Development of Sn–Zn lead-free solders bearing alloying elements

Sn–Zn solder alloys have been considered as one of the more attractive lead-free solders since it can easily replace Sn–Pb eutectic alloy without increasing the soldering temperature. However, there are still some problems to be resolved, such as the argument about the poor oxidation resistance and embrittlement behavior. In order to overcome these drawbacks, and further enhance the properties of Sn–Zn lead-free solder alloys, a small amount of alloying elements (rare earths, Bi, Ag, Al, Ga, In, Cr, Cu, Sb, Ni, Ge) added into Sn–Zn alloys were selected by many researchers. For example, a small amount of Al, P, Bi, Ga can improve the high-temperature oxidation resistance of Sn–Zn solders remarkably as well as Cr. This paper summarizes the effects of alloying elements on the wettability, oxidation resistance, melting behavior, mechanical properties, creep properties, microstructures and intermetallic compounds layer of Sn–Zn lead-free solders.     

Thermal creep and fracture behaviors of the lead-free Sn–Ag–Cu–Bi solder interconnections under different stress levels

In our previous study, the creep behavior of the lead-free Sn–Ag–Cu–Bi solder joints has been proven to follow the Arrhenius power-law relationship, and the thermal fatigue behavior of the solder joints exhibits the typical creep deformation characteristics with a superposition of the pulsating features. In this study, the thermal creep and fracture behaviors of the lead-free Sn–Ag–Cu–Bi solder interconnections were characterized under different stress levels, with a systematical comparison to that of a traditional Sn60Pb40 near-eutectic solder. The results show that the creep strain rate of both solder connections follows Weertman-Dorn equation, and the calculated creep stress exponent for two solders is reasonably close to other published data. The SEM inspection and analysis of fractographies of creep fractured solder joints manifest that the creep failure of the lead-free Sn–Ag–Cu–Bi solder joint shows obviously intergranular fracture mechanism, while the Sn60Pb40 joint ruptures dominantly by a transgranular sliding mechanism.     

Creep and fatigue behaviors of the lead-free Sn–Ag–Cu–Bi and Sn60Pb40 solder interconnections at elevated temperatures

Creep and fatigue behaviors of the interconnections soldered by the lead-free Sn–Ag–Cu–Bi solder were investigated at different elevated temperatures (with the homologue temperature in the range of 0.71– 0.82), with a comparison to that of a traditional Sn60Pb40 solder. The results show that the lead-free Sn–Ag–Cu–Bi solder shows a superior anti-creep performance over the Sn60Pb40 solder, in terms of a much lower creep strain rate and a vastly elongated creep fracture lifetime; in the secondary creep regime, the calculated creep-activation energy for two solders is reasonably close to other published data. In addition, it has also been shown that the joints soldered by the lead-free Sn–Ag–Cu–Bi solder exhibits a superb fatigue property.     

Synthesis and DSC study on Sn3.5Ag alloy nanoparticles used for lower melting temperature solder

The traditional Sn–Pb eutectic solder alloys are being phased out from the electronics industry due to the toxicity of lead (Pb), leading to the development and implementation of lead-free solders. Sn3.5Ag lead-free solder alloy, considered to be one of the promising alternatives to replace the traditionally used Sn–Pb solder, however, still has some weaknesses, such as its higher melting temperature than that of the Sn–Pb solder alloy. A possible way to decrease the melting temperature of a solder alloy is to decrease the alloy particle size to the nanometer range. Sn3.5Ag nanoparticles with different size distribution were synthesized using chemical reduction method by applying NaBH₄ as reduction agent. The melting properties of these synthesized nanoparticles were studied by differential scanning calorimetry (DSC), and size-dependent melting temperature depression of these nanoparticles has been observed. Gibbs–Thomson equation was used to analyze the size-dependent melting temperature property, giving a good prediction of the melting temperature depression for the Sn-based lead-free solder alloy nanoparticles.     

Development of Sn–Ag–Cu and Sn–Ag–Cu–X alloys for Pb-free electronic solder applications

The global electronic assembly community is striving to accommodate the replacement of Pb-containing solders, primarily Sn–Pb alloys, with Pb-free solders due to environmental regulations and market pressures. Of the Pb-free choices, a family of solder alloys based on the Sn–Ag–Cu (SAC) ternary eutectic (T _eut. = 217°C) composition have emerged with the most potential for broad use across the industry, but the preferred (typically near-eutectic) composition is still in debate. This review will attempt to clarify the characteristic microstructures and mechanical properties of the current candidates and recommend alloy choices, a maximum operating temperature limit, and directions for future work. Also included in this review will be an exploration of several SAC + X candidates, i.e., 4th element modifications of SAC solder alloys, that are intended to control solder alloy undercooling and solidification product phases and to improve the resistance of SAC solder joints to high temperature thermal aging effects. Again, preliminary alloy recommendations will be offered, along with suggestions for future work.     

