Mechanical fatigue of Sn-rich Pb-free solder alloys

Recent fatigue studies of Sn-rich Pb-free solder alloys are reviewed to provide an overview of the current understanding of cyclic deformation, cyclic softening, fatigue crack initiation, fatigue crack growth, and fatigue life behavior in these alloys. Because of their low melting temperatures, these alloys demonstrated extensive cyclic creep deformation at room temperature. Limited amount of data have shown that the cyclic creep rate is strongly dependent on stress amplitude, peak stress, stress ratio and cyclic frequency. At constant cyclic strain amplitudes, most Sn-rich alloys exhibit cycle-dependent and cyclic softening. The softening is more pronounced at larger strain amplitudes and higher temperatures, and in fine grain structures. Characteristic of these alloys, fatigue cracks tend to initiate at grain and phase boundaries very early in the fatigue life, involving considerable amount of grain boundary cavitation and sliding. The growth of fatigue cracks in these alloys may follow both transgranular and intergranular paths, depending on the stress ratio and frequency of the cyclic loading. At low stress ratios and high frequencies, fatigue crack growth rate correlates well with the range of stress intensities or J-integrals but the time-dependent C* integral provides a better correlation with the crack velocity at high stress ratios and low frequencies. The fatigue life of the alloys is a strong function of the strain amplitude, cyclic frequency, temperature, and microstructure. While a few sets of fatigue life data are available, these data, when analyzed in terms of the Coffin–Mason equation, showed large variations, with the fatigue ductility exponent ranging from −0.43 to −1.14 and the fatigue ductility from 0.04 to 20.9. Several approaches have been suggested to explain the differences in the fatigue life behavior, including revision of the Coffin–Mason analysis and use of alternative fatigue life models.     

Three-dimensional (3D) visualization of reflow porosity and modeling of deformation in Pb-free solder joints

The volume, size, and dispersion of porosity in solder joints are known to affect mechanical performance and reliability. Most of the techniques used to characterize the three-dimensional (3D) nature of these defects are destructive. With the enhancements in high resolution computed tomography (CT), the detection limits of intrinsic microstructures have been significantly improved. Furthermore, the 3D microstructure of the material can be used in finite element models to understand their effect on microscopic deformation. In this paper we describe a technique utilizing high resolution (< 1 µm) X-ray tomography for the three-dimensional (3D) visualization of pores in Sn-3.9Ag-0.7Cu/Cu joints. The characteristics of reflow porosity, including volume fraction and distribution, were investigated for two reflow profiles. The size and distribution of porosity size were visualized in 3D for four different solder joints. In addition, the 3D virtual microstructure was incorporated into a finite element model to quantify the effect of voids on the lap shear behavior of a solder joint. The presence, size, and location of voids significantly increased the severity of strain localization at the solder/copper interface.     

Microstructure-based modeling of the deformation behavior of particle reinforced metal matrix composites

A review is provided of the use of analytical models and two dimensional (2D) and three dimensional (3D) microstructure based FEM models to accurately predict the properties of particle reinforced composite materials. It is shown that analytical models do not account for the microstructural factors that influence the mechanical behavior of the material. 2D models do capture the anisotropy in deformation behavior induced by anisotropy in particle orientation. The experimentally-observed dependence of Young's modulus and tensile strength is confirmed by the 2D microstructure-based numerical model. However, because of the 2D stress state, a realistic comparison to actual experimental values is not possible. A serial sectioning process can be used to reproduce and visualize the 3D microstructure of particle reinforced metal matrix composites. The 3D microstructure-based FEM accurately represents the alignment, aspect ratio, and distribution of the particles. Comparison with single particle and multiparticle models of simple shape (spherical and ellipsoidal) shows that the 3D microstructure-based approach is more accurate in simulating and understanding material behavior.     

Estimation of liquidus temperature of Sn-based alloys and its application to the design of Pb-free solder

An interesting relation between the melting temperature and the alloy composition was discovered in Sn-based alloys through the evaluation of a series of experimental data on the liquidus temperatures (LTs) for 134 types of multi-component Sn alloys. In these Sn alloys, the degree of liquidus temperature drop with alloying was not affected by the kind of individual elements but by their total atomic fraction alone. The compositional dependence on the LT could be expressed by the equation, LT(K)=499.79–1.799X, where X was the total mole percentage of alloying elements. It was demonstrated that the use of this equation would make it possible to develop Pb-free solder alloys more efficiently.     

