Influence of minor Bi additions on the interfacial morphology between Sn–Zn–xBi solders and a Cu layer

The influence of minor Bi additions on the interfacial morphology between Sn–Zn–xBi (x = 0, 1, 3) solders and a Cu layer after reflowing were investigated by microstructural observations. The addition of minor amount of Bi into Sn–Zn solder reduced the tendency to form cracks at the solder/Cu interface. This is because alloying with Bi reduced the mismatch of the coefficient of thermal expansion (CTE) between the solder alloys and the Cu plate. Moreover, the Sn–Zn solder with Bi reduced the melting temperature of the solder alloy, and, this resulted in the coarsening of the gains and thickening of the intermetallic compound (IMC) layers because solder alloys with a lower melting temperature experienced a longer molten period during reflow. Because the Bi atoms accumulated at the surface of the IMC layers in homogeneously, partially impeding the IMC dissolving into the molten solder, a serrated-like Cu–Zn–Sn IMC layer was formed at the Sn–8Zn–3Bi/Cu interface.     

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.     

Effect of temperature and substrate surface texture on wettability and morphology of IMCs between Sn–0.7Cu solder alloy and copper substrate

In the present work, the effect of soldering temperature (270 and 298?°C) and substrate surface texture (0.02 and 1.12?μm) on wetting characteristics and morphology of intermetallic compounds (IMCs) between Sn–0.7Cu lead-free solder on copper substrates was investigated. It was found that increase in temperature and substrate surface roughness improved the wettability of solder alloy. However, the effect of surface roughness on wettability was significant as compared to that of temperature. The spreading of solder alloy was uniform on smooth substrate, whereas spreading of the alloy on rough substrate resulted in an oval shape. The morphology of IMCs transformed from long needle shaped to short and thick protrusions of IMCs with increase in surface roughness of the substrate. Needle shaped and thick protruded intermetallics formed at the solder/Cu interface were identified as Cu₆Sn₅ compounds. The formation of Cu₃Sn IMC was observed only for the spreading of solder alloy at 298?°C which contributed to improvement in the wettability of solder alloy on both smooth and rough substrate surfaces.     

Activation energies of intermetallic compound growth at interface between Sn–5Bi–3.5Ag solder and Cu substrate

Abstract

The growth kinetics of intermetallic compound layers formed between Sn–5Bi–3.5Ag solder and Cu substrate were investigated at temperatures between 70°C and 200°C for 0 to 60 days. A quantitative analysis of the intermetallic compound layer thickness as a function of time and temperature was performed. Diffusion couples showed a composite intermetallic layer comprised of Cu₆Sn₅ and Cu₃Sn. The growth of intermetallic compounds followed diffusion controlled kinetics and the layer thickness reached only 10 μm after 60 days of aging at 150°C. The apparent activation energies calculated for the growth of the total intermetallic compound (Cu₆Sn₅ + Cu₃Sn), Cu₆Sn₅ and Cu₃Sn intermetallic are 88.6, 84.3 and 70.28 kJ mol^-1, respectively.     

Effects of reflow atmosphere and flux on Sn whisker growth of Sn–Ag–Cu solders

We evaluated the Sn whisker growth behavior of Sn–Ag–Cu solder fillets on lead frames of quad flat packages (QFPs) upon OSP printed circuit boards that were exposed to 85 °C/85% relative humidity (RH) exposure. Three different concentrations of halogen flux for activated Sn-3.0wt%Ag–0.5wt%Cu were used to solder in air and in an inert N₂ reflow atmosphere. The lead frames of the QFPs consisted of Sn plated Cu and Fe-42wt%Ni (alloy 42). Sn whiskers were observed on the surface of the QFP solder fillet joints that were reflowed with halogen containing flux in an air atmosphere. A substantial amount of Sn oxides were formed in those solder fillets while whisker growth and the amount of Sn oxides increased with the halogen content. Sn oxide formation apparently enhanced whisker formation. The combination of air reflow atmosphere and high halogen flux was the worst combination for solder fillet oxidation resulting in Sn whisker formation regardless of the electrode’s lead frame composition of Cu or alloy 42. In contrast, an inert N₂ reflow atmosphere obviously prevented Sn whisker formation on Sn–Ag–Cu solder fillets under all conditions used in this work.     

Interfacial reactions between eutectic Sn–Pb solder and Co substrate

This study examined the reaction couples of Co substrate with liquid and solid Sn-37 wt% Pb alloys, respectively. In the liquid/solid Sn–Pb/Co couples, one single-phase CoSn₃ layer was found at the interface at temperatures from 210 to 250 °C. The layer thickness was measured as a function of the reaction time. The layer growth rate increased with increasing temperature. The CoSn₃ layer growth was linear in the early stage and then became parabolic. In addition, the similar CoSn₃ growth behavior results were observed in the solid/solid Sn–Pb/Co couples from 150 to 170 °C. Nevertheless, one Pb-rich phase layer accumulated at the solder/CoSn₃ interface due to Sn consumption. Also, the reaction kinetics of Sn-37 wt% Pb/Co was symmetrically studied at various temperatures. The chemical reaction constants, diffusion constants, and the relevant activation energies were reported.     

