Epitaxial growth of Cu6Sn5 formed at Sn-based lead-free solder/non-textured polycrystalline Cu plate interface

This study examines the epitaxial growth of the intermetallic compound (IMC) of Cu₆Sn₅ (or (Cu,Ni)₆Sn₅) that forms at the interface between molten Sn-based lead-free solders and non-textured polycrystalline Cu substrates. Sn, Sn–Cu, Sn–Cu–Ni and Sn–Ag–Cu solders were investigated. The dominant growing planes in a hexagonal structure of this IMC on Cu substrates are (101) and (102). Addition of trace Ni into Sn–Cu solders leads to an increase in (101) growth and a decrease in (102) growth. The presence of Ag in Sn–Ag–Cu solders facilitates (102) growth and suppresses (101) growth. Such an epitaxial growth should have a large influence on the mechanical and electrical characteristics of the Sn-based solder/Cu joints.     

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.     

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.     

Thermophysical Properties of Sn–Ag–Cu Based Pb-Free Solders

Lead–tin (Pb–Sn) alloys are the dominant solders used for electronic packaging because of their low cost and superior properties required for interconnecting electronic components. However, increasing environmental and health concerns over the toxicity of lead, combined with global legislation to limit the use of Pb in manufactured products, have led to extensive research and development studies of lead-free solders. The Sn–Ag–Cu ternary eutectic alloy is considered to be one of the promising alternatives. Except for thermal properties, much research on several properties of Sn–Ag–Cu alloy has been performed. In this study, five Sn–xAg–0.5Cu alloys with variations of Ag content x of 1.0 mass%, 2.5 mass%, 3.0 mass%, 3.5 mass%, and 4.0 mass% were prepared, and their thermal diffusivity and specific heat were measured from room temperature to 150 °C, and the thermal conductivity was calculated using the measured thermal diffusivity, specific heat, and density values. Also, the linear thermal expansion was measured from room temperature to 170 °C. The results show that Sn–3.5Ag–0.5Cu is the best candidate because it has a maximum thermal conductivity and a low thermal expansion, which are the ideal conditions to be a proper packaging alloy for effective cooling and thermostability.     

The effects of suppressed beta tin nucleation on the microstructural evolution of lead-free solder joints

Most lead-free solders comprise tin (Sn) as the majority component, and nominally pure β-Sn is the majority phase in the microstructure of these solders. It is well established that nucleation of β-Sn from Sn-base liquid alloys is generally difficult. Delays in the onset of β-Sn formation have a profound effect upon the microstructural development of solidified Sn-base alloys. Utilizing stable and metastable phase diagrams, along with solidification principles, the effects of inhibited β-Sn nucleation on microstructural development are discussed, employing the widely studied Sn–Ag–Cu (SAC) alloy as a model system. This analysis shows that the main effect of suppressed β-Sn nucleation on near-eutectic SAC solders is to increase the number and/or volume fraction of primary or primary-like microconstituents, while simultaneously decreasing the volume fraction of eutectic microconstituent. General strategies are outlined for avoiding unwanted microconstituent development in these materials, including the use of metastable phase diagrams for selecting alloy compositions, employment of inoculants to promote β-Sn nucleation, and utilization of high cooling rates to limit solid phase growth. Finally, areas for future research on the development of inoculated Sn-base solder alloys are outlined.     

Interfacial reactions in the Sn–Sb/Ag and Sn–Sb/Cu couples

Sn–Sb and Sn–Sb-based alloys are important high temperature solders. Interfacial reactions are examined in the Sn-5.0 at% Sb/Ag and Sn-10.0 at% Sb/Ag at 200 °C, and Sn-5.0 at% Sb/Cu and Sn-10.0 at% Sb/Cu at 250 °C. Only the Ag₃Sn phase is formed in the Sn–Sb/Ag couples, and Cu₆Sn₅ and Cu₃Sn phases are formed in the Sn–Sb/Cu couples. The reaction products all grow linearly with the square root of reaction time which suggests the interfacial reactions are diffusion controlled.     

Effects of strain rate and temperature on the stress–strain response of solder alloys

To ensure reliable design of soldered interconnections as electronic devices become smaller, requires greater knowledge and understanding of the relevant mechanical behavior of solder alloys than are presently available. The present paper reports the findings of an investigation into the monotonic tensile properties of bulk samples of three solder alloys; a lead–tin eutectic and two lead-free solders (tin–3.5 copper and a tin–3.5 silver alloy). Temperatures between–10 and 75°C and strain rates between 10^–1 and 10^–3 s^–1 have been studied. Both temperature and strain rate may have a substantial effect on strength, producing changes well in excess of 100%. Strength is reduced by lowering strain rate and increasing temperature, and Sn–37 Pb is usually most sensitive to the latter. Expressions for strain and strain rate hardening have been developed. The Sn–0.5 Cu alloy is usually the weakest and most ductile. Sn–37 Pb is strongest at room temperature but with increasing temperature and lower strain rates it becomes inferior to Sn–3.5 Ag. Ductility changes with temperature and strain rate for all three alloys are generally small with inconsistent trends. The role of such data in stress analysis and modeling is considered and the paramount importance of employing data for conditions appropriate to service, is emphasized.     

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%)     

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.     

