Fatigue crack growth behavior in 63Sn-37Pb and 95Pb-5Sn solder materials

Fatigue crack growth behavior of two typical solders 63Sn-37Pb and 95Pb-5Sn has been investigated under room temperature (20°C) and frequency of 10 Hz. Fatigue crack growth behavior of the 63Sn-37Pb specimen tested at stress ratios up to 0.5 and the 95Pb-5Sn specimen tested at stress ratios up to 0.7 were dominantly cyclic dependent. Intergranular crack growth was observed for the 63Sn-37Pb specimen tested at stress ratio of 0.7, which indicates that a crack grows under influence of creep. The difference of effect of stress ratio on crack growth behavior between two solders may results from the difference of homologous temperature. Fatigue crack growth mechanisms for solder materials are summarized as follows: A crack propagates in a transgranular manner at low homologous temperature and low stress ratio, while it propagates along eutectic grain boundaries at high homologous temperature and high stress ratio under influence of creep. At the intermediate homologous temperatures, a crack propagates in a transgranular manner with some region of intergranular fracture.     

Evaluation of the mechanical properties of a ternary Sn-20In-2.8Ag solder

The Sn-20In-2.8Ag solder alloy is a potential lead-free solder for replacing the traditional Sn-Pb solders. In this study, the mechanical properties of the bulk material are reported by tensile test at various strain rates and temperatures. The Sn-20In-2.8Ag solder possessed a solidus and liquidus between 170.8°C and 195.5°C. The ultimate tensile strength (UTS) and elongation were 59.3 MPa and 50.2% at a strain rate of 10⁻³ s⁻¹ at room temperature. Moreover, the UTS of this alloy decreased, but its elongation increased, with increasing testing temperature. Stress exponents of Sn-20In-2.8Ag alloy varied from 6.5 at room temperature to 4 at 100°C, and the activation energy for creep was 51.0 kJ/mol at the higher temperature range from 50°C to 100°C. The typical intergranular creep fracture mode was observed in Sn-20In-2.8Ag solder during tensile creep deformation.     

Fatigue crack-growth behavior of Sn-Ag-Cu and Sn-Ag-Cu-Bi lead-free solders

Fatigue crack-growth behavior and mechanical properties of Sn-3Ag-0.5Cu, Sn-3Ag-0.5Cu-1Bi, and Sn-3Ag-0.5Cu-3Bi solders have been investigated at room temperature (20°C). The tensile strength and hardness of the solders increased with increasing Bi content. However, the yield strengths of Sn-3Ag-0.5Cu-1Bi and Sn-3Ag-0.5Cu-3Bi solders were nearly similar, but the 3Bi solder exhibited the lowest ductility. Fatigue crack-growth behavior of the solders was dominantly cycle dependent in the range of stress ratios from 0.1–0.7 at a frequency of 10 Hz, except for the Sn-3Ag-0.5Cu solder tested at a stress ratio of 0.7. Mixed intergranular/transgranular crack propagation was observed for the Sn-3Ag-0.5Cu solder tested at the stress ratio of 0.7, indicating the importance of creep in crack growth. The Sn-3Ag-0.5Cu-1Bi and Sn-3Ag-0.5Cu-3Bi solders had higher resistance to time-dependent crack growth, resulting from the strengthening effect of the Bi constituent. It appears that the addition of Bi above a certain concentration is harmful to the mechanical properties of Sn-3Ag-0.5Cu.     

Microstructure and Creep Deformation of Sn-Ag-Cu-Bi/Cu Solder Joints

Sn-Ag-Cu solder is one of the candidate alternatives to Sn-Pb-based solders. In order to improve its performance, different materials have been added to Sn-Ag-Cu-based solders. Several studies on Sn-Ag-Cu-based solders with Bi additions have shown Sn-Ag-Cu-Bi to be a class of solders with good wetting behavior and good performance that show great promise for use in the electronics assembly and packaging industry. To investigate the mechanical reliability of the Sn-Ag-Cu-Bi solders further, single-lap shear creep characteristics have been studied in this work. Dog-bone-type solder joint specimens were formed using five types of solder alloys, Sn-3.0Ag-0.5Cu and Sn-3.0 Ag-0.5Cu-xBi (x = 1 wt.% to 4 wt.%) with Cu substrates, and creep tests were performed at temperatures of 120°C and 150°C under stresses of 5 MPa to 10 MPa. Results indicate that the rupture times for Sn-3.0Ag-0.5Cu-xBi solder joints up to 4 wt.% of Bi are longer than the rupture time for Sn-3.0Ag-0.5Cu. Stress exponents ranged from 3 to 7 for temperatures of 150°C and 120°C with stresses under 10 MPa. Microstructural analyses using scanning electron microscopy (SEM) were performed and related to the creep behavior of the solder joints.     

