Solder Matrix Research Articles

An investigation of microstructure and properties of Sn3.0Ag0.5Cu-XAl2O3 composite solder

Purpose The purpose of this paper is to investigate microstructure and properties of Sn3.0Ag0.5Cu-XAl2O3 composite solder which were prepared through powder metallurgy route. Design/methodology/approach Sn3.0Ag0.5Cu (SAC305)-XAl2O3 (X = 0.2, 0.4, 0.6, 0.8 Wt. %) composite solders were prepared through the powder metallurgy route. The morphology of composite solder powders which consists of Al2O3 particles and SAC solder powders after ball milling was observed. The retained ratio of Al2O3 nanoparticles in composite solder billets and solder joints were also quantitatively measured. Furthermore, the as-prepared composite solder alloys were studied extensively with regard to their microstructures, thermal property, wettability and mechanical properties. Findings After ball milling, the Al2O3 nanoparticles added were observed embedded into the surface of SAC solder powders. Only about 5-10 per cent of the initial Al2O3 nanoparticles added were detected in the composite solder joints after reflow. In addition, finer ß-Sn grains were achieved with addition of Al2O3 nanoparticles; the Al2O3 nanoparticles were found retained in the composite solder matrix. Besides, negligible changes in melting temperature and the considerably reduced undercooling were obtained in composite solder alloys. Wettability was improved by appropriate addition of Al2O3 nanoparticles. Microhardness and shear strength of composite solders were both improved after Al2O3 nanoparticles addition. Originality/value This paper indicated that powder metallurgy route offered a feasible approach to produce nanoparticle reinforced composite solder. In addition, the quantitative analysis of the actual retained ratio of the Al2O3 nanoparticles in solder joints provided practical implications for the manufacture of composite solders.

Microstructure, mechanical and electrical performances of zirconia nanoparticles-doped tin-silver-copper solder alloys

The present study investigates the influence of ZrO2 nanoparticles addition on microstructure, material properties i.e., elastic moduli, creep behavior, hardness, shear strength and electrical resistivity of Sn–3.0Ag–0.5Cu (wt%)-based interconnected alloys. From microstructure observation it was revealed that in the plain Sn–Ag–Cu bulk solder a needle-shaped Ag3Sn and irregular shaped Cu6Sn5 IMC particles were formed in the elongated-like β-Sn grains. However, in the composite solder alloy, very fine needle-shaped Ag3Sn and irregular shaped Cu6Sn5 IMC particles were formed in equiaxed β-Sn grain matrix. Further, material properties like elastic moduli, hardness, creep behavior and electrical resistivity of Sn–Ag–Cu solder were improved significantly after adding the ZrO2 nanoparticle. Sn–Ag–Cu-based composite solder joints were prepared on Au/Ni-metallized Cu pad ball grid array (BGA) substrate and investigated their performance after exposing a thermal shock chamber at –40 to 90 °C. From this test, it was confirmed that the degradation behavior of plain solder joints was much faster as compared to the composite solder joints and also detected some micro-cracks at the interface and solder matrix. Further, the shear strength of plain solder joints was decreased significantly as compared to the composite solder joints.

Effects of Ag content on the interfacial reactions between liquid Sn–Ag–Cu solders and Cu substrates during soldering

The effects of Ag content on the interfacial reactions between Sn–Ag–Cu (SAC) solders and Cu substrates during soldering were studied based on an experiment in which the liquid solder is removed after a long exposure time. This removal of the solder allows for the capture and visualization of the interfacial IMCs formed during soldering and avoids the influence of Cu6Sn5 precipitated from the solder matrix during cooling. The results showed that a Cu6Sn5 scallop type layer with round grains having a strong texture was formed at the interface of the SAC solders/Cu systems. The addition of Ag decreased the IMC/liquid SAC solder interfacial energy and improved the wettability of the solder on the Cu. In addition to the reaction energy, the interfacial microstructure also affected the interfacial IMC growth extent. By changing the growth environment near the interface, Ag affected the orientation of Cu6Sn5 grains formed at the interface, leading to the evolution of a microstructure that generates more Cu6Sn5 grain boundaries at the interface. Consequently, under the same reflow conditions, SAC solders with a higher Ag content in contact with the Cu substrate form thicker IMCs interfacial layers.

