Creep behavior in Cu and Ag particle-reinforced composite and eutectic Sn-3.5Ag and Sn-4.0Ag-0.5Cu non-composite solder joints

Creep properties were determined for small, geometrically realistic Pb-free solder joints. Solder joints were prepared with eutectic Sn-3.5Ag and Sn-4.0Ag-0.5Cu solder alloys. Composite solder joints were made using the eutectic Sn-3.5Ag alloy as the matrix with 15 vol % of mechanically added 6 m size Cu and 4 m size Ag reinforcing particles. Creep tests were conducted on these joints at 25 °C, 65 °C and 105 °C representing homologous temperatures ranging from 0.61 to 0.78. Qualitative and quantitative evaluations of creep behavior were obtained from the distortion of excimer laser-induced surface ablation markings on the solder joint. Various creep parameters, such as global and localized creep strain, variation of creep strain and strain-rate, activation energy for creep, and the onset of tertiary creep were determined. General findings in this study revealed that the creep resistance in composite solder joints is significantly improved with Cu particle reinforcements. In contrast, the improvement in the creep properties of Ag particle-reinforced composite solder joints was far less even though highly uniform deformation in the joint was observed. The strain noted at the onset of tertiary creep for Cu and Ag reinforced composite solder joints was typically lower compared to non-composite solder joints. The activation energies for creep were similar for all the solder materials investigated in this study. © 2001 Kluwer Academic Publishers     

Some mechanical properties of Sn–3.5 Ag eutectic alloy at different temperatures

The eutectic alloy Sn–3.5 wt % Ag has been examined as one of the lead-free solder alloys. Microhardness tests as a function of temperature were performed to calculate the effective activation energy of the solder alloy Sn–Ag and compared with the pure elements Sn and Ag. Various creep parameters such as, exponent n_tr and the parameter in the transient creep stage and the values of the stress exponent n from the steady-state stage were calculated under different constant applied stresses at different working temperatures. The structure changes of the alloy were reported before and after creep test.     

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.     

Reliability of lead-free solder joints in CSP device under thermal cycling

Finite element method and Garofalo–Arrheninus creep model were combined and used to evaluate the reliability of different lead-free solder joints (SnAgCu, SnAg, SnSb and SnZn) and SnPb solder joints in chip scale package (CSP) 14 × 14 device under thermal cyclic loading. The results show that von Mises stress and equivalent creep strain in each of the four lead-free solder joints and SnPb solder joints were strongly different, increasing in the order SnPb < SnAg < SnSb < SnZn < SnAgCu. It is found that maximum stress–strain concentrates on the top-surface of corner solder joints in the CSP device for all solder joints, and SnAgCu solder joints shows the highest fatigue life among those five kinds of solder joints.     

Surface damage accumulation in Sn–Ag solder joints under large reversed strains

During service, solder joints may encounter repeated reversed stress states. Since realistic solder joint specimen geometry employed for creep/stress relaxation studies does not facilitate stress reversal, shear test specimens consisting of 3/8×3/8×1 copper blocks joined with eutectic Sn–Ag solder of realistic thickness of about 400 m were used for this study. The mechanical behavior and microstructural features of the solder joint subjected to repeated reversed straining were investigated by imposing large shear strains. Damage accumulation on the surface of the solder joints due to a few such large shear strain reversals is very similar to that observed on the surface of thermomechanically fatigued joints made with the same solder.     

Evolution of thermal property and creep resistance of Ni and Zn-doped Sn–2.0Ag–0.5Cu lead-free solders

The effect of small amount of Ni and Zn additions on thermal behavior and creep properties of Sn–2Ag–0.5Cu (SAC 205) lead free solder alloy was investigated. Additions of Ni and Zn, respectively, into SAC (205) solder caused its undercooling temperature to decrease significantly from 26.1 °C to 18.0 and 1.4 °C. The results show that the formation of new (Cu,Ni)₆Sn₅ and Cu₅Zn₈ hard particles in the β-Sn matrix are drastically strengthen the SAC (205) solder. The lead-free SAC (205) – 0.5Zn solder joints showed superior creep resistance in terms of much lower creep rate and elongated creep fracture lifetime over the traditional SAC (205) and SAC (205) – 0.05Ni solders. However, the ductility of SAC (205) – 0.5Zn alloy is lower than that of the other two alloys. This difference was attributed to the lower precipitate coarsening and higher precipitate density of Cu₅Zn₈ and Ag₃Sn IMCs in Zn-doped SAC (205) cast alloy, which could provide more obstacles for dislocation pile up in the adjacent β-Sn grains. Nevertheless, the strengthening effect depends on the undercooling of the solder.     