Effects of nano-structured particles on microstructure and microhardness of Sn–Ag solder alloy

In this research, the typical nano-structured Polyhedral Oligomeric Silsesquioxane (POSS) particles were incorporated into the Sn–3.5Ag eutectic solder paste by mechanically mixing to form lead-free composite solder. The effects of nano-structured POSS additions on the microstructure and mechanical properties of as-fabricated composite solder alloys were systematically investigated. Experimental results indicated that the average size and spacing distance of Ag₃Sn intermetallic compounds (IMCs) in composite solder matrix decreased as compared to the Sn–3.5Ag eutectic solder. The 3 wt% addition of nano-structured POSS particles could enhance the microhardness of composite solder by 18.4% compared with the Sn–3.5Ag eutectic solder matrix. The average grain size and spacing distance of Ag₃Sn IMCs in Sn–Ag + 3 wt% POSS composite solder matrix reduced from 0.35 to 0.23 μm and from 0.54 to 0.32 μm, respectively. The refined Ag₃Sn IMCs, acting as a strengthening phase in the solder matrix, could enhance the microhardness of the composite solders.     

Impression creep of monolithic and composite lead free solders

Even though several EMS (Electronic manufacturing services) companies are currently producing “lead free” products, a general notion of apprehension still exists in the industry, primarily due to the lack of sufficient mechanical reliability data supporting the use of lead free alloys. The current study was an effort to generate an understanding of the mechanisms of creep deformation in monolithic and composite (Ag and Cu reinforced) Sn–3.5Ag and Sn–3.0Ag–0.5Cu lead free alloys in the high stress high temperature regime. Small volume solder samples were reflowed using a custom built computer controlled resistance furnace. Impression creep testing was employed to determine the activation energy and stress exponent. A careful analysis of the collected data revealed the underlying creep mechanisms and the following conclusions could be made. Both Sn–3.5Ag and Sn–3.0Ag–0.5Cu exhibited higher creep resistance as compared to the eutectic tin–lead solder under all tested conditions, with the ternary lead free alloy marginally outperforming the binary lead free alloy. Composite solders performed better as compared to monolithic solders. Furthermore, Cu reinforced solders demonstrated higher creep resistance as compared to Ag reinforced solders.     

Effects of microstructure and temperature on corrosion behavior of Sn–3.0Ag–0.5Cu lead-free solder

The heterogeneous microstructure of solder could be obtained when cooling rate of the solder joint was not even, which would affect the corrosion behavior of solder during service. The ambient temperature would also affect the corrosion behavior of solder joint. In this paper, the effects of microstructure and temperature on the corrosion behavior of Sn–3.0Ag–0.5Cu (SAC305) lead-free solder were investigated. The various microstructures of SAC305 lead-free solder were obtained by cooling specimens in air and furnace. Compared to the fine-fibrous Ag₃Sn phase inside the commercial SAC305 solder, platelet-like Ag₃Sn formed as cooling speed decreasing. The polarization behavior of SAC305 solders in 3.5 wt.% NaCl solution was not significantly affected by various microstructures, but sensitive to temperature.     

Comparison of low-melting lead-free solders in tensile properties with Sn–Pb eutectic solder

The tensile properties of Sn–8 mass % Zn–3 mass % Bi and Sn–58 mass % Bi low-melting lead-free solders were investigated and compared with those of a Sn–Pb eutectic solder. The tensile strength decreases with decreasing strain rate and increasing temperature. The tensile strength of each solder is approximately double that of the Sn–Pb solder at room temperature (RT). The ductility of each solder is inferior to that of the Sn–Pb solder. From the results of strain-rate-change tests, the stress exponents and the activation energies for creep of Sn–58Bi and Sn–8Zn–3Bi were also examined.     