Comparative study of the microstructure and mechanical strength of tin-copper (Sn0.7Cu) solder modified with silver (Ag) by both alloying and doping methods

In order to compare the effectiveness of alloying methods with doping methods, we have studied the microstructure and mechanical strength of Sn0.7Cu solder and two kinds of Ag modified Sn0.7Cu solders, i.e., Sn0.7Cu0.3Ag (prepared by alloying pure Sn, Cu, and Ag metals) and Sn0.7Cu + 0.3Ag (prepared by doping Ag nanoparticles into Sn0.7Cu). Solder joints were sent for mechanical tests and microstructure observation after aging at 90 °C for up to 500 h. Electromigration (EM) test models were built to evaluate the resistance of solders to EM degradation at an average current density of 0.79 × 104 A/cm2. Experimental results revealed that both Ag modified Sn0.7Cu solders had higher mechanical strength and stronger resistance to EM than Sn0.7Cu. This improvement was due to the introduction of dispersed Ag3Sn intermetallic compounds (IMCs), which acted as a second phase to refine the bulk solder, stop dislocations and hinder atomic migration. Sn0.7Cu0.3Ag was found to be more effective in reinforcing the mechanical strength than Sn0.7Cu + 0.3Ag, since the shear strength and microhardness of the former solder were respectively 12.79 % and 1.01 % higher than the latter one. The products of diffusivity and effective charge number DZ*, which represented the EM rate, for Cu atoms were measured to be 3.49 × 10−9, 1.91 × 10−9, 2.64 × 10−9 cm2/s for Sn0.7Cu, Sn0.7Cu0.3Ag, and Sn0.7Cu + 0.3Ag, respectively. The superior performance of Sn0.7Cu0.3Ag was ascribed to the smaller size and wider distribution of Ag3Sn IMCs in the solder matrix. To conclude, the alloying method produced greater improvement than the doping method in the microstructure and mechanical strength of Sn0.7Cu solder.

Effects of post-reflow cooling rate and thermal aging on growth behavior of interfacial intermetallic compound between SAC305 solder and Cu substrate

The interfacial reactions between Cu and Sn3Ag0.5Cu (SAC305) solder reflowed under various cooling rates were investigated. It is found that the cooling rate is an important parameter in solder reflow process because it influences not only microstructure of solder alloy but also the morphology and growth of intermetallic compounds (IMCs) formed between solder and Cu substrate. The experimental results indicate that only scallop-like Cu6Sn5 IMC layer is observed between solder and Cu substrate in case of water cooling and air cooling, while bilayer composed of scallop-like Cu6Sn5 and thin layer-like Cu3Sn is detected under furnace cooling due to sufficient reaction time to form Cu3Sn between Cu6Sn5 IMC and Cu substrate which resulted from slow cooling rate. Samples with different reflow cooling rates were further thermal-aged at 423 K. And it is found that the thickness of IMC increases linearly with square root of aging time. The growth constants of interfacial IMC layer during aging were obtained and compared for different cooling rates, indicating that the IMC layer thickness increased faster in samples under low cooling rate than in the high cooling rate under the same aging condition. The long prismatic grains were formed on the existing interfacial Cu6Sn5 grains to extrude deeply into solder matrix with lower cooling rate and long-term aging, and the Cu6Sn5 grains coarsened linearly with cubic root of aging time.

Rapid microstructure evolution of structural composite solder joints induced by low-density current stressing

The microstructure evolution of solder matrix and interfacial intermetallic compounds (IMCs) of Cu/Sn37Pb/Sn3.0Ag0.5Cu/Sn37P/Cu structural composite joints were investigated under low-density current stressing of 2.0×103A/cm2 at 100°C. Cu flux diffusion induced by electro-migration was delayed by Ag3Sn particles in eutectic phase of Sn3.0Ag0.5Cu solder, by which Cu6Sn5 particles could not reach the interfacial IMC at anode side, and then the growth of interfacial IMC or regular polarity effect at node was inhibited. Coarsened Cu6Sn5 particles were accumulated at the fusion zone between SnAgCu and SnPb close to anode side. Micro-cracks were found at cathode side after current stressing for as short as 100h, and increased with current stressing time until the failure of solder joint.