Tensile creep behavior of Sn–Ag–Cu–Ni multicomponent lead-free solder alloy

In the process of electronic packaging, the dissolution of under bump metallizations, such as Cu and Ni, into liquid solder occurs during soldering, which can change the original solder to a multicomponent one. Under the trend of miniaturization, it is quite necessary to evaluate the properties of multicomponent solder with excessive Cu and Ni compositions. In this study, the tensile creep behavior of Sn–3.5Ag–2.0Cu–0.5Ni multicomponent lead-free solder alloy is investigated at three temperatures, i.e., 303, 348 and 393 K. The steady-rate creep rates are obtained in the range of 10^?4–10^?8 s^?1, when the normalized stress, σ/E, is in the range of 10^?4–10^?3. Based on the Dorn equation, the apparent stress exponent (n _a), threshold stress (σ _th), and activation energy of creep (Q _C) are calculated at the three temperatures. It is found that the Sn–3.5Ag–2.0Cu–0.5Ni solder alloy shows a better creep performance than pure tin and eutectic Sn–3.5Ag solder due to the strengthening effect of Ag₃Sn and (Cu,Ni)₆Sn₅ IMC precipitations. The true stress exponent for creep is identified to be 7, indicating that the creep behave is controlled by the dislocation-pipe diffusion in the tin matrix.     

The effect of joint size on the creep properties of microscale lead-free solder joints at elevated temperatures

Solder joints in electronic packaging systems are becoming smaller and smaller to meet the miniaturization requirements of electronic products and high density interconnect technology. Furthermore, many properties of the real solder joints at the microscale level are obviously different from that of bulk solder materials. Creep, as one of the key mechanical properties at elevated temperatures, can impair the reliability of miniature solder joints in electronic devices. However, there is a lack of knowledge about the comparative creep properties of microscale solder joints of different sizes. Most previous studies have focused on the creep properties of bulk solder materials or solder joints of the same size. In this research, to determine whether a size effect exists for creep properties of solder joints or not, we characterized the creep behaviors of Sn–3.0Ag–0.5Cu lead-free solder joints under tensile loading modes using microscale butt-joint specimens with a copper-wire/solder/copper-wire sandwich structure with two different sizes. Also, the creep failure mechanisms were investigated. Experimental results show that the creep activation energy and creep stress exponent are very similar for both sizes of solder joint. However, under the same testing conditions, the joints with a larger size exhibit a much higher steady-state creep rate and a shorter creep lifetime than the smaller joints.     

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.     

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

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

Creep rupture of lead-free Sn-3.5Ag and Sn-3.5Ag-0.5Cu solders

The aim of this study is to investigate the creep rupture behavior of lead-free Sn-3.5Ag and Sn-3.5Ag-0.5Cu solders at three temperatures ranging from room temperature (RT) to 90 °C, under a tensile stress range of σ/E=10⁻⁴ to 10⁻³. The ultimate tensile strength (UTS) and creep resistance were found to be decreased with increasing temperature for each given lead-free solder. Both the binary and ternary Ag-containing alloys exhibited superior UTS and creep strength to the conventional Sn-37Pb solder at a similar temperature. Due to a more uniform distribution of eutectic phases and a larger volume fraction of intermetallic compounds (IMCs), the Sn-3.5Ag-0.5Cu alloy had greater UTS and creep strength than did the eutectic Sn-3.5Ag solder at each testing temperature. The stress exponents (n) of minimum strain rate (˙ε_min) were decreased from 7 and 9 at RT to 5 and 6 at 60 and 90 °C, for the binary and ternary lead-free alloys, respectively. Fractography analyses revealed typical rupture by the nucleation and growth of voids/microcracks at IMCs on the grain boundaries. Both Monkman-Grant and Larson-Miller relationships showed good results in estimating the rupture times under various combinations of applied stress and temperature. A model, using a term of applied stress normalized by Young’s modulus, was proposed to correlate the rupture times at various temperatures and could explain the rupture time data reasonably well for the given two lead-free solders.     

