Pb Eutectic Solder Research Articles

Role of Cu percentage in the Sn-xCu alloy on the mechanical performance at Sn-xCu/ENIG interface produced by laser soldering

PurposePb-free solders have been developed to replace the standard Sn–Pb eutectic solder since the prohibition on Pb used in solders. The Sn–Ag–Cu series of lead-free solders is the most extensively used in the electronics industry. The Ag3Sn, which forms during isothermal ageing, can significantly degrade solder joint reliability. Sn–Ag–Cu solder’s high price further hindered its use in the electronics industry. This paper aims to investigate different copper percentages into Sn–xCu solder alloy to improve its microstructure and strength performance.Design/methodology/approachThe solder alloys used in this work were Sn–xCu, where x = 0.0, 0.3, 0.5, 0.7, 1.0 Wt.%, which was soldered onto electroless nickel immersion gold (ENIG) substrate using carbon dioxide (CO2) gas laser. Then these samples were subjected to isothermal aging for 0, 200, 500, 1,000 and 2,000 h. The Sn–xCu solder alloy was fabricated through a powder metallurgy process.FindingsMicrostructure characterization showed that Cu addition resulted in fine and rounded shape of Cu–Sn–Ni particles. Shear strength of Sn–xCu solder joints was increased with increasing Cu content, but at aging duration of 1,000 h, it dropped slightly. It is believed that the strength improved due to the increment of diffusion rate during isothermal aging.Practical implicationsIn a Cu–Sn solder, the recommended amount is 1.0 Wt.% of Cu. In extensive aging procedures, it was discovered that Sn1.0Cu solder improved the reliability of solder joints. The findings indicated that the innovative solder alloys might satisfy the needs of high-reliability applications.Originality/valueThe study shows that the right amount of Cu enhances the solidification of Sn–Cu solder, increasing the shear force of the Cu–Sn solder joint. The Sn1.0Cu exhibits a ductile fracture on the top microstructure, improving the joint’s average shear strength.

Influence of aging on microstructure and hardness of lead-free solder alloys

Purpose The purpose of this paper is to carry out a study of the evolution of the microstructure and the microhardness of Sn-Cu-Ag alloys from as-cast condition and under artificial isothermal aging at different temperatures (100ºC and 180ºC) for a treatment time up to 500 h. A comparison with Sn-37% Pb eutectic solder samples was also made. Design/methodology/approach Sn-3.5%Ag, Sn-0.7%Cu and Sn-3.5%Ag-0.9%Cu were poured in two different cooling rate conditions and then aged at 100ºC (373ºK) and 180 °C (453ºK) during 500 h. Microstructural changes were observed by optical microscopy, scanning electron micrograph and energy dispersive X-ray microanalysis. Differential scanning calorimetry technique (DSC) was also used to confirm the obtained results. Findings A decrease up to 20% in microhardness respect to the value of the as-cast alloy was observed for both aging temperatures. These changes can be explained considering the coarsening and recrystallization of Sn dendrites present in the microstructures of all the systems studied. Originality/value There is no evidence of dissolution or precipitation of new phases in the range of studied temperatures that could be detected by DSC calorimetry technique. The acting mechanisms must be the result of coarsening of Sn dendrites and the residual stresses relaxation during the first stages of the isothermal aging.

Development and characterization of eutectic Sn-Zn, Sn-Ag, Sn-Bi and Sn-Cu solder alloys

AbstractThis paper reports the comparative study of the microstructure, melting point, hardness and electrical resistivity of as-cast eutectic Sn-8.8 wt.% Zn, Sn-3.5 wt.% Ag, Sn-43 wt.% Bi and Sn-0.7 wt.% Cu lead-free solder alloys. Sn-3.5 wt.% Ag eutectic solder alloy was found to have the lowest electrical resistivity of 1.91 μΩ · cm whereas the highest electrical resistivity of 14.17 μΩ · cm was observed in the case of Sn-43 wt.% Bi eutectic solder alloy. The lowest melting point of 141.60°C was observed in the case of Sn-43 wt.% Bi eutectic alloy whereas the highest melting point of 243.28°C was observed in the case of Sn-3.5 wt.% Ag solder alloy. The results of the various solder alloys were compared with the properties of commercially available Sn-38.1 wt.% Pb eutectic solder alloy.