The effect of aging temperature on the phenomena occurring at the interface of solder SnZn with Na on Cu substrate

The aim of this study was to show the effect of aging time, temperature and Na addition to eutectic SnZn on the kinetics of formation and growth of the Cu5Zn8 and CuZn4 phases on Cu substrate. After a wetting test, the samples were aged at different temperatures for different times. Analysis of the microstructure of a cross section of the connection showed an NaZn13 phase in the solder matrix, which can reduce the diffusion rate of Zn atoms to the interface.

Effect of adding 0.5 wt% ZnO nanoparticles, temperature and strain rate on tensile properties of Sn–5.0 wt% Sb–0.5 wt% Cu (SSC505) lead free solder alloy

This study investigates the effect of adding 0.5wt% ZnO nanoparticles into Sn–5wt% Sb–0.5wt% Cu (SSC505) solder alloy on the growth of intermetallic compounds (IMCs) as well as the associated changes of microstructure. It was found that, the morphologies of the IMCs transformed to refinement forms due to the adsorption effect of ZnO nanoparticles. Systematic study of tensile properties was performed for bulk solders over a wide range of strain rates at various temperatures to determine the plastic deformation mechanism. Obviously, the ultimate tensile strength (UTS) and the yield stress (σy) were improved. This can be attributed to reinforcement of ZnO nanoparticles, refined β-Sn grains and IMCs that could obstruct the dislocation slipping. The obtained results are consistent with the prediction of the classic theory of dispersion strengthening. Furthermore, the average activation energy (Q) for plain and composite solders were 57 and 59kJ/mol, respectively, which close to that of pipe-diffusion mechanism in Sn based solder matrix.

Improving shear strength of Sn-3.0Ag-0.5Cu/Cu joints and suppressing intermetallic compounds layer growth by adding graphene nanosheets

This study investigated the effects of adding 0.1wt% of graphene nanosheets (GNSs) into Sn-3.0Ag-0.5Cu solder matrix on the growth of intermetallic compounds (IMC) layer with Cu substrates during isothermal aging at temperatures of 100, 125, 150 and 175°C for up to 7 days. The results indicated that the GNSs could provide huge diffusion barrier for the diffusion of Sn and Cu atoms in solder matrix, and the activation energy for IMC layer growth was calculated as 46.20 and 59.72kJ/mol for Sn-3.0Ag-0.5Cu solder and Sn-3.0Ag-0.5Cu-0.1GNSs composite solder, respectively. Compared with Sn-3.0Ag-0.5Cu solder joints, the IMC layer morphology transformation from scallop-type to layer-type and the IMC layer growth for composite solder joints were suppressed. After aging 7 days at 175°C, the shear strength of composite solder joints, whose IMC layer was comparatively thin, was 30% higher than that of Sn-3.0Ag-0.5Cu solder joints.

The Dissolution and Supersaturation of Zn in the Sn9Zn Solder Under Current Stressing

The Sn-Zn phase diagram indicates that Sn and Zn exhibit nearly zero mutual solubility. However, this study found that Zn shows certain solubility in the Sn matrix of Sn9Zn solder under current stressing with 6000–8000 A/cm2. The dissolution of the Zn crystal induced by electric current was evidenced by the diminishing of the x-ray diffraction orientation of Zn(002) and the cavity formation on the Zn crystal. The Zn concentration reaches supersaturation of 4–5 wt.% at 8000 A/cm2. The dissolution behavior was further investigated with scanning electron microscopy and a focus ion beam of the specimen quenched with liquid nitrogen after current stressing. The morphologies of the cavities on the Zn crystals were examined and three dissolution mechanisms were identified.