The short and long term properties of a liquid crystalline polymer at elevated temperatures: Characterization and modeling

Tensile and short term (24 h) creep tests were performed on Xydar G930, a liquid crystalline polymer (LCP) with 30 wt.% glass filler, at temperatures and stress levels ranging from room temperature to 175°C and 0.3 fraction ultimate tensile strength (UTS) to 0.8 fraction UTS, respectively. Temperature was found to have an affect on the short term tensile properties. The resulting strain vs time creep curves showed the expected dependence of creep strain on temperature and stress level. Creep compliance curves were derived from the creep curves and showed distinctively nonlinear viscoelastic behavior at all stress levels and temperatures. Creep compliance was found to follow a power law in time. The power law was used to model the stress dependence of creep and the Arrhenius equation was employed to model the temperature dependence up to 120°C. A significant reduction in creep resistance was observed at 175°C. Time-temperature-stress-superposition was used to show that the material followed power law behavior up to 1000 h.     

Creep Properties of Sn-1.0Ag-0.5Cu Lead-Free Solder with Ni Addition

In this work, tensile creep tests for Sn-1.0Ag-0.5Cu-0.02Ni solder have been conducted at various temperatures and stress levels to determine its creep properties. The effects of stress level and temperature on creep strain rate were investigated. Creep constitutive models (such as the simple power-law model, hyperbolic sine model, double power-law model, and exponential model) have been reviewed, and the material constants of each model have been determined based on experimental results. The stress exponent and creep activation energy have been studied and compared with other researchers’ results. These four creep constitutive models established in this paper were then implemented into a user-defined subroutine in the ANSYS™ finite-element analysis software to investigate the creep behavior of Sn-1.0Ag-0.5Cu-0.02Ni solder joints of thin fine-pitch ball grid array (TFBGA) packages for the purpose of model comparison and application. Similar simulation results of creep strain and creep strain energy density were achieved when using the different creep constitutive models, indicating that the creep models are consistent and accurate.     

Temperature Dependence of Creep and Hardness of Sn-Ag-Cu Lead-Free Solder

The creep behavior and hardness of Sn-3.5Ag-0.7Cu solder were studied using Berkovich depth-sensing indentation at temperatures of 25°C to 125°C. Assuming a power-law relationship between the creep strain rate and stress, an activation energy of 40 kJ/mol and stress exponents of 7.4, 5.5, and 3.7 at 25°C, 75°C, and 125°C, respectively, were obtained. The results revealed that, with increasing temperature, the creep penetration and steady-state creep strain rate increased whereas the stress exponent decreased. The stress exponent and activation energy results also suggested that the creep mechanism is dislocation climb, assisted by diffusion through dislocation cores in Sn. Furthermore, the hardness results exhibited a decreasing trend with increasing temperature, which is attributed to softening at high temperature.     

Characterization of eutectic Sn-Bi solder joints

This report presents experimental results on 58Bi-42Sn solder joints, optical and SEM microstructures of their matrix and of their interface with copper, solidification behavior studied by differential scanning calorimetry, wettability to copper, creep, and low cycle fatigue. These results are discussed in comparison with 60Sn-40Pb solder, and with three low temperature solders, 52In-48Sn, 43Sn-43Pb-14Bi, and 40In-40Sn-20Pb. The 58Bi-42Sn solder paste with RMA flux wets Cu matrix with a wetting angle of 35° and had a 15° C undercooling during solidification. The constitutive equation of the steady state shear strain rate, and the Coffin-Manson relation constants for the low cycle shear fatigue life at 65° C have been determined. The test results show that this solder has the best creep resistance but the poorest fatigue strength compared with the other four solders.     

Effects of prestrain,rate of prestrain,and temperature on the stress-relaxation behavior of eutectic Sn-3.5Ag solder joints

Stress-relaxation studies on eutectic Sn-Ag solder (Sn-3.5Ag in wt.%) joints were carried out at various temperatures after imposing different amounts and rates of simple shear strain. Stress-relaxation parameters were evaluated by subjecting geometrically realistic solder joints with a nominal joint thickness of ∼100 μm and a 1 mm × 1 mm solder-joint area. The peak shear stress during preloading and residual shear stress resulting from stress relaxation were higher at the low-temperature extremes than those at high-temperature extremes. Also, those values increased with increasing simple shear strain and the rate of simple shear strain imposed prior to the stress-relaxation events. The relaxation stress is insensitive to simple shear strain at 150°C, but at lower temperatures, a faster rate of simple shear strain causes a higher relaxed-stress value. The resulting deformation structures observed from the solder-joint side surfaces were also strongly affected by these parameters. At high temperature, grain-boundary sliding effects were commonly observed. At low temperature, intense shear bands dominated, and no grain-boundary sliding effects were observed.     