Annealing optimization for tin–lead eutectic solder by constitutive experiment and simulation

Abstract

Effects of Sb and Zn Addition on Mechanical Properties and Corrosion Resistance of Sn–Ag–Cu Solders

Pb–free solders have been widely developed by materials researchers as mainly focused on the replacement for Sn–Pb eutectic solder. In this study, Sn–3.0Ag–0.5Cu, Sn–3.0Ag–0.5Cu–xSb and Sn–3.0Ag–0.5Cu–xZn solders (x = 1.0, 2.0 and 3.0 wt%) were prepared using melting method. The effect of Sb and Zn addition on microstructure and properties, such as tensile strength, microhardness and corrosion resistance of Sn–Ag–Cu solders was investigated. The microstructure analysis revealed that addition of Sb and Zn to the Sn–Ag–Cu solders obviously decreased phase fraction of the dendritic β–Sn phase and the finer eutectic phase was obtained, resulting in superior mechanical properties of the Sn–3.0Ag–0.5Cu–xSb and Sn–3.0Ag–0.5Cu–xZn solders compared to the Sn–3.0Ag–0.5Cu solder. The results of potentiodynamic polarization tests indicated that the corrosion potentials of the Sn–3.0Ag–0.5Cu–xSb solders continuously increased with increasing of Sb content. While Zn content had little effect on the corrosion potentials of the Sn–3.0Ag–0.5Cu–xZn solders.

Microstructure and properties of AgCu/2 wt% Ag-added Sn–Pb solder/CuBe joints fabricated by vapor phase soldering

The purpose of this paper is to investigate the effect of 2 wt% Ag addition in Sn–Pb eutectic solder on microstructure and mechanical properties of AgCu/solder/CuBe joint fabricated by vapor phase soldering. 63Sn37Pb and 62Sn36Pb2Ag solder pastes were used to join Cu(1.7 wt%–2.5 wt%)Be and Ag(2 wt%–5 wt%)Cu alloys. Two fracture modes are observed in 62Sn36Pb2Ag and 63Sn37Pb joints after lateral shear tests at room temperature or 120 °C, and shear forces of 62Sn36Pb2Ag joints are far higher than those of 63Sn37Pb joints. No obvious difference is observed in morphology and thickness of intermetallic compounds (IMCs) at interfaces of the 63Sn37Pb and 62Sn36Pb2Ag joints. Within the two kinds of joints, formation of big blocky or plate-like Ag3Sn is restrained. However, many Ag3Sn IMCs particles (1–3 μm in width) in isolated and dispersed distribution are observed within 62Sn36Pb2Ag joints. The Ag3Sn particles are responsible for the better mechanical properties of 62Sn36Pb2Ag joints than those of 63Sn37Pb joints. The results present in this paper may provide a guide for restraining formation of big blocky or plate-like Ag3Sn in joints with Ag pads or thick Ag surface finish on pads by utilizing vapor phase soldering process, and improving mechanical properties of Cu/SnPb/Ag joints by adding Ag in SnPb eutectic solder.

Development and Characterization of Sn–Zn–Bi Lead Free Solder

Sn–Zn based lead free solder is a promising candidate to replace the Sn–Pb eutectic solder due to its low cost and melting temperature (198 °C) near to the Sn–Pb eutectic solder (183 °C). So far eutectic or near eutectic Sn–Pb alloys have been used as solder material but due to harmful effect on environment and human health Pb has to be eliminated from solder and prevent our environment from being polluted. Here the near eutectic composition of Sn-8 wt% Zn has been chosen. Apart from Sn-8 wt% Zn four different alloys containing different wt% of Bi were considered. They are Sn-8Zn-3Bi, Sn-8Zn-6Bi, Sn-8Zn-8Bi and Sn-8Zn-10Bi. The results of the differential scanning colorimetry (DSC) analysis show that addition of Bi to the near eutectic composition of Sn-8 wt% Zn lowers the melting temperature. The microstructure of the alloys were investigated using a scanning electron microscope and energy dispersive x-ray spectroscopy analysis. Tensile test shows that both tensile strength and elongation of the alloys reduce on addition of high wt% Bi to the Sn–8Zn solder alloy.

