Sn-Bi Solder Research Articles

Significant enhancement of comprehensive properties of SnBi solder through the addition of Cu@Ag core-shell particles

As a critical low-temperature lead-free solder, SnBi eutectic solder was known for its abundant lamellar structures of Bi-rich phase, which often resulted in problems such as brittleness, low electrical conductivity, and low thermal conductivity. In this study, a method was introduced to enhance the comprehensive properties of SnBi solder by adding Cu@Ag core-shell particles. Our results revealed that the modified SnBi solder exhibited significant refinement in both grain size and intragranular eutectic structures, resulting in a shift from the original brittle fracture mode to a mixed brittle and tough mode and a consequent increase in tensile strength by 21%. Moreover, the modified solder demonstrated a significant 59% increase in electrical conductivity by forming interconnected highspeed channels for electron migration. In addition, the addition of Cu@Ag core-shell particles to SnBi solder resulted in remarkable contributions of both electrons and phonons to thermal conductivity, with its value of 41.33 W m−1 K−1. Our research addressed partial property deficiencies of SnBi solder, which could be valuable in expanding its application in low temperature electronic packaging.

Finite Element Analysis of Sn-58Bi Shear Test

Finite element analysis (FEA) of quasi-static single lap shear test was conducted to study the mechanical properties of Sn-58Bi in terms of deformation, strain, stress, yield strength and ultimate strength. The study generated results of simulated behaviour of low temperature Sn-Bi solder alloy to address the demand of low temperature solder alloys in the electronic industry. The geometry was constructed in SolidWorks based on ASTM D1002 standard before being imported to ANSYS. The simulation was conducted at shear velocity of 0.1, 0.5, 1 and 1.27 mm/min and temperature of 138, 178, 208 and 238 °C using ANSYS. The simulation results were compared with the corresponding experimental results. Simulation results showed that an increase in shear velocity caused a decrease in ultimate strength and an increase in ductility of Sn-58Bi. As the velocity increased from 0.5 to 1.27 mm/min, the ultimate strength decreased from 339.87 to 291.24 MPa. This is predicted due to the Bi-rich layer within the lamellar structure of Sn-Bi alloy. An increase in temperature caused an increase in ultimate strength and brittleness in Sn-58Bi due to the trans-granular propagation of brittle failures along the weak Bi-abundant phase. As the temperature increased from 138 to 238°C, ultimate strength increased from 290.46 to 298.93 MPa.

Effect of multi-walled carbon nanotubes on the microstructure and properties of Sn-58Bi solder alloys

The influence of multi-walled carbon nanotube (MWCNT) content on the melting properties, wettability, microstructure, and intermetallic compounds (IMCs) of Sn-58Bi-xMWCNT (x = 0, 0.01, 0.02, 0.03, 0.05, and 0.10 wt.%) and the effects of MWCNT on the tensile strength of Cu/Sn-58Bi-xMWCNT/Cu solder joint were analyzed. The results show that MWCNT addition at a small trace amount into Sn-58Bi solder could increase the spreading area of Sn-58Bi solder, refine the solder matrix microstructure, inhibit the growth of interfacial IMCs, and significantly improve the tensile strength of Sn-58Bi solder joints. However, the melting point and melting range remained the same or slightly changed. The wettability of Sn-58Bi alloys increased first and then decreased with the addition of MWCNT. The spreading area and the tensile strength of solder joint were the maximum when the MWCNT content was 0.01 wt.%, and they were 42.23 mm2 and 50.07 MPa, respectively. The reason may be related to the improved microstructure of solder alloys and the morphology of IMCs due to MWCNT addition at trace level.

