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This paper aims to review recent reports on mechanical properties of Sn-Bi and Sn-Bi-X solders (where X is an additional alloying element), in terms of the tensile properties, hardness and shear strength. Then, the effects of alloying in Sn-Bi solder are compared in terms of the discussed mechanical properties. The fracture morphologies of tensile shear tested solders are also reviewed to correlate the microstructural changes with mechanical properties of Sn-Bi-X solder alloys.

A brief introduction on Sn-Bi solder and reasons to enhance the mechanical properties of Sn-Bi solder. The latest reports on Sn-Bi and Sn-Bi-X solders are combined in the form of tables and figures for each section. The presented data are discussed by comparing the testing method, technical setup, specimen dimension and alloying element weight percentage, which affect the mechanical properties of Sn-Bi solder.

The addition of alloying elements could enhance the tensile properties, hardness and/or shear strength of Sn-Bi solder for low-temperature solder application. Different weight percentage alloying elements affect differently on Sn-Bi solder mechanical properties.

This paper provides a compilation of latest report on tensile properties, hardness, shear strength and deformation of Sn-Bi and Sn-Bi-X solders and the latest trends and in-depth understanding of the effect of alloying elements in Sn-Bi solder mechanical properties.

The purpose of this study is to investigate the effect of electromigration (EM) on solder alloy joint on copper with nickel surface finish. Sn-Bi solder alloy has been used in this research.

The EM process was completed with the duration of 0, 24, 48, 72 and 96 h under direct current (DC) of 1,000 mA. Tensile stress on the substrates was assessed after EM at a tension rate of 0.1 mm/min. Microscopy was used to observe the formation and size of voids and conduct an analysis between copper and nickel substrates.

Four types of intermetallic compounds (IMCs), namely, Cu-Sn, Cu3Sn, Cu6Sn5, and Sn-Bi, were detected between the Sn-Bi/Co solder joint. Voids appear to be at the anode and the cathode for 96 h of EM for Sn-Bi/Ni solder join; however, there seem to be more voids at the cathode.

EM is one of the crucial keys to produce a good integrated circuit (IC). When the current density is extremely high and will cause the metal ions to move into the electron direction flow, it will be characterised based on the ion flux density. In this research, the effect of EM on the Sn-Bi solder alloy joint on copper with nickel surface finish was studied.

Overall, selection maps about the extent of the eutectic growth projects the solidification velocities leading to given microstructures. This is because of limitations of most of the set of results when obtained for single thermal gradients within the experimental spectrum. In these cases, associations only with the solidification velocity could give the false impression that reaching a given velocity would be enough to reproduce a result. However, that velocity must necessarily be accompanied by a specific thermal gradient during transient solidification. Therefore, the purpose of this paper is to not only project velocity but also include the gradients acting for each velocity.

Compilation of solidification velocity, v, thermal gradient, G, and cooling rate, Ṫ, data for Sn-Cu and Sn-Bi solder alloys of interest is presented. These data are placed in the form of coupled growth zones according to the correlated microstructures in the literature. In addition, results generated in this work for Sn-(0.5, 0.7, 2.0, 2.8)% Cu and Sn-(34, 52, 58)% Bi alloys solidified under non-stationary conditions are added.

When analyzing the cooling rate (Ṫ = G.v) and velocity separately, in or around the eutectic composition, a consensus cannot be reached on the resulting microstructure. The (v vs. G) + cooling rate diagrams allow comprehensive analyzes of the combined v and G effects on the subsequent microstructure of the Sn-Cu and Sn-Bi alloys.

The present paper is devoted to the establishment of (v vs. G) + cooling rate diagrams. These plots may allow comprehensive analyses of the combined v and G effects on the subsequent microstructure of the Sn-Cu and Sn-Bi alloys. This microstructure-processing mapping approach is promising to predict phase competition and resulting microstructures in soldering of Sn-Cu and Sn-Bi alloys. These two classes of alloys are of interest to the soldering industry, whereas manipulation of their microstructures is considered of utmost importance for the metallurgical quality of the product.