Synthesis of Sn nanoparticles and their size effect on the melting point

Recently, the use of Sn---Pb eutectic solder has been prohibited by restriction of the use of hazardous substances (RoHS), since Pb may have a bad influence on human and the environment. Instead, Tin---Silver---Copper (Sn---Ag---Cu) system is adopted as the lead free solder. However, the melting temperature of Sn---Ag---Cu is higher than that of Sn---Pb, which may induce damages in the electronic packages. To solve this problem, Sn nanoparticles were fabricated in this study. Morphology observation and component analysis of Sn nanoparticles were carried out by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectroscopy, respectively. The melting temperature of Sn nanoparticles was measured by differential scanning calorimeter (DSC822e). The lowest melting temperature of the present Sn nanoparticles is 440 K, which is approximately 65 K lower than that of the bulk Sn.

Application of Synchrotron Radiation X-Ray Microtomography to Nondestructive Evaluation of Thermal Fatigue Process in Flip Chip Interconnects

New nondestructive inspection methods with high spatial resolution are expected to support the evaluation and enhancement of the reliability of microjoints on printed circuit boards. An X-ray microtomography system, the SP-μCT has been developed at the Super Photon ring-8 GeV (SPring-8), the largest synchrotron radiation facility in Japan. In this work, the SP-μCT was first applied to the nondestructive evaluation of thermal fatigue phenomena, namely microstructure evolution (i.e., phase growth) and microcrack propagation, appearing in actual solder microbumps of flip chip interconnects due to thermal cyclic loading. In addition, a refraction-contrast imaging technique was simultaneously applied to visualize the fatigue cracks with an actual opening of less than 100 nm. The observed specimen has a flip chip structure joined by Sn-37wt%Pb eutectic solder bumps 150 μm in diameter. Consequently, the process of phase growth and crack propagation was determined via observation of consecutive computed tomography (CT) images obtained in the same plane of the same specimen. As the thermal cycle proceeded, remarkable phase growth was clearly observed, followed by the appearance of fatigue cracks in the corners of the interfaces between the solder bump and Cu pad. Moreover, the CT images also enabled us to evaluate the fatigue lifetime of the bumps, as follows. The lifetime to fatigue crack initiation was estimated by quantifying the increase in the phase growth. The crack propagation lifetime to failure was then determined by measuring the average crack propagation rate. Such results have not been obtainable at all by X-ray CT systems for industrial use and demonstrate the possibility of nondestructive inspection by a synchrotron radiation X-ray microtomography system.

Electrochemical corrosion behaviour of Sn–Ag–Cu (SAC) eutectic alloy in a chloride containing environment

AbstractThe corrosion behaviour of the Sn94.5Ag3.8Cu1.5 (SAC) eutectic alloy was investigated in 0.1 M NaCl solution by potentiodynamic polarization and impedance spectroscopy measurements and compared with that of the conventional Sn73.9Pb23.1 eutectic solder employed for a long time in the packaging of microelectronic components and devices. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to characterize the SAC eutectic alloy prior to and after the electrochemical tests. The electrochemical results indicated that the Sn–Ag–Cu eutectic alloy exhibits better corrosion behaviour than the Sn–Pb eutectic solder in NaCl solution. The presence of a corrosion products layer constituted by tin oxy‐chloride was detected at the surface of both alloys investigated after the electrochemical tests. The better corrosion behaviour of SAC eutectic alloy compared to Sn–Pb eutectic solder is ascribed to the formation of a more compact surface film of corrosion products with improved protective properties owing to the presence of copper and silver, as revealed by EPMA.

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.

Interfacial reaction of gas-atomized Sn–Zn solder containing Ni and Cu additives

Sn–Zn-based alloy is expected as an alternative low-temperature lead-free solder because of its melting temperature being close to that of Sn–Pb eutectic solder. The Sn–Zn solder is, however, inferior in terms of interfacial reaction with Cu substrate; that is, a hard and brittle Cu–Zn intermetallic compound (IMC) layer is formed at the joint interface, which leads to degradation of adhesion strength. In order to prevent the formation of Cu–Zn IMCs, we have investigated the effects of atomizing treatment on the Sn–Zn solders with additives Ni and Cu. In the solder containing Cu and Ni additives, the Cu–Zn IMC layer commonly observed is formed for the non-atomized solder, whereas the Cu–Sn IMC layer is formed for the atomized solder as well as Sn–Pb solder alloy. This is significant improvement for practical use of Sn–Zn based solder alloy.