Hot tear cracks on the suppression of Sn–Bi alloy for low-temperature assembly

PurposeIn the electronic assembly industry, low-temperature soldering holds great potential to be used in surface mounting technology. Tin–bismuth (Sn–Bi) eutectic alloys are lead-free solders applied in consumer electronics because of their low melting point, high strength and low cost. This paper aims to investigate how to address the problem of hot tear crack formation during Sn–Bi low-temperature solder (LTS) in the mass production of consumer electronics.Design/methodology/approachThis paper explored the development of hot tear cracks during Sn–Bi soldering in the fabrication of flip chip ball grid arrays. Experiments were designed to simulate various conditions encountered in Sn–Bi soldering. Quantitative analysis was conducted on the number of hot tear cracks observed in different alloy compositions and solder volumes to explore the primary cause of hot tear cracks and possible methods to suppress crack formation.FindingsHot tear cracks existed in Sn–Bi solders with different bismuth (Bi) contents, but increasing the solder volume reduced the number of hot tear cracks. Experiments were designed to test the degree of chip transient thermal warpage with temperature change, and, according to the results, glue was dispensed in specific areas to reduce chip warpage deformation. Finally, the results of combined process experiments pointed to an effective method of low-temperature soldering to suppress hot tear cracks.Research limitations/implicationsThe study focuses on Sn–Bi solders only without other solder pastes such as SAC305 or Sn–Zn series.Practical implicationsWith the growing popularity of smart electronics, especially in intelligent terminals, new energy vehicles electronics, solar photovoltaic and other field, there will be more and more demand for low- temperature, energy-saving, lead-free solders. Therefore, this study will help the industry to roll out LTS (Sn–Bi) solutions rapidly.Social implicationsIn the long term, lean and green manufacturing is expected to be essential for maintaining an advanced manufacturing industry across the world. Developing new LTSs and soldering processes is the most effective, direct solution for energy conservation and emission mitigation. With the growing popularity of smart electronics, especially in intelligent terminals, new energy vehicles and solar photovoltaics, there would be an increased demand for low-temperature, energy-saving, lead-free techniques.Originality/valueAlthough there are many methods that can be used to suppress hot tear cracks, there is little research on how to control the hot tear cracks caused by the low-temperature soldering of Sn–Bi in laptop applications. The authors studied the hot tear cracks that developed during the world’s first mass production of 50 million personal laptops based on low-temperature Sn–Bi alloy solder pastes. By controlling the Bi content, redesigning the solder paste printing process (e.g. through a printer’s stencil) and adding dispensing processes, the authors obtained reliable and stable experimental data and conclusions.

Effects of Near-amorphous multiphase intermetallic compound particles on the microstructure and mechanical properties of SnBi solders

Effects of Near-amorphous multiphase intermetallic compound particles on the microstructure and mechanical properties of SnBi solders

Preparation and performance of Sn-based composite solder joints by solid-liquid low-temperature solder bonding technology

In the era of big data, the advantages of 3D packaging are becoming increasingly significant. In addition, the mismatch between the coefficient of thermal expansion (CTE) of the PCB and BGA components leads to dynamic warpage of the system, and the expansion and innovation of low-temperature soldering have been a new trend. We developed a fully mixed solder joint of a SAC305(Sn-3.0Ag-0.5Cu wt.%) solder ball and Sn–58Bi(wt.%) solder paste by solid-liquid low-temperature solder bonding technology. The mixing degree and diffusion process of the two were calculated and simulated by mathematical calculation and COMSOL Multiphysics. The results show that a completely mixed solder joint can be obtained after reflow at 210 °C for 5 min. The average shear strength of the composite solder joints was 63.68 ± 1.12 MPa, which was about twice that of the single SAC305 and Sn58Bi solder joints, and the shear fracture was a composite mode of brittle and ductile fracture, which appeared in the IMC and the solder, respectively. After complete mixing, the solder wettability angle was less than 26.68°, with outstanding wetting properties. No phase transition occurred after aging, the distribution of each phase remained uniform, and no enrichment of the brittle phase was observed at the interface. The shear strength decreased slightly with aging time, and the reduction value was less than 10 MPa, which has good reliability.

Microstructure changes in Sn-Bi solder joints reinforced with Zn@Sn particles in thermal cycling and thermomigration

BackgroundIn the field of electronic packaging, Sn-58Bi solder paste is a promising low-temperature option for step soldering. However, the tendency for the Bi-rich phase to coarsen and segregate poses a significant challenge to the long-term reliability of the solder joints. MethodsThe Sn coated Zn (Zn@Sn) core-shell particles were prepared through a chemical plating process and incorporated into the Sn-58Bi low-temperature solder paste to enhance its reliability. The microstructural and mechanical characteristics of the solder joints were evaluated under thermal cycling and thermomigration conditions to assess their long-term service reliability. Significant findingsThe addition of Zn lowers the nucleation energy and promotes the refinement of the eutectic structure, avoiding a significant reduction in shear strength after thermal cycling. In thermomigration, the heat transfer Q* of Bi is calculated to be 22.5 kJ/mol. The Zn-rich phases hardly move and are evenly distributed within the solder joints, acting like a barrier to prevent the Bi phase diffusing from the hot end to the cold end.

