Eutectic Sn-Bi Research Articles

The Influence of Stirring Time on Mechanical Properties of Sn–58Bi Alloy Treated in the Semi-Solid State by Mechanical Stirring Process

In this paper, the technology of semi-solid casting was to used are produced Sn-Bi alloy. The influence of agitating time at 125 °C on the microstructure and mechanical properties of of Sn-58Bi casting alloy were discussed. The eutectic Sn-Bi alloy was mechanically agitated during the casting solidification process. The results indicated that non-lamellar Sn-Bi structure with long plate shape, short strip, small size block and suborbicular block shape Sn phase was obtained for different stirring time. The elongation of Sn-Bi alloy with non-lamellar eutectic structure is improved prominently. Compared with the elongation of lamellar eutectic Sn-Bi alloy prepared by conventional die casting, the elongation of semi-solid Sn-Bi alloy with suborbicular block shape Sn phase is enhanced up to 45%.

Microstucture and Mechanical Properties of Semi-Solid Sn–58Bi Alloy

Preparation of semi-solid Sn–58Bi alloys by mechanical stirring casting method was investigated. The effect of temperature below the melting temperature on the microstructures and mechanical properties of semi-solid Sn–58Bi alloys was discussed. The experimental results showed that long plate shape, block shape and suborbicular Sn phases were formed by mechanical agitation during the casting solidification process. The non-lamellar Sn-Bi Microstructure improves prominently the elongation of Sn-Bi alloy. The elongation of semi-solid Sn-Bi alloy with suborbicular Sn phase is enhanced up to 37% compared with the elongation of lamellar eutectic Sn-Bi alloy prepared by conventional die casting. After the agigation above 130 °C, non-lamellar Sn-Bi eutectic structure is formed and lamellar Sn-Bi eutectic precipitates. When the agitating temperature was 135 °C, the elongation of semi-solid Sn-Bi alloy mixed with non-lamellar and lamellar Sn-Bi alloy microstructure is enhanced up to 24%.

Features of island nanostructures formed by melting Sn, Bi and Sn–Bi thin films on C substrates

We report the results of studying the morphological structure of island systems formed by melting Sn, Bi and eutectic Sn–Bi polycrystalline films 2–70 nm thick deposited on carbon substrates. Substrate coverage with a film is found to be non-uniform against film mass thickness after its melting and crystallization. The interval of film thickness is determined where the island systems with narrow enough particle size distribution are formed under melting.

Current enhanced wettability of eutectic SnBi melt on Cu substrate

The effect of a direct electric current on the wetting behaviour of eutectic SnBi melt on Cu substrate at 220°C was investigated in the present study. It is found that the application of current enhanced the growth rate and lateral growth of interfacial reaction layer between eutectic SnBi melt and Cu substrate. Furthermore, with the increase in the current, the spread of eutectic SnBi melt on Cu is accelerated significantly and the steady state contact angle is decreased markedly. The reason for these is also discussed in the present paper.

Electrodeposition of the Sn-58 wt.%Bi layer for low-temperature soldering

The electroplating characteristics and deposition layer of a Sn-Bi eutectic alloy applicable to low-temperature soldering were investigated. A methane sulphonate electrolyte was fabricated and the current density was focused in order to achieve an eutectic Sn-Bi composition. The electroplating characteristics, including the polarization curve and deposition thickness with plating time, were also studied. As experimental results, a polarization curve showed a Sn-Bi codeposition occurred under a −0.5 V electropotential. The Bi content in the Sn-Bi deposit decreased with an increasing current density, such as 99.42 wt.% for −10 mA/cm2 and 42.27 wt.% for −40 mA/cm2. The thickness of the electroplated layer increased with the plating time, while a relatively uniform thickness was obtained in a short plating time, such as less than 40 min. The eutectic deposition of Sn-58.2 wt.%Bi was successfully fabricated by plating at −30 mA/cm2 for 30 min.

Effects of electromigration on resistance changes in eutectic SnBi solder joints

The motivation of this study is tried to explore the relationship between resistance changes and microstructure evolution of eutectic SnBi solder joints under 104 A/cm2 of current density and 50 °C of ambient temperature. A novel type of one-dimensional solder joints was employed to achieve a true uniform distribution of current density, and a real-time data acquisition system was employed to investigate the voltage changes of the eutectic SnBi solder joints during electromigration process. This study suggested that the resistance remained initially due to the interaction between coarsened phase and Joule heating effect, and then increased due to the formation of continuous Bi-rich phase at anode interface, finally remained again due to the phase segregation of Bi- and Sn-rich phases.