Impact of board and component metallizations on microstructure and reliability of lead-free solder joints

Aging and accelerated thermal cycling (ATC) have been performed on 2512 chip resistors assembled with Sn3.8Ag0.7Cu (wt.%) solder. The boards were finished with immersion Ag (IAg), electroless nickel/immersion gold (ENIG), and hot air solder leveling Sn–Pb eutectic solder (HASL), and the components’ terminations were finished with 100% Sn and Sn8.0Pb (wt.%). The boards were reflowed with an average cooling rate of 1.6 °C/s. It was found that the microstructure and reliability of the solder joints depended on the board surface finish. The boards containing small amounts of Pb (from board/component terminations) were the most reliable. Solder joints to copper showed a significantly higher number of cycles to first failure than the joints on nickel. Better reliability of the Sn3.8Ag0.7Cu/Cu joints was attributed to an increased copper content in the bulk due to substrate dissolution.

Nondestructive Evaluation of Thermal Phase Growth in Solder Ball Microjoints by Synchrotron Radiation X-Ray Microtomography

In high-density packaging technology, one of the most important issues is the reliability of the microjoints connecting large scale integrated circuit chips to printed circuit boards electrically and mechanically. The development of nondestructive testing methods with high spatial resolution is expected to enhance reliability. An X-ray microtomography system called SP-μCT has been developed in Super Photon ring-8 GeV (SPring-8), the largest synchrotron radiation facility in Japan. In this work, SP-μCT was applied in the nondestructive evaluation of microstructure evolution, that is, the phase growth due to thermal cyclic loading in solder ball microjoints. Simulating solder microjoints used in a flip chip, specimens were fabricated by joining a Sn–Pb eutectic solder ball 100 μm in diameter to a steel pin in the usual reflow soldering process. The phase growth process was determined by observing the computed tomography (CT) images obtained consecutively at the fixed point of the target joining. In the reconstructed CT images, the distribution of the constituent phases in the Sn–Pb eutectic solder was identified based on the estimation value of the X-ray linear attenuation coefficient. Consequently, the microstructure images obtained nondestructively by SP-μCT provided us with the following useful information for evaluating the reliability of the solder microjoints. First, each phase involves not dispersing particles but a three-dimensional monolithic structure like a sponge. Second, the phase growth proceeds in such a way that the average phase size to the fourth power increases proportionally to the number of cycles. Finally, in the vicinity of the joining interface, more rapid phase growth occurs compared to the other regions because local thermal strain due to the mismatch of thermal expansion leads to a remarkable phase growth.

Inelastic Deformation and Fatigue of Solder Alloys Under Complicated Load Conditions

Abstract Fundamental study of deformation and fatigue fracture behavior of solder alloys under complex load conditions is a key to enabling implementation of sophisticated three-dimensional (3D) time-dependent nonlinear finite-element stress and strain analyses for the life assessment for electronic packages and assemblies. In this study, the rate-dependent deformation and fatigue fracture behavior of Sn3.8Ag0.7CuPb-free alloy and Sn–Pb eutectic alloy was investigated with thin-walled specimens using a biaxial servo-controlled tension–torsion material testing system, with solder alloys subjected to a variety of complex load conditions: pure shearing at strain rates between 6.7×10−7∕s and 1.3×10−1∕s, creep at temperatures ranging from room temperature up to 125°C, and cyclic loading with frequencies of 0.001Hz to 3Hz. Biaxial stress conditions were imposed to investigate the effects of multiaxial stresses on deformation behavior. The effects of frequency and temperature on cyclic deformation and fatigue facture were investigated for lead-free Sn3.8Ag0.7Cu and Sn–Pb eutectic solder. Fractography of fatigue tested samples was also conducted to determine possible fatigue failure mechanisms.