Superplastic creep of low melting point solder joints

In prior work, we showed that eutectic Sn-Pb solder joints exhibit superplastic behavior after rapid solidification. Further examples of superplasticity in nominally air-cooled solder joints are reported in this study of three low-melting point alloys: 40In-40Sn-20Pb (wt. %), eutectic 52In-48Sn, and 43Sn-43Pb-14Bi, which were creep-tested in shear at 20°, 65°, and 90° C. The test results indicate that above 65° C, the indium-containing solders have stress exponents between 2.4 to 2.9, a possible overall shear strains of 500%, and an absence of primary creep; at 90° C, 43Sn-43Pb-14Bi solder has a stress exponent close to 2.3. Optical microstructures of the three solders are presented; they help to explain the superplastic behavior.     

The creep of lead-free solders at elevated temperatures

Full implementation of the new generation of lead-free solders requires a detailed knowledge and understanding of their mechanical behavior. This paper reports an investigation of the creep behavior of three lead-free alloys: Sn-0.5 Cu, Sn-3.5 Ag, and Sn-3.8Ag-0.7Cu, at 75°C, and compares their response to that of Sn-37Pb at the same temperature. In terms of stress and time to rupture, the Sn-0.5Cu alloy behaves similarly to the eutectic Sn-Pb over the range of rupture lives considered (up to 1000 h). The silver-containing alloys exhibit much greater creep resistance, typically a hundred fold and a thousand fold for the binary and tenanary, respectively. These alloys are less ductile but their creep strains to failure are generally above ten percent. Their minimum creep rates are at least 100 times slower. When testing at the same homologous temperature (0.76), the silver-containing alloys retain the substantial superiority. The relationship between applied steady-state (or minimum) creep rate behavior is best described by a power law equation, although the steady state domain generally occupies less than 30 percent of life. The microstructural changes induced by creep are briefly described and used to explain some of the creep characteristics of the alloys.     

Effect of TiO<Subscript>2</Subscript> Nanoparticles Addition on the Thermal,Microstructural and Room-Temperature Creep Behavior of Sn-Zn Based Solder

The effect of adding TiO₂ nanoparticles as well as aging time on the thermal, microstructural and creep properties of Sn-6.5Zn solder was studied. The Sn-6.5Zn composite solders were prepared by mechanically dispersing different weight percentages (0.0 wt.%, 0.25 wt.%, 0.50 wt.%, 0.75 wt.% and 1.0 wt.%) of TiO₂ nanoparticles into Sn-6.5Zn solder. After being solution heat treated at 453 K for 4 h, specimens were cooled by water quenching at 273 K. Specimens were artificially aged at 393 K for durations ranging from 15 to 120 min, followed by water quenching at 273 K to cease further aging. The thermal behavior of the composite solders was investigated using differential scanning calorimetry (DSC). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to observe the microstructure of the solders. The mechanical properties were characterized using tensile creep tests and correlated with microstructural features. The investigation revealed that the minimum creep rate of solders decreased with the increase in the content of TiO₂, while it increased with increasing aging time. The data from microstructure-properties analysis showed that the nano-TiO₂ particles had significantly refined the microstructure and improved the creep resistance in comparison with the Sn-Zn solder. The calculated stress exponent values were close to 7.     

Predicting tensile properties of the bulk 96.5Sn-3.5Ag lead-free solder

Equations are presented for predicting tensile properties as functions of temperature and strain rate for the bulk-eutectic 96.5Sn-3.5Ag lead-free solder. At 25°C, we obtained 49.0 GPa for Young’s modulus based on acoustic measurements, which is higher than most of those measured by tensile tests that are subject to viscoelastic creep; 23.1 MPa and 26.3 MPa for yield stress and ultimate tensile strength (UTS) of specimens that are cast, annealed, and aged at a strain rate of 2.0×10⁻⁴ s⁻¹; 48.7% for total elongation, which is larger than most of the reported values. The presence of “initial defects” in the specimens, such as porosity and void, might cause the reduction in measured total elongations. Contribution of NIST, an agency of the U.S. government, not subject to copyright in the United States.     

Effect of Strain Rate and Temperature on the Tensile Properties of Tin-Based Lead-Free Solder Alloys

The tensile properties of Sn-3Ag-0.5Cu, Sn-3.5Ag, and Sn-0.7Cu lead-free solders were investigated on small-scale specimens and compared with those of Sn-37Pb eutectic solder at various strain rates from 1 × 10⁻⁴ s⁻¹ to 1 × 10⁻² s⁻¹ and over a wide temperature range from 25°C to 150°C. The tests were under true strain-rate-controlled conditions. The ductility of each lead-free solder is relatively constant while that for Sn-Pb eutectic solder strongly depends on strain rate and temperature. The strain rate sensitivity index m for lead-free solders is relatively stable and showed little dependence on temperature, whereas the values of m for Sn-37Pb increased linearly with increasing temperature.     