An Investigation of Diffusion Barrier Characteristics of an Electroless Co(W,P) Layer to Lead-Free SnBi Solder

Diffusion barrier characteristics for eutectic SnBi solder/electroless Co(W,P) couples were investigated via liquid-state aging at 250°C and solid-state aging at 120°C. At the couple interface, CoSn3 intermetallic compound (IMC) spallation was observed for the SnBi/amorphous Co(W,P) couple subjected to liquid-state aging. In contrast, no spallation of IMCs was observed for the SnBi/amorphous Co(W,P) couples subjected to solid-state aging. For the SnBi/polycrystalline Co(W,P) couple, a thick IMC layer was observed adjacent to a tungsten-enriched amorphous interfacial layer regardless of aging conditions. IMC formation in all samples indicated that Co(W,P) is essentially a sacrificial barrier to SnBi solder. However, amorphous Co(W,P) might also exhibit stuffed-type barrier behavior due to its relatively high phosphorus (P) content. Analytical results indicated that the P content in Co(W,P) is a crucial factor affecting the structural evolution at the SnBi/electroless Co(W,P) interface.

Low-frequency damping properties of eutectic Sn–Bi and In–Sn solders

Eutectic Sn–Bi and In–Sn solders possess much higher damping capacities than Sn–Ag–Cu solders at temperatures above room temperature. The strain amplitude dependence of damping capacities in Sn–Bi solder determined at room temperature is comparable to that of Mg alloys measured at the same temperature. Sn–Bi and In–Sn solders both exhibit conspicuous high-temperature damping backgrounds, with activation energies of 0.77 and 0.27 eV, respectively.

Effect of Ni Addition on the Formation and Growth of Intermetallic Compound at Eutectic SnBi/Cu Interface

The reactions between Cu and the eutectic SnBi (Sn58wt.%Bi) solder alloy with and without 1wt.%Ni addition were investigated in this paper. After as-reflowed process, the IMCs formed in the Sn58wt.%Bi/Cu and Sn58wt.%Bi1wt.%Ni/Cu joints were Cu6Sn5 and (CuNi)6Sn5, respectively. During aging process, the thickness of the IMC layers formed at each solder/Cu joint increased, and a new layer Cu3Sn formed adjacent to the Cu substrate. It was found that 1wt.%Ni addition in Sn58wt.%Bi solder alloy could slightly enhance the growth rate of the total IMC layer, but effectively reduce the growth rate of Cu3Sn layer during aging process. The growth behavior of IMC layer for each joint followed the diffusion-controlled mechanism during aging.

Influence of electrochemical properties on electrochemical migration of SnPb and SnBi solders

SnPb solders have been widely used in many fields because of their excellent solderability. To respond to environmental and health concerns regarding SnPb solders, it is necessary to develop lead-free solder for electronic assembly. In this paper, the resistance to the electrochemical migration of eutectic SnPb and SnBi solder alloys was evaluated and analyzed to detect the difference in the electrochemical migration behavior between these solders by their electrochemical properties. Pb and Bi additions to a Sn-base solder improved the resistance to electrochemical migration because of enhanced polarization properties. The resistance was closely related to the cathodic deposition efficiency and the anodic dissolution behavior. In addition, the composition of the dendrite formed by the electrochemical migration was related to the standard electrode potential of the alloying element.

Retarding the electromigration effects to the eutectic SnBi solder joints by micro-sized Ni-particles reinforcement approach

Electromigration (EM) has become one of the reliability concerns to the electronic solder joint due to its increasing capacity to bear the high current density (10 4 A/cm 2). Although the failure induced by EM can trigger a large void across the entire cathode interface, no effective solutions are presented throughout years of effort on this problem. Here, the composite solder joints are addressed to demonstrate their potential roles on solving the EM issue in the eutectic SnBi solder joints. Micro-sized Ni particles were selected to intentionally add into the solder matrix due to their extensive application as a barrier layer in the under-bump-metallization (UBM) of flip chip solder joints. The ultimate results illustrated that the Ni particles can react with Sn to form the cluster-type Sn–Ni intermetallic compounds (IMCs) inside the solder matrix after the first reflow. Accordingly, the phase segregation of Sn and Bi was significantly inhibited during the current stressing, demonstrating the Sn–Ni IMCs can act as the obstacles to obstruct the movement of dominant diffusion entity (Bi atoms/ions) along the phase boundaries.