Microstructural Evolution of Lead-Free Solder Joints in Ultrasonic-Assisted Soldering

Solder joint reliability greatly depends on the microstructure of the solder matrix and the morphology of intermetallic compounds (IMCs) in the joints. Addition of strengthening phases such as carbon nanotubes and ceramic particles to solder joints to improve their properties has been widely studied. In this work, ultrasonic vibration (USV) of casting ingots was applied to considerably improve their microstructure and properties, and the resulting influence on fluxless soldering of Cu/Sn-3.0Ag-0.5Cu/Cu joints and their microstructural evolution was investigated. It was demonstrated that USV application during reflow of Sn-based solder had favorable effects on β-Sn grain size refinement as well as the growth and distribution of various IMC phases within the joints. The β-Sn grain size was significantly refined as the ultrasound power was increased, with a reduction of almost 90% from more than 100 μm to below 10 μm. Long and large Cu6Sn5 tubes in the solder matrix of the joints were broken into tiny ones. Needle-shaped Ag3Sn was transformed into flake-shaped. These IMCs were mainly precipitated along β-Sn phase boundaries. High-temperature storage tests indicated that the growth rate of interfacial IMCs in joints formed with USV was slower than in conventional reflow joints. The mechanisms of grain refinement and IMC fragmentation are discussed and related to the ultrasonic effects.

Whisker growth behaviors in POSS-silanol modified Sn3.0Ag0.5Cu composite solders

Abstract As an ideal modifier, the cage-type polyhedral oligomeric silsesquioxane (POSS)-silanol creates a strong bonding with the solder without agglomeration. It has been shown that the addition of POSS-silanol improves both the mechanical performance and the solder's resistance to electromigration. This study further examined the effect of the addition of POSS-silanol on whisker growth in a Sn3.0Ag0.5Cu (SAC305) solder. The whisker growth was studied for pure SAC305 and SAC305 modified with POSS-silanol under isothermal aging and thermal cycling conditions. The results indicate that the added POSS-silanol not only reinforces the solder joint, but also inhibits whisker formation under both thermal aging and thermal cycling conditions. POSS-silanol can therefore be considered a promising modifier material which enhances the reliability of solders. During the thermal aging process, the added POSS-silanol was found to inhibit the interdiffusion of Sn and Cu atoms at the interface region, thereby preventing the formation of Cu 6 Sn 5 but promoting the conversion of Cu 6 Sn 5 into Cu 3 Sn. During the thermal cycling process, the added POSS-silanol significantly inhibited whisker formation and the deformation of the interface by stiffening the solder matrix and decreasing the driving force for whisker formation.

Microstructures and properties of new Sn–Ag–Cu lead-free solder reinforced with Ni-coated graphene nanosheets

This paper deals with microstructures and properties of SAC305 lead-free solder reinforced with graphene nanosheets (GNS) decorated with Ni nanoparticles (Ni-GNS). These Ni-coated GNS nanosheets were synthesized by an in-situ chemical reduction method. After morphological and chemical characterization, Ni-GNS were successfully integrated into SAC305 lead-free solder alloy with different weight fractions (0, 0.05, 0.1 and 0.2 wt.%) through a powder metallurgy route. The obtained composite solders were then studied extensively with regard to their microstructures, wettability, thermal, electrical and mechanical properties. After addition of Ni-GNSs, cauliflower-like (Cu,Ni)6 Sn5 intermetallic compounds (IMCs) were formed at the interface between composite solder joint and copper substrate. Additionally, blocky Ni–Sn–Cu IMC/GNS hybrids were also observed homogenously distributed in the composite solder matrices. Composite solder alloys incorporating Ni-decorated GNSs nanosheets showed slightly reduced electrical resistivity compared to the unreinforced SAC305 solder alloy. With an increase in the amount of Ni-GNS, the composite solders showed an improvement in wettability with an insignificant change in their melting temperature. Mechanical tests demonstrated that addition of 0.2 wt.% Ni-GNS would result in 19.7% and 16.9% improvements in microhardness and shear strength, respectively, in comparison to the unreinforced solders. Finally, the added Ni-GNS reinforcements in the solder matrix were assessed with energy-dispersive X-ray spectroscopy, scanning electron microscopy and Raman spectroscopy.