Fine pitch (0.4 mm) surface mount assembly studies were conducted with several lead‐free solder pastes formulated with both traditional RMA (∼6% residue level) and low residue (1%) flux vehicles. The lead‐free solder alloys evaluated included the two baseline eutectic binary alloys, Sn‐Bi and Sn‐Ag, and three new lead‐free solder compositions: (1)91.8Sn–4.8Bi–3.4Ag (wt%) developed at Sandia Laboratories, (2) 77.2Sn–20ln–2.8Ag (Indalloy 227) developed at Indium Corporation of America and (3) 96.2Sn–2.5Ag–0.8Cu–0.5Sb (Castin) provided by AIM, Inc. The basic physical properties pertinent to assembly performance (melting temperature and wetting behaviour) were determined for each of the new alloys. Assembly performance was assessed as a function of circuit board surface finishes, thermal reflow profiles and solder paste flux composition. The feasibility of 0.4 mm pitch assembly was established with each of the lead‐free solder alloys investigated. No issues particular to the combined use of low residue flux vehicles and lead‐free solder powders were identified. The circuit board laminates did not suffer any thermal degradation effects (reflow was performed in an inert atmosphere). All lead‐free solders, compared with the Sn‐Pb eutectic solder, exhibited reduced spreading on the circuit board lands after reflow. It was concluded that the performance of the new solder formulations is adequate for surface mount applications. Further differentiation among these solders will have to be based on their long‐term reliability performance. These studies are currently under way.

This study aims to analyze the wettability of the self-developed Sn–Bi–Zn solder and to conduct a series of analysis on the wetting kinetics, diffusion phenomenon and interfacial reaction of Sn–Bi–Zn solder on Cu substrate.

The wetting kinetics, diffusion phenomenon and interfacial reaction of Sn–Bi–Zn solder on Cu substrate were analyzed by experiments. The interface was observed by scanning electron microscope to study the effect of Zn content on its interface.

With the increase in brazing temperature, the final spreading equivalent radius of the solder increases significantly, and the final contact angle of the solder decreases significantly. In addition, when the Zn content is 1%, the spreading effect of solder is the best, the equivalent radius is the largest and the contact angle is the smallest. According to the microstructural analysis, the thick intermetallic compounds layer of the Sn–15Bi–xZn solders on the Cu substrate can be effectively decreased by adding appropriate Zn content.

The wetting kinetics, diffusion phenomenon and interfacial reaction of Sn–15Bi–xZn solder on Cu substrate at different temperatures have not been studied yet.

The purpose of this paper is to investigate the effects of Sn-Ag-x leveling layers on the mechanical properties of SnBi solder joints. Four Sn-Ag-x (Sn-3.0Ag-0.5Cu, Sn-0.3Ag-0.7Cu, Sn-0.3Ag-0.7Cu-0.5 Bi-0.05Ni and Sn-3.0Ag-3.0 Bi-3.0In) leveling layers were coated on Cu pads to prepare SnBi/Sn-Ag-x/Cu solder joints. The microstructure, hardness, shear strength and fracture morphology of solder joints before and after aging were studied.

The interfacial brittleness of the SnBi low-temperature solder joint is a key problem affecting its reliability. The purpose of this study is to improve the mechanical properties of the SnBi solder joint.

Owing to the addition of the leveling layers, the grain size of the ß-Sn phase in the SnBi/Sn-Ag-x/Cu solder joint is significantly larger than that in the SnBi/Cu eutectic solder joint. Meanwhile, the hardness of the solder bulk in the SnBi/Cu solder joint shows a decrease trend because of the addition of the leveling layers. The SnBi/Cu solder joint shows obvious strength drop and interfacial brittle fracture after aging. Through the addition of the Sn-Ag-x layers, the brittle failure caused by aging is effectively suppressed. In addition, the Sn-Ag-x leveling layers improve the shear strength of the SnBi/Cu solder joint after aging. Among them, the SnBi/SACBN/Cu solder joint shows the highest shear strength.