Electromigration in Sn-Bi Modified with Polyhedral Oligomeric Silsesquioxane

Electromigration (EM) behavior of eutectic Sn-Bi modified with cage-type polyhedral oligomeric silsesquioxane (POSS) trisilanol was investigated. A direct current (DC) was applied to solder joints newly designed for uniform current distribution throughout the joint. For this study, a current density of 104 A/cm2 was applied at 25°C and 50°C. The evolution of surface and interior microstructure due to current stressing was observed periodically using optical and scanning electron microscopy. The results revealed that the EM behavior was retarded significantly in solder joints with the addition of POSS trisilanol. Different from eutectic Sn-Bi solder joints, no continuous hillocks formed at the anode side and no cracks occurred at the cathode side in solder joints modified with POSS trisilanol even after 336 h of current stressing at 25°C. In addition, the accumulation of Bi/Sn phases at regions near the anode/cathode was also effectively limited. Joints stressed at 50°C also exhibited similar behavior. It is postulated that POSS trisilanol near the phase boundary provided significant restriction to the mass transport due to DC current stressing.

Effect of Y2O3 particles on microstructure formation and shear properties of Sn-58Bi solder

In the present study, varying weight percentages of Y2O3 particles (3–5 μm) from 0 to 3% were incorporated into eutectic Sn-Bi solder matrix to form composite solders. It is found that the reinforcement particles were well dispersed in the solder matrix. They depressed the growth of intermetallic compound (IMC) layers and reduced the size of IMC grains. Since the Y2O3 particles serve as additional nucleation sites for the formation of primary Bi-rich phase, the size of both Bi-rich phase and the IMCs were decreased gradually with the Y2O3 content increasing. Shear tests were also conducted on as—soldered joints. The growth of solid-state intermetallic compounds layer was examined by thermal aging of the solder/Cu couple for a temperature range from 60 to 120°C and time periods from 50 to 500 h. Compared with Sn-58Bi solder, finer eutectic microstructures were obtained with Y2O3 addition after long time aging. The apparent activation energies calculated for the growth of the intermetallic compound layers were 72 ± 5 kJ/mol of Sn-58Bi, 74 ± 4 kJ/mol of Sn-58Bi-0.5wt%Y2O3, 81 ± 5 kJ/mol of Sn-58Bi-1wt% Y2O3 and 81 ± 7 kJ/mol of Sn-58Bi-3wt.% Y2O3, respectively.

Resistance Changes in Eutectic Sn-Bi Solder Joints During Electromigration

The resistance of electronic interconnects made with Sn-based solders experiences significant deterioration during service in high-performance electronic products. During electromigration damage, metal atoms/ions can migrate in the direction of electron flow. Accordingly, accumulation of metal atoms/ions at the anode interface can induce surface bulging. On the other hand, depletion of metal atoms/ions at the cathode interface can induce surface dimpling. In addition, the different α-rich and β-rich phases in a binary eutectic system were found to separate in the bulk region of eutectic Sn-Bi solder joints. Such atomic movement leads to complex interactions between the resistance changes of electronic solder joints during the different stages of electromigration. In order to clarify such scenarios, a LabVIEW ® -controlled system was employed to quantitatively measure the instantaneous resistance values after an electric current was applied. This study investigated the effects of a high current density (10 4 A/cm 2 ) at two different ambient temperatures (25°C and 80°C) on the deterioration of resistance changes during the early stage of the electromigration process.

Effect of Electromigration and Isothermal Aging on the Formation of Metal Whiskers and Hillocks in Eutectic Sn-Bi Solder Joints and Reaction Films

Numerous electronic system failures have been attributed to short circuits caused by metal whiskers that bridged closely spaced circuit elements when they were maintained in a high-current-density environment. Typically, in single-phase interconnect materials, atoms are driven from the cathode to the anode and a compressive stress can build up at the anode end of the stripe, which can induce the formation of whiskers and hillocks. However, the electronic solders used in interconnects are multiphase materials where primary and secondary diffusion atoms/ions exist. In order to accelerate the growth of whiskers and hillocks, a high current density (10 4 A/cm 2 ) combined with high ambient temperatures (80C) was applied to eutectic Sn-Bi solder joints. Metal whiskers and hillocks were observed on the overflowed solder film above the Cu substrate. However, in the absence of the electric field, metal whiskers and hillocks could also be squeezed out from the solder reaction films after several days of isothermal aging (125C), demonstrating that the chemical reaction between Sn atoms and Cu atoms can provide the driving force for the formation and growth of metal whiskers and hillocks.