Life expectancies of Pb-free SAC solder interconnects in electronic hardware

The transition from lead (Pb) bearing solder to Pb-free solder has arisen in response to government restrictions on the use of lead (Pb) by the European Union. As a result, electronic manufacturers have sought a material comparable to the conventional 63Sn37Pb solder that has been traditionally used to assemble electronic hardware. Based on extensive review of various solder combination, the majority of electronic manufacturers appear to be adopting a tin–silver–copper (SAC) solder as a popular Pb-free solder replacement. Significant investments have been made by many researchers to characterize the material behavior and durability of this solder system. While the exact composition of the SAC solder is still in question, it now appears that the 96.5Sn3.0Ag0.5Cu (SAC305) solder is gaining wider acceptance as the favored Pb-free replacement, for surface mount assemblies that are going to be subjected predominantly to cyclic thermal environments. This paper presents a review of our current understanding of the life expectancy of Pb-free SAC solder interconnects for electronic hardware. To this end, the paper focuses on material characterization of SAC solder, as well as its temperature cycling and vibration fatigue reliability. From this review, SAC solder interconnects are shown to be suitable for providing adequate life expectancies for temperature cycling in electronic hardware. However, it is clear that there are differences between SAC and the conventional Sn37Pb solder, that need to be understood in order to design reliable electronic hardware.     

Effect of cerium addition on wetting, undercooling, and mechanical properties of Sn-3.9Ag-0.7Cu Pb-free solder alloys

In this article, we report the effect of cerium (Ce) addition on wettability, microstructure, and mechanical properties of Sn-3.9Ag-0.7Cu (SAC) solders. It was found that the wettability of Ce-containing solder on Cu substrate is comparable to that of SAC solder for compositions up to 0.5 wt% Ce. The microstructure of lap-shear joints containing 0.5 wt% Ce rare-earth elements showed a finer microstructure and a thinner Cu₆Sn₅ intermetallic layer at the Cu/solder interface. Using differential scanning calorimetry (DSC), it was found that the magnitude of undercooling was significantly reduced with the addition of Ce. It has been reported that SAC–0.5Ce alloy exhibited a much higher elongations compared with SAC alloy, however, the mechanism for the enhanced ductility is still not clear. Especially, the effect of Cu₆Sn₅ intermetallic layer thickness on ductility need to be further investigated. The results in this study showed that the presence of CeSn₃ intermetallic particles, and not the thin Cu₆Sn₅ intermetallic layer, was responsible for the improved ductility demonstrated by Ce-containing solders. This finding revealed a new approach of enhancing the ductility, as well as the shock performance of Pb-free solder with additional alloying elements for the future solder alloy design.     

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.     

Inelastic deformation of (Ti,V)C alloys

The densification behaviour of (Ti, V) C alloys was studied in order to prepare fully dense, single phase alloys. The hot-pressing experiments were conducted on two compositions, TiC and TiC-75% VC, at temperatures between 1300 and 1900° C and pressure up to 120 M Pa. The effects of temperature and composition on final porosity, grain size and phases present were determined. The diffusion coefficients determined from densification data agree well with the grain-boundary diffusion coefficient of carbon. This observation, combined with microstructural evidence, indicates that the final stage of densification is controlled by grain-boundary diffusion of carbon and/or metal. The enhanced densification and grain growth in VC rich samples are attributed to the presence of second phase material along the grain boundaries. The rapid interdiffusion of titanium and vanadium in (Ti, V) C to form a single phase, raises the possibility of faster diffusion along the migrating boundaries in contrast to the stationary boundaries.     

Microstructure-based fatigue modeling of cast A356-T6 alloy

High cycle fatigue (HCF) life in cast Al-Mg-Si alloys is particularly sensitive to the combination of microstructural inclusions and stress concentrations. Inclusions can range from large-scale shrinkage porosity with a tortuous surface profile to entrapped oxides introduced during the pour. When shrinkage porosity is controlled, the relevant microstructural initiation sites are often the larger Si particles within eutectic regions. In this paper, a HCF model is introduced which recognizes multiple inclusion severity scales for crack formation. The model addresses the role of constrained microplasticity around debonded particles or shrinkage pores in forming and growing microstructurally small fatigue cracks and is based on the cyclic crack tip displacement rather than linear elastic fracture mechanics stress intensity factor. Conditions for transitioning to long crack fatigue crack growth behavior are introduced. The model is applied to a cast A356-T6 Al alloy over a range of inclusion severities.     

Microstructure-based multistage fatigue modeling of aluminum alloy 7075-T651

The multistage fatigue model for high cycle fatigue of a cast aluminum alloy developed by McDowell et al. is modified to consider the structure-property relations for cyclic damage and fatigue life of a high strength aluminum alloy 7075-T651 for aircraft structural applications. The multistage model was developed as a physically-based framework to evaluate sensitivity of fatigue response to various microstructural features to support materials process design and component-specific tailoring of fatigue resistant materials. In this work, the model is first generalized to evaluate both the high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes for multiaxial loading conditions, with appropriate modifications introduced for wrought materials. The particular microstructural features of relevance to fatigue in aluminum alloy 7075-T651 include micron-scale Fe-rich intermetallic particles and rolling textures. The model specifically addresses the role of local constrained cyclic microplasticity at fractured inclusions in fatigue crack incubation and microstructurally small crack growth, including the effect of crystallographic orientation on crack tip displacement as the driving force. The model is able to predict lower and upper bounds of the fatigue life based on measured inclusion sizes.     