Metallurgy and Kinetics of Liquid–Solid Interfacial Reaction during Lead-Free Soldering

The wetting of a molten solder on metallic surfaces is a rather complex phenomenon. In addition to physical spreading due to surface tension reduction, there are interfacial metallurgical and flux chemical reactions with the metallic substrate surface. Substrate dissolution and intermetallic formation take place rapidly during soldering. Since lead-free soldering requires substantially higher soldering temperatures (around 250 � C), the rates of intermetallic growth and substrate dissolution for lead-free solders are expected to be significantly greater than those for the current Sn–Pb eutectic solder. This study systematically investigates the metallurgy of the solid–liquid interface reactions and intermetallic growth kinetics for three lead-free solders: Sn–Ag eutectic (96.5%Sn–3.5%Ag), Sn–Cu eutectic (99.3%Sn–0.7%Cu) and Sn–Ag– Cu eutectic (Sn–3.8Ag–0.7Cu, SAC 387) with three metallic substrates: Cu, Ni, and Alloy 42 (42%Ni–52%Fe) over temperatures ranging from 225 to 280 � C for reaction time from 10 s to 16 h. Wetting behavior of these three alloys on PCBs with OSP, immersion Sn, and Ni/Au finishes, was also examined from 220 � C up to 260 � C. A thorough understanding of lead-free solder/substrate interfacial reactions should give guidance to the optimum lead-free soldering processes and to the optimum lead-free coating thicknesses for component and PCB terminal finishes, as well as for under-chip metallurgical coatings for flip-chip and BGA applications.

The effect of strain rate and temperature on the tensile properties of Sn–3.5Ag solder

The tensile response of Sn–3.5% Ag solder was investigated and compared with that of a Sn–37% Pb eutectic solder at various strain rates from 2.38×10 −6 s −1 to 2.38×10 −3 s −1 over the temperature range from −50 °C to 150 °C. The relationship between tensile strength, σ UTS, and strain rate, ɛ´, for Sn–3.5Ag can be expressed by the equation σ UTS= Aɛ´ m . The influence of temperature on the strain rate sensitivity index m was very slight for Sn–3.5Ag, whereas the m values of Sn–37Pb increased strongly with increasing temperature. The relationship between the tensile strength of the Sn–3.5Ag alloy and temperature follows an Arrhenius law, and the activation energy for creep was found to be 78 kJ/mol, close to that for the pipe diffusion controlled creep of tin. The microstructure and fracture morphologies of both solders were observed with a scanning electron microscope. Ag 3Sn particles were observed in the primary β-Sn in the Sn–3.5Ag solder by transmission electron microscope.

Aging effects on fracture behavior of 63Sn37Pb eutectic solder during tensile tests under the SEM

This study investigates the influence of aging treatment on fracture behavior of Sn–Pb eutectic solder alloys at different loading rate regime during tensile tests under the scanning electron microscope. In high homologous temperature, the solder exhibit the creep behavior that could be confirmed through the phenomena of grain boundary sliding (GBS) to both as-cast and aged specimens. Owing to the large grain scale after high temperature storage, boundary behavior was limited to some extent for the difficulty in grain rotation and boundary migration. Instead, drastic intragranular deformation occurred. Also, the phase coarsening weakened the combination between lead-rich phase and tin matrix. Consequently, surface fragmentation was detected for the aged specimens. Furthermore, the fracture mechanism changed from intergranular dominated to transgranular dominated with increasing loading rate to both specimens during early stage.

Aging effects on fracture behavior of 63Sn37Pb eutectic solder during tensile tests under the SEM

Abstract This study investigates the influence of aging treatment on fracture behavior of Sn–Pb eutectic solder alloys at different loading rate regime during tensile tests under the scanning electron microscope. In high homologous temperature, the solder exhibit the creep behavior that could be confirmed through the phenomena of grain boundary sliding (GBS) to both as-cast and aged specimens. Owing to the large grain scale after high temperature storage, boundary behavior was limited to some extent for the difficulty in grain rotation and boundary migration. Instead, drastic intragranular deformation occurred. Also, the phase coarsening weakened the combination between lead-rich phase and tin matrix. Consequently, surface fragmentation was detected for the aged specimens. Furthermore, the fracture mechanism changed from intergranular dominated to transgranular dominated with increasing loading rate to both specimens during early stage.

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.