The mechanical properties and microstructures of copper and brass joints soldered with eutectic tin-bismuth solder

The mechanical properties and microstructures ofcopper and brass soldered with eutectic tin-bismuth solder have been determined and the joints examined using metallographic techniques. Joints made with copper were stronger than those made with brass. At the copper/solder interface a uniform layer 2m thick of Cu_5.2Sn₅ was formed and at the brass/solder interface a uniform layer 2 m thick of (Cu, Zn)_2.9Sn and an irregular layer 2 to 5m thick of (Cu, Zn)_5.7Sn₅ were formed. Copper joints fractured etthocopper/solder interface and brass joints fractured in the internmetalic layer. Copper joints soldered with eutectic Sn-Bi were stronger than copper joints soldered with eutectic Sn-Pb and the reverse was true for brass joints. Results are also given for the effect of thermal shock on copper and brass joints soldered with Sn-Bi and Sn-Pb solders, and also for We fatigue and creep behaviour of joints soldered with eutectic Sn-Bi solder.     

Sn-9Zn-xP无铅焊料合金性能研究

以Sn-9Zn合金为研究对象,考察了P的添加对其性能的影响,并对其机制作了初步探讨。通过二次离子质谱(SIMS)分析发现,P的添加显著降低Sn-9Zn合金中的含氧量,从而提高了合金在Cu上的润湿性。P的改善润湿性的效果不仅有利于其工艺性能,也提高了Sn-9Zn/Cu焊点界面的结合强度,只使其塑性略有下降。同时,微量P的添加不改变Sn-9Zn合金与Cu形成的焊点的界面结构。另外,蠕变强度测试结果表明,P的添加能显著提高合金的抗蠕变性能。     

Steady state creep and creep recovery behaviours of pre-aging Al–Si alloys

The creep and creep recovery of pre-aging Al–1 wt.%Si and Al–1 wt.%Si–0.1 wt.%Zr–0.1 wt.%Ti alloys have been investigated at room temperature under different constant stresses. The aging temperature dependence of steady creep rate, _st, and the recovery strain rate, π, show that under the same test conditions first alloy yields creep or creep recovery rates much higher as compared with those of second alloy. The stress exponent n was found to change from 2.5 to 7.43 and 4.57 to 11.99 for two alloys, respectively, characterizing dislocation slipping mechanism. The activation energies of steady state creep of the two alloys were found to be 78.4 kJ/mol and 32.8 kJ/mol for Al–Si and Al–Si–Zr–Ti alloys, respectively. The microstructure of the samples studied was investigated by optical and transmission electron microscopy (TEM).     

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

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

Simulated thermal cycling of surface components on FR4 printed wiring boards using standard tin–lead solder

Thermal cycling is used as a diagnostic test for electronic equipment. It is particularly severe on solder joints that are under both compressive and tensile strain during a cycle as a result of differential thermal expansion. The electronics industry document IPC-SM-785 addresses this problem and provides means whereby joint failure can be predicted as a function of the type and number of cycles used. The method can be applied to both non-compliant ceramic chips and compliant packages on FR4 board. This paper describes the applicability of SM-785 for both compliant and non-compliant packages. Due to the change in metallurgy of solder at temperatures approaching –20 °C, the approach is restricted to temperatures where near eutectic tin–lead solder is in its non-elastic form. A new approach will be needed when lead-free joining becomes mandatory.     