Effect of small amount of rare earth addition on electromigration in eutectic SnBi solder reaction couple

Effect of small amount of rare earth (RE) addition on electromigration behavior of Cu/SnBi/Cu solder reaction couple (SRC) was investigated with current density of 5 × 103 A/cm2 at room temperature and 100 °C, respectively. Results indicate that tiny RE addition to eutectic SnBi solder alloy can make the energy of interfaces and grain boundaries decrease, restrain the movement of dislocations and grain boundary sliding. Therefore, phase segregation and IMC growth will be effectively suppressed which enhances the electromigration resistance.

Effects of a Third Element on Microstructure and Mechanical Properties of Eutectic Sn-Bi Solder

The effects of a third element, namely silver, copper, zinc, or antimony, on the microstructure and mechanical properties of eutectic tin-bismuth (Sn–Bi) solder were investigated. The investigation showed that, except for zinc, the addition of a trace amount of the third element improves the ductility of the Sn–Bi solder owing to the formation of a fine, homogeneous ternary eutectic microstructure. In particular, the antimony addition is the most effective in improving solder ductility. That is to say, the addition of 0.5 wt% antimony minimizes the grain size of the eutectic microstructure and increases the elongation up to about 40%. Moreover, an intermetallic compound, namely, SnSb, precipitated finely from the solid tin solution near the grain boundaries with bismuth. This fine precipitated intermetallic compound suppresses the coarsening of the eutectic structure and thus improves solder ductility.

Temperature-Dependent Phase Segregation in Cu/42Sn-58Bi/Cu Reaction Couples under High Current Density

The effects of current stressing at 10 4 A/cm 2 on Cu/42Sn-58Bi/Cu reaction couples with a one-dimensional structure at 23°C, 50°C, and 114°C were investigated. The microstructural evolution during electromigration was examined using scanning electron microscopy. The temperature dependence of the coarsening of the Bi-rich phase, the dominant migrating entity, and hillock/whisker formation in eutectic Sn-Bi were investigated under high current density. During current stressing at 10 4 A/cm 2 , the average size of the Bi-rich phase remained the same at 23°C, increased at 50°C, and shrank at 140°C. Bi accumulated near the anode side at both high (50°C, 140°C) and low temperature (23°C). At high temperatures, both Sn and Bi diffused towards the anode side, but Bi moved ahead of Sn during current stressing. However, at low temperatures, Sn reversed its direction of migration to the cathode side. Pure Bi hillocks/whiskers and a mixed structure of Sn and Bi hillocks were extruded as a consequence of compressive stress from electromigration-induced mass flow towards the anode side.

Electromigration in Line-Type Cu/Sn-Bi/Cu Solder Joints

In this study, the different electromigration (EM) behaviors of eutectic Sn-Bi solder in the solid and molten states were clarified using line-type Cu/Sn-Bi/Cu solder joints. When the eutectic Sn-Bi solder was in the solid state during the EM test, a Bi-rich layer formed at the anode side while a Sn-rich band formed at the cathode side, and the intermetallic compound (IMC) at the cathode side was thicker than that at the anode side. The growth of the Bi-rich layer exhibited a linear dependence on the time of stressing. While the actual temperature of the solder joint increased to 140°C and the solder was in a molten state or partially molten state, two separate Bi-rich layers formed at the anode side and a great many Cu6Sn5 IMC precipitates formed between the two Bi-rich layers. Also, the IMC layer at the cathode side was thinner than that at the anode side. With a current-crowding-reduced structure, the products of diffusivity and effective charge number of Bi in the eutectic Cu/Sn-Bi/Cu solder joints stressed with current density of 5 × 103 A/cm2 at 35°C, 55°C, and 75°C were calculated.

Directional growth of Cu 3Sn at the reactive interface between eutectic SnBi solder and (1 0 0) single crystal Cu

The growth mechanisms of Cu 3 Sn at the interface between eutectic SnBi solder and Cu were investigated by transmission electron microscopy. On (1 0 0) Cu the interfacial reaction during reflow resulted in a columnar growth of Cu 3 Sn along Cu [1 0 0] with a special crystallographic relationship of ( 2 1 ¯ 0 ) Cu 3 Sn ∥ ( 4 ¯ 0 2 ) Cu and [ 1 2 2 ] Cu 3 Sn ∥ [ 0 1 0 ] Cu . In the subsequent solid-state aging new triangular Cu 3 Sn grains nucleated and grew at the triple junction sites of the interface between Cu and two adjacent Cu 3 Sn grains.