Study of electromigration resistance of (Pd,Ni)Sn4 phase in lead-free solder joints

A Pd layer is frequently deposited on a Ni substrate as a surface finish in electronic packaging. During soldering process, the (Pd,Ni)Sn4 phase is formed at the solder/Ni interface and it is able to reduce electromigration damage. In this study, various interfacial structures, Sn/Pd(1 or 3 μm)/Ni and Sn/Cu(3 μm)/Pd(1 μm)/Ni, were designed to conduct electromigration tests. The results revealed that the electromigration resistance of the (Pd,Ni)Sn4 was closely associated with the Ni content. In the Sn/Pd(1 or 3 μm)/Ni couple, the (Pd,Ni)Sn4 layer with a high Ni solubility of ∼17 at% cannot retard electromigration. The (Pd,Ni)Sn4 phase was massively migrated into the solder matrix and the Ni substrate was considerably consumed. In the Sn/Cu(3 μm)/Pd(1 μm)/Ni couple, the formed (Pd,Ni)Sn4 layer with a low Ni content (<3 at%) can effectively retard electromigration; the interfacial microstructure was similar to that without current stressing. In addition, various Sn–Pd–Ni alloys were prepared to obtain the (Pd,Ni)Sn4 precipitates with different Ni contents. Under current stressing, the (Pd,Ni)Sn4 phase clearly exhibited a better electromigration resistance when the Ni content decreased. Moreover, for the interface with Cu substrate, the (Pd,Ni)Sn4 layer (<5 at%Ni) can act as an effective barrier layer to suppress the electromigration-induced Cu dissolution.

A study of green Sn–xZn photovoltaic ribbons for solar cell application

Sn–xZn (x=9, 25, and 50wt%) alloy solders were applied in a photovoltaic (PV) ribbon. The effects of intermetallic compounds (IMCs) on series resistance and bond reliability of Sn–xZn PV modules were investigated. Cu5Zn8 and AgZn3 IMCs were found at the interfaces (solder/Cu and solder/Ag). The Zn content in the solder dominated the growth behavior of IMCs at the interface. The IMC thickness increased with increasing Zn content in the solder, and thus the bond reliability of the PV module deteriorated. After a bias was applied for a long duration, the interfacial characteristics and solder matrix were affected by the bias-induced thermal diffusion. In addition, the evolution in IMCs due to thermal diffusion during bias changed the original series resistance of the ribbon material and affected the series resistance of the PV module.

Effects of trace amount praseodymium and neodymium on microstructure and mechanical properties of Sn–0.3Ag–0.7Cu–0.5Ga solder

The effects of trace mount of rare earth elements Pr and Nd addition on Sn–0.3Ag–0.7Cu–0.5Ga lead free solder were investigated by observation of microstructure and the morphology of interface layer, as well as the test of shearing strength of the solder joints. The results show that the microstructure of the solder matrix can be optimized by appropriate addition of Pr and Nd, but the scalloped and “region-like” compounds (IMCs) appear in the solder matrix with excessive Pr and Nd addition respectively which may result in the stress concentration and become the birthplace of the cracks. The behavior of absorption of rare earth elements can retard the interface reaction between solder and Cu substrate, refine the morphology of the interface layer. The shearing strength of the solder joints can be enhanced better with appropriate addition of Pr than Nd by improvement of nucleation rate and control of growth rate of the grains near the interface layer.

The size effect on intermetallic microstructure evolution of critical solder joints for flip chip assemblies

Purpose – The purpose of this paper is to study the intermetallic compound (IMC) thickness, composition and morphology in 100-μm pitch and 200-μm pitch Sn–Ag–Cu (SAC305) flip-chip assemblies after bump reflow and assembly reflow. In particular, emphasis is placed on the effect of solder joint size on the interfacial IMCs between metal pads and solder matrix. Design/methodology/approach – This work uses 100-μm pitch and 200-μm pitch silicon flip chips with nickel (Ni) pads and stand-off height of approximately 45 and 90 μm, respectively, assembled on substrates with copper (Cu) pads. The IMCs evolution in solder joints was investigated during reflow by using 100- and 200-μm pitch flip-chip assemblies. Findings – After bump reflow, the joints size controls the IMC composition and dominant IMC type as well as IMC thickness and also influences the dominant IMC morphology. After assembly reflow, the cross-reaction of the pad metallurgies promotes the dominant IMC transformation and shape coarsened on the Ni pad interface for smaller joints and promotes a great number of new dominate IMC growth on the Ni pad interface in larger joints. On the Cu pad interface, many small voids formed in the IMC in larger joints, but were not observed in smaller joints, combined with the drawing of the IMC growth process. Originality/value – With continued advances in microelectronics, it is anticipated that next-generation microelectronic assemblies will require a reduction of the flip-chip solder bump pitch to 100 μm or less from the current industrial practice of 130 to150 μm. This work shows that as the packaging size reduced with the solder joint interconnection, the solder size becomes an important factor in the intermetallic composition as well as morphology and thickness after reflow.