Rate-dependent indentation behavior of solder alloys

Finite element (FE) simulations of visco-plastic indentation in Sn-37Pb eutectic solder alloy are performed to investigate the influence of loading rate on its creep characteristic. The resulting indentation load-displacement curves are rate-dependent and have varying creep penetration depths during the same hold time. Creep indentation hardness H, defined from the concept of work of indentation, varies with volume strain occurring during the creep hold time, which is a measure of creep strain rate _cr. Thus, creep stress sensitivity can be determined from the H versus _cr curve. This analysis can be verified by the good agreement between the derived value and the predefined value, and then be used to analyze the Berkovich indentation load-displacement curves of Sn-3.5Ag-0.75Cu lead-free solder. Such indentation tests and physical analysis provide a cheaper and more convenient method to determine the mechanical properties of the upcoming lead-free solder alloys.     

Experimental force modeling for deformation machining stretching mode for aluminum alloys

Deformation machining is a hybrid process that combines two manufacturing processes—thin structure machining and single-point incremental forming. This process enables the creation of complex structures and geometries, which would be rather difficult or sometimes impossible to manufacture. A comprehensive experimental study of forces induced in deformation machining stretching mode has been performed in the present work. A table-type force dynamometer has been used to record the deforming forces in three Cartesian directions. The influence of five process parameters—floor thickness, tool diameter, wall angle, incremental step size, and floor size on the deforming forces—is investigated. Individual as well as combined empirical models of the parameters with regard to the forces have been formed. The results of this study indicate that the average resultant force primarily depends on the floor thickness to be deformed and the incremental depth in the tool path. This could be due to the variation in local stiffness of the sheet with change in floor thickness. The effect of tool diameter, deforming wall angle, and floor size is not significant.     

Thermal properties of Sn-based solder alloys

During last few decades, emerging environmental regulations worldwide, more notably in Europe and Japan, have targeted the elimination of Pb usage in electronic assemblies due to the inherent toxicity of this element. This situation drives to the replacement of the Sn–Pb solder alloy of eutectic composition commonly used as joining material to suitable lead-free solders for microelectronic assembly. Sn-based alloys containing Ag, Cu, Bi, and Zn are potential lead-free solders, usually close to the binary or ternary eutectic composition. For this reason a great effort was directed to establish reliable thermophysical data fundamental to interpret the solidification process and fluidity of alloys belonging to these systems. In this work, an analysis of the solidification process of pure Sn, binary Sn–Ag, Sn–Cu, Sn–Bi, Sn–Zn, Sn–Pb and ternary Sn–Ag–Cu eutectic alloys was carried out using computer aided-cooling curve analysis and differential scanning calorimetry.     

Plastic deformation of titanium alloys under simple loading and its mathematical modeling

Results of an experimental investigation of the regularities of plastic deformation of titanium alloys in a plane stress state are analyzed. Tests were performed by loading thin-walled tubular specimens by an axial force and internal pressure under conditions of a proportional increase in the loads. The alloys are found to be transversely isotropic materials whose isotropic surface coincides with the cylindrical surface of the specimen. The process of plastic deformation of the alloys under simple loading is shown to be described well by equations of a previously proposed deformation theory of the plasticity of transversely isotropic media. Translated from Problemy Prochnosti, No. 5, pp. 27–35, September–October, 1999.     

Recrystallized grain rotation behavior in a Pb-free BGA solder joint under electron current stress

The grain orientations of a Pb-free ball grid arrays solder joint after thermomechanical fatigue (TMF) were characterized quantitatively using electron backscattered diffraction. Due to subgrain rotation, the small recrystallized grains evolved from the reflowed orientations appeared in a Pb-free solder joint subjected to thermomechanical stress. Also, these recrystallized grains rotated under electron current stress, indicating that the tin orientations of the Pb-free solder joints can significantly affect the response of the solder joints to service conditions such as TMF and electromigration. Meanwhile, two types of double twinning of tin in solder joints were observed in Pb-free solder joints. The change in orientation between the two groups of double twined orientations was in the range of 80°–90° (79.1°, 82.9° and 86.5° corresponding with 57.2° and 62.8°). Four orientations of tin grains, having the same twin grain with a [100] or [010] direction were observed in one of the systems, while the other one presented with two groups of tricrystals perpendicular to each other.     

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.     

Fatigue crack initiation and growth in solder alloys

In modern electronic packaging, especially surface mount technology (SMT), thermal strain is usually induced between components during processing, and in service, by a mismatch in the thermal expansion coefficients. Since solder has a low melting temperature and is softer than other components in electronic packaging, most of the cyclic stresses and strains take place in the solder. Fatigue crack initiation and fatigue crack propagation are likely to occur in the solder even when the cyclic stress is below the yield stress. It is an objective of this research to study the behaviour of fatigue crack initiation and propagation in both lead‐containing solder (63Sn‐37Pb), and lead‐free solders (Sn‐3.5Ag). The effect of alloying (Cu and Bi addition), frequency, tensile hold time and temperature on low cycle fatigue (LCF) behaviour of the solders is discussed. Mechanisms of LCF crack initiation and propagation are proposed and LCF life prediction, based on the various models, is carried out.