A review of mechanical properties of lead-free solders for electronic packaging

The characterization of lead-free solders, especially after isothermal aging, is very important in order to accurately predict the reliability of solder joints. However, due to lack of experimental testing standards and the high homologous temperature of solder alloys (T _h > 0.5T _m even at room temperature), there are very large discrepancies in both the tensile and creep properties provided in current databases for both lead-free and Sn–Pb solder alloys. Some recent researches show that the room temperature aging has significant effects on mechanical properties of solders. This paper is intended to review all available data in the field and give rise to the possible factors including room temperature effects which causes the large discrepancies of data. This review of the research literatures has documented the dramatic changes that occur in the constitutive and failure behavior of solder materials and solder joint interfaces during isothermal aging. However, these effects have been largely ignored in most previous studies involving solder material characterization or finite element predictions of solder joint reliability during thermal cycling. It is widely acknowledged that the large discrepancies in measured solder mechanical properties from one study to another arise due to differences in the microstructures of the tested samples. This problem is exacerbated by the aging issue, as it is clear that the microstructure and material behavior of the samples used in even a single investigation are moving targets that change rapidly even at room temperature. Furthermore, the effects of aging on solder behavior must be better understood so that more accurate viscoplastic constitutive equations can be developed for SnPb and SAC solders. Without such well-defined relationship, it is doubtful that finite element reliability predictions can ever reach their full potential.     

Multiple reflow study of ball grid array (BGA) solder joints on Au/Ni metallization

This paper evaluates the shearing behavior of ball grid array (BGA) solder joints on Au/Ni/Cu pads of FR4 substrates after multiple reflow soldering. A new Pb-free solder, Sn–3Ag–0.5Cu–8In (SACI), has been compared with Sn–3Ag–0.5Cu (SAC) and Sn–37Pb (SP) solders, in terms of fracture surfaces, shearing forces and microstructures. Three failure modes, ball cut, a combination of solder shear and solder/pad bond separation, and pad lift, are assessed for the different solders and reflow cycles. It is found that the shearing forces of the SP and SAC solder joints tend to increase slightly with an increase in the number of reflow cycles due to diffusion-induced solid solution strengthening of the bulk solder and augmentation of the shearing area. However, the shearing forces of the SACI solder joints decrease slightly after four cycles of reflow, which is ascribed to the thermal degradation of both the solder/intermetallic compound (IMC) and IMC/Ni interfaces. The SACI solder joints yield the highest strengths, whereas the SP solder joints give the smallest values, irrespective of the number of reflow cycles. Thickening of the interfacial IMC layer and coarsening of the dispersing IMC particles within the bulk solders were also observed. Nevertheless, the variation of shearing forces and IMC thickness with different numbers of reflow cycles was not so significant since the Ni under layer acted as an effective diffusion barrier. In addition, the initially-formed IMC layer retarded the further extensive dissolution of the pad material and its interaction with the solder.     

Properties of SnAgCu/SnAgCuCe soldered joints for electronic packaging

For quad flat packages (QFP256), lead-free soldered joints reliability in service is a critical issue. In this paper, soldering experiments of quad flat package (QFP256) devices were carried out by means of infrared reflow soldering system with Sn–3.8Ag–0.7Cu and Sn–3.8Ag–0.7Cu–0.03Ce lead-free solders, respectively, and the mechanical properties of micro-joints of the QFP devices were tested and studied by STR micro-joints tester. The results indicate that the tensile strength of Sn–Ag–Cu–Ce soldered joints is better than that of Sn–Ag–Cu soldered joints. In particular, the addition of trace Ce to the Sn–Ag–Cu solder can refine the microstructures and decrease the thickness of the intermetallic compound layer of Sn–Ag–Cu solder alloys. In addition, the stress–strain response of Sn–Ag–Cu/Sn–Ag–Cu–Ce soldered joints in quad flat packaging was investigated using finite element method based on Garofalo–Arrhenius model. The simulated results indicate creep distribution of soldered joints is not uniform, the heel and toe of soldered joints, the area between soldered joints and leads are the creep concentrated sites. The creep strain of Sn–Ag–Cu–Ce soldered joints is lower than that of Sn–Ag–Cu soldered joints.     

