Reverse polarity effect and cross-solder interaction in Cu/Sn–9Zn/Ni interconnect during liquid–solid electromigration

The diffusion behavior of Zn atoms and Cu–Ni cross-solder interaction in Cu/Sn–9Zn/Ni interconnects during liquid–solid electromigration were investigated under a current density of 5.0 × 10³ A/cm² at 230 °C. Under the combined effect of chemical potential gradient and electron wind, Zn atoms with positive effective charge number would directionally diffuse toward the Cu interface under both flowing directions of electrons. When electrons flowed from Cu substrate to Ni substrate, EM significantly enhanced the diffusion of Cu atoms to the opposite Ni interface, resulting in the formation of interfacial Cu₅Zn₈; while no Ni atoms diffused to the opposite Cu interface. When electrons flowed from Ni substrate to Cu substrate, only a small amount of Cu atoms diffused to the opposite Ni interface, resulting in the formation of a thin interfacial (NiCu)₃(SnZn)₄ (containing 3 wt% Cu); EM significantly accelerated the diffusion of Ni atoms to the Cu interface, resulting in the formation of a large amount of (NiCu)₃(SnZn)₄ at the Cu interface. Even under downwind diffusion, no apparent consumption of Cu substrate was observed due to the formation of a thick and dense Cu₅Zn₈ layer at the Cu interface. It is more damaging with electrons flowing from Ni to Cu than that from Cu to Ni.     

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.     

Study on properties of Sn–9Zn–Ga solder bearing Nd

The effects of Nd on wettability, microstructure and mechanical properties of Sn–9Zn–Ga–xNd lead-free solder were investigated. The results indicate that adding moderate amount of rare earth Nd, the wettability as well as mechanical properties of Sn–9Zn–0.5Ga solder were evidently improved, and when the content of Nd is at 0.08 wt%, the best wettability and comprehensive properties of soldered joint were obtained. It was also found that the addition of rare earth Nd could refine the microstructure of the solder, but some dark NdSn₃ phase appeared when the addition of Nd exceeded 0.15 wt%. Moreover, the IMCs thickness at the solder/Cu interface was reduced with the addition of Nd which gave a favorable influence on the mechanical property of the soldered joints.     

Effect of Cu/Zn on microstructure and mechanical properties of extruded Mg–Sn alloys

In this paper, the effect of Cu and Zn addition on mechanical properties of indirectly extruded Mg–2Sn alloy was investigated. Mg–2Sn–0.5Cu alloy exhibits a moderate yield strength (YS) of 225?MPa and an ultimate strength of 260?MPa, which are much higher than those of the binary Mg–2Sn alloy, and the elongation (EL) evolves as ～15.5%. Mechanical properties of the Mg–2Sn–0.5Cu alloy are deteriorated with more 3 wt-% Zn addition, and YS and EL are reduced as 160?MPa and ～10%. The detailed mechanism is discussed according to the work-hardening rate and strengthening effect related to the grain sizes, second phases and macro-textures. Grain refinement and proper texture are believed to play a critical role in both strength and ductility optimisation.     

Growth mechanisms of interfacial intermetallic compounds in Sn/Cu–Zn solder joints during aging

The interfacial reactions of Sn/Cu–xZn (x = 15 and 30 at.%) solder joints were investigated. Before aging, [Cu₆(Sn,Zn)₅] and [Cu₆(Sn,Zn)₅/Cu–Zn–Sn] intermetallic compounds (IMCs) formed at the [Sn/Cu–15Zn] and [Sn/Cu–30Zn] interfaces, respectively. After thermal aging at 150 °C for 80 days, [Cu₆(Sn,Zn)₅/Cu₃(Sn,Zn)/Cu(Zn,Sn)/CuZn] and [Cu₆(Sn,Zn)₅/Cu(Zn,Sn)/CuZn] IMCs, respectively, formed at the [Sn/Cu–15Zn] and [Sn/Cu–30Zn] interfaces. Increasing the amount of Zn in the Cu–Zn substrates evidently suppresses the growth of Cu₃Sn and Kirkendall voids at the solder joint interfaces. Transmission electron microscopy images show the different microstructure of CuZn and Cu–Zn–Sn phases in Sn/Cu–Zn joints. These Cu–Zn phases act to inhibit the growth of Cu₆Sn₅ and Cu₃Sn IMCs. As the content of Zn increased in Cu–Zn substrates, both CuZn and Cu(Zn,Sn) grew significantly. In addition, the growth of the Cu₆(Sn,Zn)₅/Cu₃Sn IMCs approached a reaction-controlled process. The formation mechanisms of the CuZn and Cu(Zn,Sn) phases were probed and proposed with regard to the interfacial microstructure, elemental distribution, and the compositional variation at Sn/Cu–xZn interfaces.     