This work suppresses the interfacial brittleness of the SnBi/Cu solder joint after isothermal aging by adding Sn-Ag-x leveling layers on the Cu pads. It provides a way to improve the mechanical performances of the SnBi solder joint.

A challenge in selecting and applying lead‐free solders lies in separating the influences of materials' properties, fluxes and processes to obtain robust assembly conditions that are compatible with PCB finishes and all component terminations. This paper discusses simple steps towards establishing a lead‐free assembly process. With reference to results of solder paste spread and wetting tests and component solderability tests, some of the current limitations in applying standard test methods to lead‐free evaluations are highlighted.

In 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.

This 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.

Hot 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.

The study focuses on Sn–Bi solders only without other solder pastes such as SAC305 or Sn–Zn series.

With 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.

In 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.

Although 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.

This paper reviews the status of lead‐free solder development works. Some of the solder systems — Bi‐Sn, Bi‐Sn‐Fe, ln‐Sn, Sn, Sn‐Ag, Sn‐Ag‐Zn, Sn‐Ag‐Zn‐Cu, Sn‐Bi‐Ag, Sn‐Cu, Sn‐Cu‐Ag, Sn‐In‐Ag, Sn‐Sb, Sn‐Zn and Sn‐Zn‐ln — are discussed in more detail, while others are briefly commented on. In general, compared with eutectic Sn‐Pb solder, all the lead‐free solder alternatives investigated more or less exhibit some shortcomings, such as price, physical, metallurgical or mechanical properties. Relatively, Sn‐ln‐containing systems are more promising in terms of solder mechanical properties and soldering performance, although the price of ln may be a concern. Eutectic Sn‐Ag solder doped with Zn, Cu or Sb exhibits good mechanical strength and creep resistance, due to refined microstructure. The Bi‐Sn systems doped with other elements may have a niche in the low temperature soldering field. Eutectic Sn‐Cu has good potential due to its good fatigue resistance. The eutectic Sn‐Zn system modified with ln and/or Ag may be promising in terms of mechanical properties. Finding a lead‐free alternative for high temperature solders presents the biggest challenge to the industry.

This paper seeks to decrease the soldering temperature of capacitors using Sn‐Bi plated Sn‐3.5 wt%Ag solder.

Sn‐Bi layers were electroplated on Sn‐3.5 wt%Ag solder. As soldering examples, type 1608 capacitors electroplated with Sn, and printed circuit boards (PCBs) with a surface coating of electroless‐plated Ni/Au, were selected. Sn‐3.5Ag foil coupons plated with Sn‐95.7 wt%Bi were inserted as solder between the capacitors and the lands on the PCBs. The samples were reflowed at 220°C, which is below the normal reflow temperatures of around 240 ∼ 250°C used with Pb‐free solders. During heating, Bi in the plated layer diffuses into the Sn‐3.5Ag core solder resulting in a transient decrease in soldering temperature based on the concept of transient liquid phase bonding.

The joints made with the Sn‐95.7%Bi plated Sn‐3.5Ag solder at 220°C showed good appearance, and evidence of significant Bi segregation was absent in the microstructure. The shear strengths of the capacitor joints bonded with Sn‐95.7%Bi plated Sn‐3.5%Ag solder were approximately 5,000‐6,000 gf. After 1,000 thermal cycles between −40 and +125°C, the shear strengths of the joints decreased approximately 5‐10 percent from the strengths in the as‐reflowed state for all plated solders. This confirmed that the soldered joints were stable and not significantly degraded by thermal cycles.

Reduced temperature soldering using Sn‐Bi plated Sn‐3.5%Ag solder was applied to attach capacitors to PCBs. In a production application, the foil coupons could be replaced by pre‐solder on the PCB pads.