Nanoparticles of Sn3.0Ag0.5Cu alloy synthesized at room temperature with large melting temperature depression

The Sn3.0Ag0.5Cu (wt%) lead-free solder alloy is considered to be one of the most promising alternatives to replace the traditionally used Sn–Pb solders. This alloy composition possesses, however, some weaknesses, mainly as a result of its higher melting temperature compared to the eutectic Sn–Pb solders. Nanoparticles of Sn3.0Ag0.5Cu lead-free solder alloy nanoparticles were prepared by chemical reduction with NaBH₄ as a reducing agent at room temperature. The melting temperature of the synthesized Sn3.0Ag0.5Cu alloy nanoparticles was determined by differential scanning calorimetry (DSC). The results showed that the calorimetric onset melting temperature of the Sn3.0Ag0.5Cu alloy nanoparticles could be as low as 200 °C, which was about 17 °C lower than that of the bulk alloy (217 °C). The field emission scanning electron microscopy (SEM) images of the as-prepared nanoparticles indicated that the major particle size of Sn3.0Ag0.5Cu nanoparticles is smaller than 50 nm. The structure and morphology of the nanoparticles were analyzed with high resolution transmission electron microscopy (HRTEM). The Ag3Sn and Sn phase were observed in the HRTEM images, which was in good agreement with the XRD results. These low melting temperature Sn3.0Ag0.5Cu alloy nanoparticles show a potential to manufacture high quality lead-free solders for electronic products.     

Effects of Ag on microstructures,wettabilities of Sn–9Zn–xAg solders as well as mechanical properties of soldered joints

The microstructures, wettabilities and mechanical properties of Sn–9Zn–xAg (x = 0, 0.1, 0.3, 0.5, 1 wt%) lead-free solders were investigated, respectively in this paper. Results show that, when the quantity of Ag added to the solder is 0.3 wt%, the microstructure of the solder becomes finer and more uniform than Sn–9Zn, and when the quantity of Ag is exceeded 0.3 wt% (upto 0.5–1 wt%), the AgZn₃ intermetallic compounds appear in the solder. In particular, adding 0.3 wt% Ag improves the wettability due to the better oxidation resistance of the Sn–9Zn–0.3Ag solder. Results also indicate that adding 0.3 wt% Ag to the solder enhances mechanical property of soldered joint, at which the fracture micrographs show that plenty of small and uniform dimples could be observed on the Sn–9Zn–0.3Ag soldered joints fractures. When the content of Ag is upto 1 wt%, some Cu–Zn and Ag–Zn intermetallic compounds appear on the bottom of dimples, and the mechanical property of the soldered joint decreases.     

Formation of interfacial structure of Sn–3.7Ag–0.9Zn eutectic solder with different Al additions

The structural evolution of interfaces between the Sn–3.5Ag–0.9Zn–xAl (x = 0.5 and 1.0) solders and Cu substrate has been investigated by microstructural observations. The results suggest that the addition of Al in the Sn–3.5Ag–0.9Zn restrains the formation of Cu₅Zn₈ intermetallic compounds (IMCs) at the soldered interface. Moreover, the formation of Al₂Cu and Cu₉Al₄ IMCs leads to a crack failure near the interface of the Sn–3.7Ag–0.9Zn–1Al and Cu pad. It is suggested that the increase of Al content (e.g. 1 wt%) in the Sn–Ag–Zn eutectic solder would do harm to the reliability of the solder joint.     

Rare-earth additions to lead-free electronic solders

The research in lead(Pb)-free solder alloy has been a popular topic in recent years, and has led to commercially available Pb-free alloys. Further research in certain properties to improve aspects such as manufacturability and long term reliability in many Pb-free alloys are currently undertaken. It was found by researchers that popular Pb-free solders such as Sn–Ag, Sn–Cu, Sn–Zn and Sn–Ag–Cu had improved their properties by doping with trace amounts of rare earth (RE) elements. The improvements include better wettability, creep strength and tensile strength. In particular, the increase in creep rupture time in Sn–Ag–Cu–RE was 7 times, when the RE elements were primarily Ce and La. Apart from these studies, other studies have also shown that the addition of RE elements to existing Pb-free could make it solderable to substrates such as semiconductors and optical materials. This paper summarizes the effect of RE elements on the microstructure, mechanical properties and wetting behavior of certain Pb-free solder alloys. It also demonstrates that the addition of RE elements would improve the reliability of the interconnections in electronic packaging. For example, when Pb-free-RE alloys were used as solder balls in a ball grid array (BGA) package, the intermetallic compound layer thickness and the amount of interfacial reaction were reduced.     