Mechanical Properties of Solderable Polymer Composites with Low- and High-Melting-Point Solder Mixed Filler

A solderable polymer composite (SPC) with a low-melting-point solder (LMPS)/high-melting-point solder (HMPS) mixed filler was formulated to solve the bonding property and reliability problems in an SPC system with an LMPS filler. In addition, the possibility of enhancing the mechanical bonding properties of SPC with LMPS/HMPS mixed fillers was investigated. Three types of SPCs (mixing ratios for LMPS and HMPS: 100:0, 50:50, and 0:100) were prepared, and two types of mechanical property investigations (i.e., ball shear and microhardness tests) were performed to measure the mechanical bonding properties of SPCs according to the LMPS/HMPS mixing ratios. The SPC with an LMPS/HMPS mixed filler exhibited enhanced mechanical bonding properties compared with the Sn-58Bi solder paste and SPC with LMPS or HMPS only due to the change in composition and subsequent microstructure transformation of the solder joint after adding HMPS.

Low-Temperature Transient Liquid Phase Bonding Technology via Cu Porous-Sn58Bi Solid–Liquid System under Formic Acid Atmosphere

This study proposes a low-temperature transient liquid phase bonding (TLPB) method using Sn58Bi/porous Cu/Sn58Bi to enable efficient power-device packaging at high temperatures. The bonding mechanism is attributed to the rapid reaction between porous Cu and Sn58Bi solder, leading to the formation of intermetallic compounds with high melting point at low temperatures. The present paper investigates the effects of bonding atmosphere, bonding time, and external pressure on the shear strength of metal joints. Under formic acid (FA) atmosphere, Cu6Sn5 forms at the porous Cu foil/Sn58Bi interface, and some of it transforms into Cu3Sn. External pressure significantly reduces the micropores and thickness of the joint interconnection layer, resulting in a ductile fracture failure mode. The metal joint obtained under a pressure of 10 MPa at 250 °C for 5 min exhibits outstanding bonding mechanical performance with a shear strength of 62.2 MPa.

Microstructure and properties of Sn58Bi/Ni solder joint modified by Mg particles

In this paper, various mass fractions (0, 0.1, 0.2, 0.4, 0.6, 0.8 wt%) of Mg particles were added to the Sn58Bi solder to achieve the purpose of modification, through which the Ni substrate was connected successfully through bonding at 250 °C for 2 min. Meanwhile, the microstructure, interfacial IMC, and properties of the joint were focused on. The experimental results indicated that the appropriate addition of Mg particles is able to decrease the melting point of the solder quite slightly and significantly improve the wettability of the solder on the Ni substrate. When the content of Mg is less than 0.4 wt%, the presence of MgO on the surface does not affect the normal use of solder. Likewise, Mg particles are capable of refining the microstructure of the solder and reducing the thickness of the interfacial Ni3Sn4 IMC layer and the grain size. Accordingly, the mechanical property of the joint is markedly enhanced by 53.48%, whose fracture is mainly characterized by brittle fracture, and the Mg2Sn phase plays the central role of strengthening. In a word, it is the Sn58Bi-0.4Mg/Ni solder joint that processes the best combination performance.

Effects of alloying elements on the interfacial segregation of bismuth in tin-based solders

Sn-Bi solders are potential candidates for replacing traditional lead-based solders in electronic packaging due to their low melting points, superior wettability, and mechanical properties. The grain coarsening due to the interfacial Bi segregation is a vital issue that affects the reliability of Sn-Bi solder joints. Through first-principles calculations, this study systematically revealed the effects of different alloying elements on the interfacial segregation of Bi towards Sn(100) surface. Bi tends to segregate to the top-surface of Sn due to the formation of vacancies on Sn surface. However, doping alloying elements into the Sn-Bi solders could effectively suppress the interfacial Bi segregation. The presence of elements such as Pd, Pt, and Au located at the adjacent surface layer of Bi could inhibit the Bi precipitation, by apparently increasing the segregation energies of Bi to the Sn surface. The results can be interpreted by the enhanced bond orders and charge transfer between Bi, alloying elements, and their neighboring Sn atoms. This study could provide valuable insights into the design of multi-component Sn-Bi solders with minor alloying elements doping in the future.

