0.9%Zn lead-free solder and Cu substrate were systematically explored for different soldering times (1, 5, and 10 min). According to microstructural observations, it is found that the longer the soldering time is, the thicker the soldered interface becomes. The interface soldered for 1 min is composed of the Cu₅Zn₈ intermetallic compounds (IMCs) layer locating above the Cu₆Sn₅ IMCs layer. The interfaces soldered for 5 and 10 min are mainly made up of the Cu₆Sn₅ IMCs with some bulk Ag₃Sn IMCs randomly distributing within it. The evolution of the IMCs layer in the soldered interface can be divided into three stages: the Cu₅Zn₈ IMCs firstly forms, then Cu₆Sn₅ IMCs separated out from the bottom (controlled by diffusion of Sn in the Cu₅Zn₈), finally, subsequent growth of the Cu₆Sn₅ IMCs layer is controlled by diffusion of Sn in Cu₆Sn₅ IMCs.     

Effects of Ga addition on microstructure and properties of Sn–0.5Ag–0.7Cu solder

The effects of rare element Ga on solderability, microstructure, and mechanical properties of Sn–0.5Ag–0.7Cu lead-free solder were investigated. The experimental results show that Ga plays a positive role in improving the wettability and the microstructure of the solder. When the content of Ga is at 0.5 wt%, the grain size of the solder is smaller and the shear force is enhanced greatly. It is also found that the thickness of the IMCs at the solder/Cu interface is reduced with proper addition of Ga. The increase of mechanical properties may be related to the refining of IMCs of the solder due to Ga addition.     

Interfacial reaction and growth behavior of IMCs layer between Sn–58Bi solders and a Cu substrate

This paper reports on the interfacial reaction and growth behavior of intermetallic compounds (IMCs) layer (η-Cu₆Sn₅ + ε-Cu₃Sn) between molten Sn–58Bi solder and Cu substrate for various liquid–solid soldering temperatures and times. In addition, the Bi segregation at the Cu₃Sn/Cu interface was also discussed, too. It was found that the Cu₆Sn₅ IMC could be observed as long as the molten solder contacted with the Cu substrate, while the Cu₃Sn IMC was formed at the interface between Cu₆Sn₅ and Cu substrate as the higher soldering temperature and/or longer soldering time were applied. Both thickness of total IMCs layer and Cu₆Sn₅ grains size increased with increased soldering temperature or time. The growth of the Cu-Sn IMCs layer during soldering exhibited a linear function of the soldering temperature and 0.27 power of soldering time. With soldering temperature increasing (above 280 °C in this present study), Bi was accumulated at the Cu₃Sn/Cu interface and resulted in some isolated Bi particles were formed.     

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.     

Reactive wetting of Sn–2.5Ag–0.5Cu solder on copper and silver coated copper substrates

In the present work, wetting characteristics and morphology of intermetallic compounds (IMCs) formed between Sn–2.5Ag–0.5Cu lead-free solder on copper (Cu) and silver (Ag) coated copper substrates were compared. It was found that, Ag coated Cu substrate improved the wettability of solder alloy. The average values of contact angles of solder alloy solidified on Ag coated Cu substrate were reduced to about 50 % as compared to contact angles obtained on Cu substrates. Flow restrictivity for spreading of solder on Ag coated Cu was found to be lower as compared to Cu substrate. The spreading of solder alloy on Ag coated Cu exhibited halo zone. Coarse needle shaped Cu₆Sn₅ IMCs were observed at the solder/Cu substrate interface whereas at the solder/Ag coated Cu interface Cu₆Sn₅ IMCs showed scallop morphology. The formation of Cu₃Sn IMC was observed for the spreading of solder alloy on both substrates. The solder/Ag coated Cu substrate interface exhibited more particulates of Ag₃Sn precipitates as compared to solder/Cu substrate interface. The improved wettability of solder alloy on Ag coated Cu substrate is due to the formation of scallop IMCs at the interface.     

Kinetics of intermetallic compound layer growth and interfacial reactions between Sn–8Zn–5In solder and bare copper substrate

Abstract

The growth kinetics of the intermetallic compound layer formed between Sn–8Zn–5In solder and bare Cu substrate by solid-state isothermal aging were examined at temperatures between 70 and 150°C for times up to 100 days. Experimental results showed that the intermetallic compound observed on the bare copper substrate was γ-Cu₅Zn₈ and its thickness increased with ageing temperature and time. The layer growth of the intermetallic compound in the couple of the Cu/Zn satisfied the parabolic law at the given temperature range. As a whole, because the values of time exponent (n) were approximately 0·5, the layer growth of the intermetallic compound was considered to be mainly controlled by diffusion mechanism over the temperature range studied. The apparent activation energy for growth of the γ-Cu₅Zn₈ intermetallic compound was 62 kJ mol^?1.     

Effects of Co additions on electromigration behaviors in Sn–3.0 Ag–0.5 Cu-based solder joint

As the miniaturization trend of electronic packing industry, electromigration (EM) has become a critical issue for fine pitch packaging. The EM effects on microstructure evolution of intermetallic compound layer (IMC) in Sn–3.0 Ag–0.5 Cu + XCo (X = 0, 0.05, 0.2 wt%) solder joint was investigated. Findings of this study indicated that current stressing of Sn–3.0 Ag–0.5 Cu–0.2 Co solder joint with 10⁴ A/cm² at 50 °C for 16 days, no remarkable EM damages exhibited in solder matrix. Whereas, after current stressing at 150 °C for 1 and 3 days, Sn–3.0 Ag–0.5 Cu specimens showed obvious polarity effect between cathode and anode. Different morphology changes were also observed at both sides. After current stressing for 1 day, two IMC layers, Cu₆Sn₅ and Cu₃Sn, with wave type morphology formed at cathode. Sn phases were also observed inside in the IMC layer. However, only Cu₆Sn₅ formed in anode. Three days later, Sn phases were found in anode. Besides, Co additions, aging treatment, Ag₃Sn, and other IMCs improved the resistance of EM by the evidence of retarding polarity effect.     

Effects of 0.1 wt% Ni addition and rapid solidification process on Sn–9Zn solder

Effects of 0.1 wt% Ni addition and rapid solidification process on Sn–9Zn solder alloy were investigated. Characteristics of Sn–9Zn–0.1Ni alloy were analyzed compared with those of as-solidified Sn–9Zn alloy. Mechanical properties and interfacial microstructure of solder/Cu joints obtained using these solders were comparatively studied. By comparison with as-solidified Sn–9Zn alloy, the wettability of solder was obviously improved with 0.1 wt% Ni addition, and the melting behavior of the solder was promoted due to the rapid solidification process. The corrosion resistance of as-solidified and rapidly solidified Sn–9Zn–0.1Ni alloys was improved due to the formation of Ni–Zn intermetallic compound (IMC) and the refining of Zn-rich phases. Formation and growth of IMCs at the interface of Sn–9Zn–0.1Ni/Cu joints was significantly depressed. Rapid solidification process promoted the interfacial reaction during soldering and improved the bonding strength of joints.     

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.