Microstructures formed from mixed solders on copper substrate

Abstract

With the development and use of a variety of Pb free solders, it is probable that some solder joints in electronic assemblies may be made with solders of two different compositions. To investigate possible microstructures resulting from such procedure, samples were prepared using small balls of four different Sn–Ag–Cu (SAC) Pb free solders, as well as Sn–Zn–Al solder, melted together with eutectic Pb–Sn solder paste and also various SAC solder pastes, on a copper substrate. It was observed that using eutectic Pb–Sn solder paste with an SAC solder ball introduced some Pb–Sn eutectic microstructure and changed the ternary eutectic present from Ag₃Sn–Cu₆Sn₅–Sn to Ag₃Sn–Pb–Sn. Use of an SAC solder paste with Sn–Zn–Al solder introduced an apparent Ag–Cu–Zn ternary compound, replacing Zn lamellae of the Sn–Zn eutectic. With eutectic Pb–Sn solder paste, the Pb–Sn–Zn ternary eutectic was formed. It was noted that use of a high Sn solder results in rapid dissolution of the copper substrate.     

Materials behaviour and intermetallics characteristics in the reaction between SnAgCu and Sn–Pb solder alloys

The paper compares theoretical calculations with experimental data, to identify the metallurgical mechanisms with respect to the rework or repair that may be encountered in the transition period from Sn–Pb to Pb-free soldering. Thermodynamic calculations have been carried out to study material behaviour and possible formation of intermetallic precipitates during the reaction between Sn–Pb and Sn–Ag–Cu Pb-free alloys. Two Sn–Ag–Cu alloys that are relevant to current industrial interests, namely Sn–3.9Ag–0.6Cu* (known as ‘405 alloy’ in Europe and North America), and Sn–3.0Ag–0.5Cu (known as ‘305’ alloy in Asia), were reacted with different contamination levels of eutectic Sn–37Pb solder. The variables examined included those related to both the materials and processes, such as composition, temperature and cooling rate. Together these are the primary drivers with respect to the resultant solder microstructures, which were studied using scanning electron microscopy (SEM). Nanoindentation, which is suitable for the ultra-fine and complex microstructures, was also used to investigate the micromechanical properties, including hardness and elastic modulus, at both ambient and elevated temperatures. The results from this work provide guidance as to the consequence for microstructural evolution and hence mechanical integrity when small amounts of Pb exist in Pb-free alloys. The composition of alloys in this paper is in weight percentage (wt%)     

Investigation on properties of Ga to Sn–9Zn lead-free solder

The influences of different Ga content on the properties of Sn–9Zn lead-free solder were investigated. The results indicate that Ga plays an important role not only in the structure and melting behavior, but also in the solderability and mechanical property. Sn–9Zn–0.5Ga shows finer and more uniform microstructure than Sn–9Zn. With the addition of low-melting-point Ga, TL (liquidus temperature) and TS (solidus temperature) of the alloys decreases with increasing of Ga content while △T (liquidus temperature minus solidus temperature) increases. Ga can improve the oxidation resistance and reduce the surface tension of solder, so the solderability of Sn–9Zn–xGa lead-free solder is significantly improved. When the content of Ga is 0.5 wt.%, the pull force of soldered joint is 16.1 N, enhanced by 11% compared to that of Sn–9Zn, and the fracture micrographs show that the joint failed in a ductile manner. The addition of 3 wt.%Ga resulted in a brittle failure. The introduction of 0.5 wt.% Ga into Sn–9Zn alloy improves creep resistance of the solder.     

Creep property of composite solders reinforced by nano-sized particles

In the present work the creep properties of Sn37Pb and Sn0.7Cu based composite solders with nano-sized metallic Cu, Ag and nano-sized oxide Al₂O₃, TiO₂ reinforcement particles have been studied. First, a series of volume percentages of reinforcements were selected for optimizing the content of particles. Then, the composite solder with optimum volume fraction of the reinforcement particles, corresponding to maximum creep rupture life, is selected for investigating the effect of applied stress level and test temperature on creep rupture life of the composite solder joints. In the creep rupture life test, small single-lap tensile-shear joints were adopted. The results indicate that all the composite solders have improved creep resistance, comparing to the eutectic Sn37Pb solder and the Sn0.7Cu lead-free solder. The creep rupture life of the composite solder joints is first increased with the increase in the volume fraction of reinforcement in the composite solders. Then, the creep rupture life is decreased, as the reinforcement content exceeds a certain value. The creep rupture life of the solder joints is decreased with the increase of applied stress and testing temperature. Moreover, the reinforced efficiency of nano-sized Ag particles is the best in all the tested nano-sized reinforcements for the Sn37Pb based and Sn0.7Cu based composite solders, when the particles contents are in their own optimum content.