Effect of Ni-modified reduced graphene oxide on the mechanical properties of Sn–58Bi solder joints

Sn–58Bi solder has great research value as a low-temperature solder. However, it still requires strengthening due to a lot of brittle Bi-phase, which reduces mechanical reliability. In this study, Ni-modified reduced graphene oxide (rGO@Ni) was prepared by heat reduction, and mechanical stirring was used to manufacture rGO@Ni-reinforced Sn–58Bi composite solder. The distribution, phase composition, and structural changes of graphene and Ni nanoparticles during enhancer synthesis were characterized in detail by Scanning Electron Microscope, Energy Dispersive Spectroscopy Raman and Fourier Transform Infrared spectra, X-ray Photoelectron Spectroscopy, X-ray Diffraction, and Transmission Electron Microscope. Besides, the morphology of intermetallic compound (IMC) layers of the composite solder/Cu interface was investigated. The thickness of the IMC layer decreases with the increase of rGO@Ni content. The microstructure analysis demonstrated that the rGO@Ni distributed near the IMC layer, which limits the diffusion of metal atoms and decreases the IMC layer's growth rate during the reflowing process. Besides, compared with Sn–58Bi solder alloys, the addition of 0.01 wt% rGO@Ni can increase the shear strength by up to 15%. However, when the content of rGO@Ni is more than 0.05 wt%, it agglomerates inside the composite solder, resulting in a decrease in the shear strength of the composite solder joint.

Effects of cobalt nanoparticle on microstructure of Sn58Bi solder joint

Effects of cobalt nanoparticle on microstructure of Sn58Bi solder joint

Phase characteristic and segregation related with corrosion on hypereutectic Sn–Bi solder under thermal storage and cycling corrosion test

Hypereutectic Sn-58%wt Bi solders were synthesized to observe phase change, corrosion and electrical endurance by various thermal storages. Initially, Cu6Sn5 IMC layer thickness is linear growth by Sn reacting with Cu joint, following, the Cu3Sn IMC starts to grow when the Cu6Sn5 is thicker enough to obstruct Cu inter-diffusing into solder. The segregation of Bi concentration gradient in eutectic phase is mitigated by homogenization treatment but corrosion resistance is deteriorated. Lump shape of SnO2 corrosive byproduct is initially observed, then the flower shape of Bi3O2 is found after Sn corroding completely. Moreover, phase change of solder related with current endurance is studied.

Mechanical properties and microstructure evolution of Cu/Sn58Bi/Cu solder joint reinforced by B4C nanoparticles

The mechanical mixed Sn58Bi solder reinforced by B4C nanoparticles for the interconnect of Cu/Cu same metal was studied. The hardness of solder alloy and shear strength of joints with Sn58Bi-xB4C (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0 wt%) composite solder were tested. The feasibility of the solution was verified by the performance of melting characteristics, wettability, microstructure, and interfacial IMC structure. It is indicated that the proper amount of B4C nanoparticles can improve the mechanical properties of the solder joint, and the hardness of the solder alloy is decreased to 246.0 HV by 13.65% with the addition of 0.6 wt% B4C and the highest shear strength of the joint reaches 52.282 MPa, which is increased by 8.9%. The failure mode of fracture is mainly considered a brittle fracture. The preferred orientation of the β-Sn phase and Bi phase are (1 0 0) and (1 0 1), respectively, and the number of low-angle grain boundary and dislocation density are increased. The melting characteristics of the solder are less affected by B4C nanoparticles, which will not interfere with its normal use. When the amount of B4C is 0.6 wt%, the spreading area of the solder is increased by 35.44%. Besides, a moderate amount of B4C nanoparticles is able to refine the matrix structure of the solder and inhibit the growth of Cu6Sn5 IMC.

Effects of Sn-Ag-x leveling layers on the microstructure and shear behavior of Sn-58Bi solder joint under thermal cycling

Effects of Sn-Ag-x leveling layers on the microstructure and shear behavior of Sn-58Bi solder joint under thermal cycling

Reliability of laser soldering using low melting temperature eutectic Sn Bi solder and electroless Ni-electroless Pd-immersion Au-finished Cu pad