Preparation and characterization of nanoporous Cu6Sn5/Cu composite by chemical dealloying of Al–Cu–Sn ternary alloy

In this article, a new ternary Al–Cu–Sn alloy system has been exploited to fabricate nanoporous Cu₆Sn₅/Cu composite slices through chemical dealloying in a 20 wt% NaOH solution at an elevated temperature. The microstructure of the sliced nanoporous Cu₆Sn₅/Cu composite was characterized using x-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and transmission electron microscopy. The experimental results show that multi-phase precursor alloy comprises α-Al, Sn, and θ-Al₂Cu phases. The new phase Cu₆Sn₅ emerges through dealloying, and the as-dealloyed samples have three-dimensional (3D) structure composed of large-sized channels (hundreds of nanometers) and small-sized channels (tens of nanometers). Both the large- and small-sized pores are 3D, open and bicontinuous. The synergetic dealloying of α-Al and θ-Al₂Cu in the three-phase Al–Cu–Sn alloy and fast surface diffusion of Cu atoms and Sn atoms result in the formation of Cu₆Sn₅/Cu composite with bimodal channel size distributions. In addition, the dealloying duration plays a significant role in the formation of Cu₆Sn₅ and the length scales of the small-sized ligament/channels at a settled temperature.     

Wettability and interfacial whiskers of Sn–9Zn–0.5Ga–0.08Nd solder with Sn,SnBi and Au/Ni coatings

The wettability and interfacial whiskers of Sn–9Zn–0.5Ga–0.08Nd solder were investigated. The results indicated that Sn–9Zn–0.5Nd–0.08Nd solder has shown good wettability using water soluble flux and self made flux based on wetting balance testing, and solder with SnBi coating has shown superiority wettability among all three coatings. Moreover, no whisker was observed in Sn–9Zn–0.5Ga–0.08Nd solder with different coatings. But in Sn–9Zn–0.5Ga–1Nd solder, fewer whiskers were observed in solder with SnBi coating compared with Sn and Au/Ni coatings, which can be attributed to the refinement effect of Bi.     

Structural and optical properties of sulfurized Cu2ZnSnS4 thin films from Cu–Zn–Sn alloy precursors

This study reports the preparation of Cu₂ZnSnS₄ (CZTS) thin films by magnetron sputtering deposition with a Cu–Zn–Sn ternary alloy target and sequential sulfurization. The effects of substrate temperatures on the structural, morphological, compositional as well as optical and electrical properties were characterized. The results showed the CZTS thin films prepared by sulfurization at substrate temperature of 570 °C yielded secondary phases along with CZTS compound. The relatively good properties of CZTS thin film were obtained after sulfurization at substrate temperature of 550 °C. This CZTS film showed compact structure with large grain size of 900 nm, direct optical band gap of 1.47 eV, optical absorption coefficient over 10⁴ cm^?1, resistivity of 4.05 Ω cm, carrier concentration of 8.22 × 10¹⁸ cm^?3, and mobility of 43.38 cm² V^?1 S^?1.     

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.     

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 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.     

Investigation on Sn grain number and crystal orientation in the Sn–Ag–Cu/Cu solder joints of different sizes

Polarized light microscopy and electron backscatter diffraction have been used to quantify the number of β-Sn grains and to examine the Sn crystallographic orientation in Sn–Ag–Cu/Cu solder joints, respectively. The effect of solder joint size on the Sn grain features was investigated due to the miniaturization of solder joints. The Sn–Ag–Cu solder joints of different sizes were found to contain only several β-Sn crystal grains and most solder joints were comprised of no more than three Sn grains. The solder joints showed a preferred crystal orientation. The c crystal axis of β-Sn grains tended to be at a small angle with solder pads. Specific orientation relationships were observed to be prevalent between neighboring β-Sn grains. The grain number, crystal orientation and misorientation were independent of solder joint size.     

Microstructure,mechanical properties and machinability of Cu–Zn–Mg and Cu–Zn–Sb brass alloys

Lead-free alloys have attracted great attentions recently due to the toxic nature of lead for the human body. In this study, low amounts of Mg and Sb were added to the Cu65–Zn35 brass and microstructure, mechanical properties and machinability of samples were compared to Cu65–Zn35 brass. Both Mg and Sb led to the promotion of β′ phase as well as the formation of new ternary copper rich intermetallic particles. It was found that these particles had a significant role in the reduction of the ultimate tensile strength, toughness, work hardening and elongation while increasing the hardness of samples. Results of machinability evaluation of samples showed that the cutting forces were decreased significantly and morphology of chips were improved compared to Cu65–Zn35 brass sample.     