Creep property of composite solders reinforced by nano-sized particles

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

Intermetallic compound layer formation between Sn–3.5 mass %Ag BGA solder ball and (Cu, immersion Au/electroless Ni–P/Cu) substrate

The growth kinetics of intermetallic compound layers formed between eutectic Sn–3.5Ag BGA (ball grid array) solder and (Cu, immersion Au/electroless Ni–P/Cu) substrate by solid-state isothermal aging were examined at temperatures between 343 and 443 K for 0–100 days. In the solder joints between the Sn–Ag eutectic solder ball and Cu pads, the intermetallic compound layer was composed of two phases: Cu₆Sn₅ (-phase) adjacent to the solder and Cu₃Sn (-phase) adjacent to the copper. The layer of intermetallic on the immersion Au/electroless Ni–P/Cu substrate was composed of Ni₃Sn₄. As a whole, because the values of the time exponent (n) are approximately 0.5, the layer growth of the intermetallic compound was mainly controlled by a diffusion-controlled mechanism over the temperature range studied. The growth rate of Ni₃Sn₄ intermetallic compound was slower than that of the total Cu–Sn(Cu₆Sn₅+Cn₃Sn). The apparent activation energy for growth of total Cu–Sn(Cu₆Sn₅+Cu₃Sn) and Ni₃Sn₄ intermetallic compound were 64.82 and 72.54 kJ mol^–1, respectively.     

On the creep rupture life prediction

Combined effects of stress, A, and fracture cavitation on the creep rupture life, tR, have been studied in conventionally cast MAR-M 002 alloy tested at 1173 K (900 °C) over a limited range of stress (A = 200–400 MPa). It is predicted that the creep fracture cavity growth is controlled by the coupled power-law creep with the grain-boundary diffusion mechanism. On the basis of this prediction the Edward–Ashby model overestimates the creep rupture life although this model correctly describes the trend in the data. The observation of a linearity between the cavity density, NA, and the product RtR4A indicates that this relationship can be used to predict the creep time, tR, where R is the rupture strain. Furthermore, another empirical method is the creep-fracture parameter, Kf = f(ac)1/2, approach, developed using the modified Griffith–Irwin type of relationship, which can also be used to predict the creep rupture life in the present alloy, where f is the creep fracture stress (or the applied stress, A) and ac the crack (or cavity) size.     

Influence of rhenium on the microstructures and mechanical properties of a mechanically alloyed oxide dispersion-strengthened nickel-base superalloy

The influence of a 3 wt% Re addition on the creep strength and microstructure of a mechanically alloyed and oxide dispersion-strengthened nickel-base superalloy was investigated. Two alloys, Ni–8Cr–6.5Al–6W–3Ta–1.5Mo–6Co–1Ti–3Re–0.15Zr–0.05C–0.01B–0.9Y₂O₃ (3Re alloy) and a non-rhenium containing (0Re) alloy were prepared for this study.The 3Re alloy showed two-fold improvement in creep life compared with that of 0Re alloy, presumably due to a change in the mode of the precipitate-dislocation interaction. For the 3Re alloy, finer, more cuboidal and aligned precipitates are formed, which force the mobile dislocations at the – interfaces to cut precipitates in order to proceed. Shearing of precipitates is evinced by the existence of stacking faults and results in an increase of creep strength. In constrast, lower creep strength was observed for 0Re alloy because a dislocation looping mode is dominant with coarser and more irregularly shaped precipitates present in this alloy. Another possible explanation for an improved creep strength of 3Re alloy is related to the tangled dislocation structure formed by the interaction between glide dislocation and interfacial dislocation, which also acts as an effective barrier for further glide dislocation motion. A 3 wt% Re addition significantly retards coarsening kinetics. Rhenium acts as a rate-controlling species upon the volume diffusion-controlled coarsening process because it is a heavy elemenet and also it almost solely partitions to the matrix. X-ray diffraction experiments showed that the magnitude of the lattice mismatch between and increased with the 3 wt% Re addition from 0% to –0.26% at room temperature. Increased lattice mismatch for 3Re alloy causes the formation of more aligned and cuboidal precipitates rather than random and odd-shaped precipitates for 0Re alloy, and it also accelerates the coalescence between cuboidal precipitates.