The demand for flexible wearable devices/substrates with miniaturization and improved integration in microelectronic devices has intensified the research interest in low-temperature laser soldering processes as an alternative to conventional reflow soldering processes owing to their advantages, such as local heating, non-contact heating, and short bonding time. In this study, we compared and evaluated the reliability of laser soldered and conventional reflow soldered joints using representative low melting temperature eutectic Sn Bi solder and thin electroless Ni-electroless Pd-immersion Au (ENEPIG)-finished Cu pads. Laser soldering was performed using various laser powers (130, 150, and 170 W) and times (2 and 4 s). Furthermore, an aging test was performed at 110 °C for 2000 h to evaluate the long-term reliability of the soldered joints. The mechanical properties, including the top and cross-sectional views and fracture surfaces, of the soldered joints were analyzed by conducting shear tests after aging. During laser soldering, various intermetallic compounds (IMCs) were formed at the joints depending on the applied energy. The metallization layer and Cu reacted with Sn in the solder after different aging durations, and additional IMCs were formed and grown. After aging for 2000 h, the shear strength decreased, and the interfacial IMC thickness increased. As the aging time increased, the fracture mode changed from an initial ductile fracture to brittle fracture (between the solder and IMCs and/or between IMCs and the Cu pad). The reflow soldered joints exhibited stable shear strength, resulting in ductile fracture until aging for 500 h. However, the shear strength decreased sharply after aging for 1000 and 2000 h, and Bi-segregation was observed after aging for 1000 h, resulting in inferior long-term reliability. After laser soldering at 150 and 170 W for 4 s, the strength of the samples decreased sharply after aging for 1000 and 250 h, respectively, and Bi-segregation was observed after aging for 2000 h. The shear strength of the sample laser soldered at 170 W for 2 s gradually decreased with increasing aging time and maintained a stable shear strength until aging for 2000 h. Therefore, laser soldering at 170 W for 2 s was considered as the optimal condition for long-term reliability. • Soldering properties of laser soldered Sn-58Bi/Au/Pd/Ni-finished Cu joints evaluated. • Laser soldering results were compared with conventional reflow soldered joints. • Various IMCs were formed depending on laser power and irradiation time. • Laser conditions with low applied energy displayed superior long-term reliability. • Selection of the optimal laser conditions is crucial to ensure reliable solder joints.

Diffusion Barrier Properties of the Intermetallic Compound Layers Formed in the Pt Nanoparticles Alloyed Sn-58Bi Solder Joints Reacted with ENIG and ENEPIG Surface Finishes

Pt-nanoparticle (NP)-alloyed Sn-58Bi solders were reacted with electroless nickel-immersion gold (ENIG) and electroless nickel-electroless palladium-immersion gold (ENEPIG) surface finishes. We investigated formation of intermetallic compounds (IMCs) and their diffusion barrier properties at reaction interfaces as functions of Pt NP content in the composite solders and duration of solid-state aging at 100 °C. At Sn-58Bi-xPt/ENIG interfaces, typical Ni3Sn4/Ni3P(P-rich layer) microstructure was formed. With the large consumption of the Ni-P layer, the Ni-P and Cu layers were intermixed and Cu atoms spread over the composite solder after 500 h of aging. By contrast, a (Pd,Ni)Sn4/thin Ni3Sn4 microstructure was observed at the Sn-58Bi-xPt/ENEPIG interfaces. The (Pd,Ni)Sn4 IMC effectively suppressed the consumption of the Ni-P layer and Ni3Sn4 growth, functioning as a good diffusion barrier. Therefore, the Sn-58Bi-xPt/ENEPIG joint survived 500 h of aging without microstructural degradation. Based on the experimental results and analysis of this study, Sn-58Bi-0.05Pt/ENEPIG is suggested as the optimum combination for future low-temperature soldering systems.

Correction: Effect of cobalt nanoparticles on mechanical properties of Sn–58Bi solder joint

Correction: Effect of cobalt nanoparticles on mechanical properties of Sn–58Bi solder joint

Growth behavior and reliability of interfacial IMC for Sn58Bi/Cu and Sn58Bi–AlN/Cu solder joints applied in IGBT modules

The IMC (interfacial intermetallic compound) produced by the metallurgical reaction between the solder and the Cu plate enables the packaging of IGBT (insulated gate bipolar transistor) modules. In this paper, the growth behavior of IMCs of Sn58Bi/Cu and Sn58Bi-0.05AlN/Cu at different times (10, 30 and 60 min) and temperatures (200, 250 and 300 °C) was investigated, which can provide a reference for the connection of IGBT. It was shown that the interfacial IMC thickness of both solders increased with time and temperature, but the growth of interfacial IMC was suppressed for Sn58Bi solder containing 0.05 wt% AlN. The effect of AlN on the inhibition of grain growth was also observed by EBSD testing. Because of the adsorption effect of AlN nanoparticles, they can adhere to the surface of IMC layer and decrease the effective the diffusion path of unreacted Cu atoms, thereby inhibiting the growth of interfacial IMC. And the effect of AlN particles on interfacial IMC was analyzed via phase field simulation. In addition, under the heating conditions with high temperature and long time, there were fewer cracks in the solder joints containing AlN particles, compared to the Sn58Bi solder. Therefore, AlN nanoparticles can enhance the reliability of Sn58Bi/Cu solder joints.