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.     

Effects of rare earth Ce on properties of Sn–9Zn lead-free solder

The influences of different Ce content on the properties of Sn–9Zn lead-free solder were investigated. The results indicate that Ce plays an important role not only in the structure and the solderability, but also in the interfacial structure of Sn–9Zn–xCe/Cu and mechanical property of soldered joint. Sn–9Zn–0.08Ce shows finer and more uniform microstructure than Sn–9Zn, and when the quantity of Ce is 0.5–1 wt%, some dark Sn–Ce compounds appear in the solder. With the addition of 0.08 wt% Ce, the solderability of solder is significantly improved because the surface tension of molten solder is decreased. Adding Ce makes the Cu5Zn8 IMCs formed at the interface of solder/Cu become much thicker than that of Sn–9Zn/Cu because much more content of Zn diffuse to the interface of solder/Cu to react with Cu. Results also indicate that adding 0.08 wt% Ce to the solder enhances mechanical property of soldered joint. When the Ce content is 0.1–0.5 wt%, some hard and brittle Cu–Zn IMCs appear in the bottom of dimples and the pull force of soldered joint decreases.     

Synthesis of Cu2ZnSnS4 films from sequentially electrodeposited Cu–Sn–Zn precursors and their structural and optical properties

The Cu₂ZnSnS₄ (CZTS) films are successfully prepared using a process of sequentially electrodeposited Cu–Sn–Zn precursors by a novel electrolyte formula and optimized parameters on Mo substrate, succeeded by annealing in saturated sulfur atmosphere. The results show that the Cu/Sn/Zn precursor sequence is strict, and optimized electro-deposition parameters are as follows: ?0.6 V, 5 min for Cu, ?1.2 V, 2 min for Sn, and ?1.35 V, 10 min for Zn. Layered precursors firstly alloy into Cu₆Sn₅ and CuZn binary phases under low annealing temperature. Then Cu₆Sn₅ and CuZn alloys decompose in sulfur atmosphere, and form CuS, SnS and ZnS binary phases. Cu₂SnS₃ ternary phase forms through reaction between CuS and SnS with increasing the temperature. Finally, the CZTS film is synthesized through reaction among binary and ternary sulfides. The photoluminescence peak from the CZTS films synthesized at 550 °C for 1 h is about at 1.49 eV.     

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.     

Tensile properties of Cu/Sn–58Bi/Cu soldered joints subjected to isothermal aging

The effects of isothermal aging on the tensile properties of Cu/Sn–58Bi/Cu soldered joints were investigated. Experimental results show that the scallop-shaped Cu₆Sn₅ and planar Cu₃Sn formed at the interface between solder and Cu substrate during reflowing and aging. The thickness of the intermetallic compounds (IMCs) increased almost linearly with the square root of aging time, and aging at 120 °C yielded a much faster growth of the IMCs layer than that of samples aged at 100 °C. The IMCs growth rate constants were 6.02 × 10^?18 and 1.85 × 10^?18 m² s^?1 for solder joints aged at 120 and 100 °C, respectively. The tensile strength of the Sn–58Bi/Cu soldered joints decreased slightly with the increasing aging time and temperature. The failure was dominated by the mixed fracturing in both the solder and the Cu₆Sn₅ grains irrespective of their thermal aging conditions. However, the fracture pattern tended to transform from ductile to brittle with increasing aging time and temperature. The Bi segregation and voids were observed around the Cu/Cu₃Sn interface as the long term aging at high aging temperature was carried out, which resulted in reduction of tensile strength of solder joints.     

Newly observed prismatic Mg2Sn laths in a Mg–Sn–Zn–Mn alloy

Lath-shaped Mg₂Sn precipitates with their habit planes parallel to the prismatic planes of the Mg matrix are characterized in a Mg–Sn–Zn–Mn alloy. The orientation relationships (ORs) between these β-Mg₂Sn precipitates and α-Mg matrix are [0 1 1]_β//[0 1 ?1 0]_α and (0 1 ?1)_β deviating 0.36° to 1.20° from (0 0 0 1)_α, in which the deviation angle of 0.39° is most frequently observed. Although the ORs vary, the laths always exhibit four side facets bearing fixed relationships with g vectors in reciprocal space. Their major side facets incline to the basal plane of Mg matrix from 4.3° to 14.3°.     

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.