Solder Joint Reliability of Sn-Cu and Sn-Ag-Cu Lead-Free Solder Alloys Solidified on Copper Substrates with Different Surface Roughnesses

In the present work, the bond strength of Sn-0.7Cu, Sn-0.3Ag-0.7Cu, Sn-2.5Ag-0.5Cu and Sn-3Ag-0.5Cu lead free solders solidified on Cu substrates was experimentally determined. The bond shear test was used to assess the integrity of Sn–Cu and Sn–Ag–Cu lead-free solder alloy drops solidified on smooth and rough Cu substrate surfaces. The increase in the surface roughness of Cu substrates improved the wettability of solders. The wettability was not affected by the Ag content of solders. Solder bonds on smooth surfaces yielded higher shear strength compared to rough surfaces. Fractured surfaces revealed the occurrence of ductile mode of failure on smooth Cu surfaces and a transition ridge on rough Cu surfaces. Though rough Cu substrate improved the wettability of solder alloys, solder bonds were sheared at a lower force leading to decreased shear energy density compared to the smooth Cu surface. A smooth surface finish and the presence of minor amounts of Ag in the alloy improved the integrity of the solder joint. Smoother surface is preferable as it favors failure in the solder matrix.

Effect of ageing time on the tensile behavior of Sn–3.5 wt% Ag–0.5 wt% Cu (SAC355) solder alloy with and without adding ZnO nanoparticles

In this work, lead-free composite solder was produced by mixing ZnO-nano-sized particles with Sn–3.5% Ag–0.5% Cu solder. The effects of nano-particle addition on the melting behavior, microstructure and tensile properties of the as-solidified composite solder are systematically investigated. In comparison with solder without the addition of ZnO nanoparticles metallographic observations revealed that the grain size of β-Sn, the intermetallic compounds (IMCs) average grain size and distance decreased significantly in the composite solder matrix. This was possibly ascribed to the strong absorption effect and high surface free energy of ZnO nanoparticles. Our results show that 0.5wt% addition of ZnO nanoparticles led to an improvement in microstructure, proof stress (ơy0.2), ultimate tensile stress (ơUTS) and fracture stress (ơf) compared with non-composite solder. Differential scanning calorimetry (DSC) revealed that the melting temperature of SAC355 composite solder is slightly higher by about 1.1K than that of the conventional SAC355 solder. The observed increase of mechanical properties was rendered to refined IMCs, acting as a strengthening phase in the solder matrix. Ageing the specimens of both solders for different aging times at all different testing temperatures, resulted in the decrease of tensile properties investigated. This was attributed to ripening of IMC particles leading to decrease of their pinning action of dislocation movements which was emphasized by SEM examinations. This interpretation is in consistence with the theoretical prediction from dispersion strengthening theory.

Microstructural discovery of Al addition on Sn–0.5Cu-based Pb-free solder design

It is important to develop Pb-free solder alloys suitable for automotive use instead of traditional Sn–Pb solder due to environmental regulations (e.g., Restriction of Hazardous Substances (RoHS)). Al addition has been spotlighted to enhance solder properties. In this study, we investigated the microstructural change of Sn–0.5Cu wt.% based Pb-free solder alloys with Al addition (0.01–0.05 wt.%). The small amount of Al addition caused a remarkable microstructural change. The Al was favored to form Cu–Al intermetallic compounds inside the solder matrix. We identified the Cu–Al intermetallic compound as Cu33Al17, which has a rhombohedral structure, using EPMA and TEM analyses. This resulted in refined Cu6Sn5 networks in the Sn–0.5Cu based solder alloy. In addition, we conducted thermal analysis to confirm its stability at a high temperature of approximately 230 °C, which is the necessary temperature range for automotive applications. The solidification results were substantiated thermodynamically using the Scheil solidification model. We can provide criteria for the minimum aluminum content to modify the microstructure of Pb-free solder alloys.