Wettability Studies of Pb-Free Soldering Materials

For Pb-free soldering materials, two main substitutes are currently being considered, consisting of Sn–Ag and Sn–Ag–Cu eutectics, both with melting points higher than that of the Sn–Pb eutectic. Therefore, both will require higher soldering temperatures for industrial applications. Also, both eutectics have a higher surface tension than the Sn–Pb eutectic, requiring wettability studies on adding Bi, Sb, and In to the eutectics to decrease the melting points and surface tension. The experimental results for the surface tension were compared with thermodynamic modeling by Butler’s method and were used to create the SURDAT database, which also includes densities for pure metals, binary, ternary, quaternary, and quinary alloys. To model the surface tension, excess Gibbs energies of the molten components were taken from the ADAMIS database. For the case of the Ag–Sn system, enthalpies of formation of Ag₃Sn from solution calorimetry were used for checking optimized thermodynamic parameters. In the study of Sn–Ag–Cu–Bi–Sb liquid alloys, the range of possible Bi compositions for practical applications has been used to formulate a generalized metric of wettability, which was checked by measurements of the influence of In on the Sn–Ag–Cu system.     

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.     

Phase Diagrams of Pb-Free Solders and their Related Materials Systems

Replacing Pb–Sn with Pb-free solders is one of the most important issues in the electronic industry. Melting, dissolution, solidification and interfacial reactions are encountered in the soldering processes. Phase diagrams contain equilibrium phase information and are important for the understanding and prediction of phase transformation and reactive phase formation at the solder joints. This study reviews the available phase diagrams of the promising Pb-free solders, and their related materials systems. The solders are Sn–Ag, Sn–Cu, Sn–Ag–Cu, Sn–Zn, Sn–Bi, Sn–In and Sn–Sb. The materials systems are the solders with the Ag, Au, Cu, Ni substrates, such as Sn–Ag–Au, Sn–Ag–Ni, Sn–Cu–Au, and Sn–Cu–Ni ternary systems. For the Pb-free solders and their related ternary and quaternary systems, preliminary phase equilibria information is available; however, complete and reliable phase diagrams over the entire compositional and temperature ranges of soldering interests are lacking.     

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.     

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.     

Constitutive models of creep for lead-free solders

The constitutive modeling of creep has been extensively studied due to the important of the creep failure mode in solder joints. However, there are very few studies that considered room temperature aging contributions in their creep modeling studies. This study investigated constitutive modeling of creep of solders by taking into account the possible contribution room temperature aging. Lead-free solder (Sn–4.0Ag–0.5Cu) was found to have a higher creep resistance than Sn–Pb solder at the same stress level and testing temperature. The higher creep resistance was contributed by the second phase intermetallic compounds, Ag₃Sn and Cu₆Sn₅. The precipitation of these intermetallic compounds can significantly block the movement of dislocations and increase the creep resistance of the material. Constitutive models of creep for both lead-free and Sn–Pb eutectic solders were constructed based on the experimental data. The activation energy for SAC405 is much higher than that of Sn–Pb, which also indicates that SAC405 possesses higher creep resistance. The constitutive models can be used in finite element analysis of actual electronic packages to predict solder joint failure. The creep mechanisms of both lead-free and Sn–Pb eutectic solders were also extensively discussed in this dissertation. Dislocation gliding and climb is believed to be the major failure mode at high stresses, while lattice diffusion and grain boundary diffusion is believed to be the major failure mode at low stress levels. Grain boundary sliding is believed to contribute to creep deformation at both high-stresses and low-stresses. For eutectic Sn–Pb, superplastic deformation is a major the creep mechanism at low-stresses and high-temperatures.     

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.     

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.     

Effective diffusivity of lead free solder alloys

A Monte Carlo simulation based algorithm is developed to compute the effective diffusivity of lead free solder alloys. Simulations are performed to determine the vacancy diffusivity for 95.5Sn–4.0Ag–0.5Cu (SAC405) solder alloy for different paths. Temperature and micro-structural influence on diffusivity are studied. A map of diffusivity versus temperature and average grain size is developed. Significant differences between diffusivity values reported in the literature for the same solder alloy is also discussed.     

The effects of third alloying elements on the bulk Ag<Subscript>3</Subscript>Sn formation in slowly cooled Sn–3.5Ag lead-free solder

The effects of third alloying elements (Cu, In, Zn) on the formation of bulk Ag₃Sn intermetallic compounds (IMCs) in slowly cooled Sn–3.5Ag lead-free solder were investigated by microstructural observation and thermal analysis technique. Microstructural observation shows that bulk Ag₃Sn IMCs existed in the microstructure of slowly cooled Sn–3.5Ag, Sn–3.5Ag–0.75Cu and Sn–3.5Ag–1.5In alloys, while no bulk Ag₃Sn IMCs formed in the slowly cooled Sn–3.5Ag–2.0Zn alloys. Thermal analysis results indicate that Ag preferably reacted with Zn to form Ag–Zn IMCs at high temperature rather than reacted with Sn to form Ag₃Sn plate.     

Formation of bulk Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> intermetallic compounds in Sn–Cu lead-free solders during solidification

The influence of cooling rate and Cu content on the microstructures of the solidified Sn–Cu alloys was investigated in terms of microstructural observation, thermal analysis and theoretical calculation. It was found that bulk Cu₆Sn₅ intermetalic compounds (IMCs) formed only in the Sn–1.0Cu alloy with lower cooling rate. At higher cooling rate, the actual eutectic point of Sn–Cu solder alloy shifts to the direction of higher Cu concentration. Thus the eutectic and hypereutectic alloys experience a metastable pseudoeutectic solidification route instead. The volume fraction of the bulk Cu₆Sn₅ IMCs formed in the Sn–1.0Cu alloy, measured by thermal analysis, is larger than that predicted by the equilibrium phase diagram. This could be attributed to eutectic Cu₆Sn₅ phases, which cling to the primary Cu₆Sn₅ crystal during the eutectic reaction due to their matching crystalline orientation relationship.