Stress relaxation behavior of eutectic tin-lead solder

The creep behavior of eutectic tin-lead solder was investigated using stress relaxation techniques. Stress relaxation experiments were performed on cast tensile specimens of commercial eutectic tin-lead solder, SN63. The sample casting conditions were controlled to produce microstructures similar to those found in typical solder joints on electronic assemblies. The stress relaxation data was analyzed to extract constitutive relations for creep. The strain rate during relaxation was found to follow two power law expressions, one with n = 3.2 at low stress levels and the other with n = 6.2 at higher stress levels. The apparent activation energy for creep and the power law exponent are discussed with relation to the published data for this alloy.     

Creep properties of Sn-8Mass%Zn-3Mass%Bi lead-free alloy

The creep properties of Sn-8mass%Zn-3mass%Bi were investigated and compared with those of other lead-free solders and a Sn-Pb eutectic solder. The creep rupture time decreases with increasing the initial stress and the temperature. For high stresses, the linear relationships between the minimum strain rate and the applied stress were observed. The stress exponents were examined to be 9.3, 7, and 4 at 298 K, 348 K, and 398 K, respectively. For low stresses, the minimum creep rate becomes relatively fast, and the creep resistance diminishes. In this range, secondary creep reduces, and the tertiary creep predominates. The creep properties of Sn-8Zn-3Bi are similar to those of Sn-Ag alloys at high stresses and those of Sn-0.5Cu at low stresses.     

Microstructure Evolution and the Constitutive Relations of High-Temperature Solders

This study characterized the temperature-dependent constitutive parameters (yield strength, ultimate tensile strength, elastic modulus, strain hardening exponent) from the mechanical behavior of five high-temperature solders, 95Sn-5Sb, 95Pb-5In, 90Pb-10Sn, 92.5Pb-5Sn-2.5Ag, and 93Pb-3Sn-2In-2Ag, chosen such that T _m > 518 K. To model appropriately their mechanical responses under high-temperature thermal cycling, where the temperatures exceed 473 K, the material’s parameters must be determined as a function of temperature. Uni-axial tensile tests were, therefore, carried out between 298 K and 473 K to determine the constitutive behavior of each solder. 95Sn-5Sb exhibited the highest strength over the temperature range tested except near 473 K. Pb-based alloys with a higher degree of solid solution (>5%) showed greater strengthening than those primarily strengthened by coarse precipitates. Additionally, microstructure changes in 90Pb-10Sn and 95Sn-5Sb were shown to be responsible for unexpected mechanical behavior at elevated temperatures.     

High strain rate compression behavior for Sn-37Pb eutectic alloy, lead-free Sn-1Ag-0.5Cu and Sn-3Ag-0.5Cu alloys

This paper emphasized on studying the high strain-rate compression behavior of the unleaded Sn-3Ag-0.5Cu (SAC305), Sn-1Ag-0.5Cu (SAC105) solders and the traditional Sn-37Pb eutectic solder. The split Hopkinson pressure bar (SHPB) apparatus was used to conduct high strain rate tests in order to characterize the associated high rate mechanical response of these alloys. Specimens were tested at strain rates ranging from 380 to 3030 s⁻¹ to obtain the dynamic stress-strain relationship for the Sn-37Pb, SAC305 and SAC105 alloys. The tested soft and ductile samples experienced a large amount of elastoplastic deformation due to impact test. In the high strain rate range studied, limited strain rate hardening effect was observed for SAC305, SAC105 and Sn-37Pb alloys studied. The strain rate sensitivity parameter (m) related to the power law creep equation was also calculated for the present solder materials at specific strain values. In addition, the saturation stresses for the leaded and lead-free solders at the strain rate range studied are also reported.     

Modeling thermomechanical fatigue behavior of Sn-Ag solder joints

Stresses that develop because of the coefficient of thermal expansion (CTE) mismatch between solder and substrate/components contribute to thermomechanical fatigue (TMF) of the solder joints. However, the relative importance of several processes that contribute to damage accumulation and its role in affecting the reliability of the solder joints are far from being understood. Aging, creep/stress relaxation, and stress/strain reversals are some of the important processes. These processes are affected by service conditions, such as the temperature extremes experienced, rates of heating and cooling, dwell times at the extreme temperatures, and so on. In addition, the elastic and plastic anisotropy of tin could also contribute to the damage accumulation during TMF of Sn-based solders. This preliminary effort to model TMF in Sn-Ag solder joints will consider the role of each of these parameters, with significant emphasis on the anisotropic-elastic behavior of Sn grains.