Physical properties of Sn58Bi–xNi lead-free solder and its interfacial reaction with copper substrate

The aims of this research are to investigate the effects of Ni on the physical properties of Sn58Bi–xNi lead-free solder, and to examine its interfacial reaction with the copper substrate. In the experiments, four concentrations of Ni (i.e. 0.05, 0.1, 0.5 and 1.0 wt.%) were individually added into Sn58Bi and their respective microstructure, tensile strength, elongation, melting temperature, wettability and electrical resistivity of Sn58Bi–xNi were subsequently measured. The results indicated that Ni refined the microstructure of the solder matrix and induced the formation of Ni₃Sn₄ intermetallic phase, and that the size and volume fraction of Ni₃Sn₄ were positively correlated to the Ni content. The optimal concentration of Ni to enhance the tensile strength of the alloy was 0.1 wt.%, but the elongation of the alloy was inversely correlated to the Ni content. The addition of Ni contributed positively to the melting temperature and wetting behavior of the alloy, whereas no significant change in the electrical resistivity of Sn58Bi–xNi was detected. In addition, Ni increased the thickness of the intermetallic layer at the interface, and only monoclinic η′-Cu₆Sn₅ phase was present at the intermetallic layer. Nevertheless, the intermetallic phase of this research was dissimilar from the findings of existing literature.     

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.     

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.     

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.     

Electromigration-enhanced intermetallic growth and phase evolution in Cu/Sn–58Bi/Cu solder joints

This paper presents some experimental observations relative to the influence of elevated current densities on the intermetallic growth and phase evolution in Cu/Sn–58Bi/Cu solder joints. Three samples were stressed with different current densities of 10⁴, 1.2 × 10⁴, and 1.4 × 10⁴ A/cm², respectively, for 80 h. The abnormal polarity effect of electromigration (EM) on chemical reactions at the cathode and the anode was investigated as well as the effect of EM on phase segregation in the two-phase eutectic microstructure. Results indicate that electric current enhances the growth of IMC layer at the cathode and retards it at the anode due to the Bi accumulation acting as a barrier layer with current density of 10⁴ A/cm². However, when current density increases, the electrical force dissolves the IMC at the cathode into the solder. More and more intermetallic precipitates formed due to the dissolution of Cu into the solder at the cathode side with increased current densities, leading to a very different morphology at the anode and the cathode interfaces, one being planar and the other being very irregular. It can be concluded that the chemical force and the electrical force are the main driving forces contributing to the IMC growth at both interfaces.     

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.     

Electromigration-induced Bi-rich whisker growth in Cu/Sn–58Bi/Cu solder joints

Effect of current stressing on whisker growth in Cu/Sn–58Bi/Cu solder joints was investigated with current densities of 5 × 10³ and 10⁴ A/cm² in oven at different temperatures. Two types of whiskers, columnar-type and filament-type, were observed on the solder film propagating along the surface of the Cu substrate and at the cathode interface, respectively, accompanied with many hillocks formation. Typically, these whiskers were 5–15 μm in length and 0.06–2 μm in diameter. EDX revealed that these whiskers and hillocks were mixtures of Sn and Bi rather than single crystal. It should be noted that the sprouted whiskers would not grow any more even if the current-stressing time increased again when the solder joint was stressed under lower current density. Nevertheless, when the current density was up to 10⁴ A/cm², the whiskers would melt along with the increasing current-stressing time. Results indicated that the compressive stress generated by precipitation of Cu₆Sn₅ intermetallics provides a driving force for whisker growth on the solder film, and the Joule heating accumulation should be responsible for whisker growth at the cathode interface.     

Growth behavior of interfacial Cu–Sn intermetallic compounds of Sn/Cu reaction couples during dip soldering and aging

The formation and growth of intermetallic compound (IMC) layer at the interface between pure Sn and a Cu substrate during dip soldering and aging were studied. The soldering was conducted at 250 °C using dipping method, followed by aging treatment at 150 °C for up to 10 days. The results showed that the IMC layer flattened with aging duration because the grooves in scallop-like IMC provide a more convenient access for Cu atoms to dissolve and react with solders and previous IMCs. And when isothermal aging was subjected, the growth rate of Cu₆Sn₅ was lower than that of Cu₃Sn due to Cu₃Sn growing rapidly with aging time by consuming Cu₆Sn₅ at the interface of Cu₃Sn/Cu₆Sn₅. Kirkendall voids were observed at Cu₃Sn/Cu interface as well as inside the Cu₃Sn layer as the Sn/Cu couple was aged at 150 °C for prolonged time, with which the Cu₃Sn IMC dominates the interfacial IMCs growth. During solid-state aging, the mean diameter (d) of interfacial Cu₆Sn₅ grains increased dramatically with the increasing time (t). The relationships between d and t were given to be d = 1.22 t ^0.291 for samples formed at 250 °C for 1 min and d = 1.53 t ^0.259 for samples formed at 250 °C for 5 min, respectively.     

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 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.     

Effect of Bi content on spalling behavior of Sn–Bi–Zn–Ag/Cu interface

The effect of the Bi content on the formation of intermetallic compounds (IMCs) layers between the Sn-xBi-0.9Zn-0.3Ag lead-free solder (with x = 1, 2, 3 and 4, in weight percent, hereafter) and Cu substrate was investigated. The structure of the IMC layer in the soldered interface varies apparently with increasing the Bi content. When the Bi content is 1 wt%, the interface soldered is consisted of CuZn and Cu₆Sn₅ IMC layers, which are separated by an intermediate solder layer. As the Bi content increases, the spalling phenomenon tends to disappear. Moreover, the layer between the Sn-2Bi-0.9Zn-0.3Ag solder and Cu substrate is thicker than others. The evolution of the soldered interfacial structure could be attributed to the existence of Bi.