Effect of Homogenization on the Indentation Creep of Cast Lead-Free Sn-5%Sb Solder Alloy

Creep behavior of cast lead-free Sn-5%Sb solder in unhomogenized and homogenized conditions was investigated by long time Vickers indentation testing under a constant load of 15 N and at temperatures in the range 321–405 K. Based on the steady-state power law creep relationship, the stress exponents were found for both conditions of the material. The creep behavior in the unhomogenized condition can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring around 11.7 × 10⁻⁴ < (H _V /E) < 18 × 10⁻⁴. The low stress regime activation energy of 54.2 kJ mol⁻¹, which is close to 61.2 kJ mol⁻¹ for dislocation pipe diffusion in the Sn, and stress exponents in the range 5.0–3.5 suggest that the operative creep mechanism is dislocation viscous glide. This behavior is in contrast to the high stress regime in which the average values of n = 11.5 and Q = 112.1 kJ mol⁻¹ imply that dislocation creep is the dominant deformation mechanism. Homogenization of the cast material resulted in a rather coarse recrystallized microstructure with stress exponents in the range 12.5–5.7 and activation energy of 64.0 kJ mol⁻¹ over the whole ranges of temperature and stress studied, which are indicative of a dislocation creep mechanism.     

Indentation Creep of Lead-Free Sn-5Sb Solder Alloy with 1.5 wt% Ag and Bi Additions

Creep behavior of the ternary Sn-5Sb-1.5Ag and Sn-5Sb-1.5Bi alloys was studied by indentation testing at 298 and 370 K, and compared to that of the binary Sn-5Sb base alloy. Among all tested materials, as indicated by their minimum creep rates, Sn-5Sb-1.5Bi showed the highest creep resistance followed by Sn-5Sb-1.5Ag and Sn-5Sb. The superiority of the Bi-containing alloy is mainly due to the microstructural refinement and solid solution hardening effects of Bi in the Sn matrix, while the improvement in the creep resistance of the Ag-containing alloy is caused by the formation of spherical and rod-shaped Ag₃Sn particles. The stress exponent values in the range 11.0–12.2 are close to those determined by tensile and impression creep testing of the same alloys reported in the literature. These high stress exponents together with the activation energies of 58–70 kJ/mol may suggest that the dominant creep mechanism is dislocation creep.     

Impression Creep Behavior of Zn-Sn High-Temperature Lead-Free Solders

This study examines the microstructure and impression creep behavior of the high-temperature Zn-20 wt.%Sn, Zn-30 wt.%Sn, and Zn-40 wt.%Sn solders under constant punch stress in the range of 25 MPa to 300 MPa and at temperatures in the range of 298 K to 425 K. Analysis of the data showed that, for all loads and temperatures, the Zn-20Sn alloy had the lowest creep rates, and thus the highest creep resistance, among all materials tested. This is attributed to the lower volume fraction of the soft Sn-rich phase with a continuous morphology which acts as the matrix encompassing the harder Zn phase. The stress exponents and activation energies were in the range of 4.0 to 6.1 and 40.0 kJ mol⁻¹ to 45.3 kJ mol⁻¹, respectively. Based on the obtained stress exponents and activation energy data, it is proposed that dislocation climb is the controlling creep mechanism. However, the observed decreasing trend of creep activation energy with stress suggests that two parallel mechanisms of lattice-diffusion-controlled and pipe-diffusion-controlled dislocation climb are competing. Dislocation climb controlled by dislocation pipe diffusion is the controlling mechanism at high stresses, whereas climb of edge dislocations is the controlling mechanism at low stresses.     

Impression Creep of a Lead-Free Sn-1.7Sb-1.5Ag Solder Reinforced by Submicron-Size Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Particles

In the present study, the Sn-1.7Sb-1.5Ag solder alloy and the same material reinforced with 5 vol.% of 0.3-μm Al₂O₃ particles were synthesized using the powder metallurgy route of blending, compaction, sintering, and extrusion. The impression creep behavior of both monolithic and composite solders was studied under a constant punching stress in the range of 20 MPa to 110 MPa, at temperatures in the range of 320 K to 430 K. The creep resistance of the composite solder was higher than that of the monolithic alloy at all applied stresses and temperatures, as indicated by their corresponding minimum creep rates. This was attributed to the dispersive distribution of the submicron-sized Al₂O₃ particles in the composite solder. Assuming a power-law relationship between the impression stress and velocity, average stress exponents of 5.3 to 5.6 and 5.8 to 5.9 were obtained for the monolithic and composite materials, respectively. Analysis of the data showed that, for all loads and temperatures, the activation energy for both materials was almost stress independent, with average values of 44.0 kJ mol⁻¹ and 41.6 kJ mol⁻¹ for the monolithic and composite solders, respectively. These activation energies are close to the value of 46 kJ mol⁻¹ for dislocation climb, assisted by vacancy diffusion through dislocation cores in the Sn. This, together with the stress exponents of about 5 to 5.9, suggests that the operative creep mechanism is dislocation viscous glide controlled by dislocation pipe diffusion.     

Effects of Ag and Al Additions on the Structure and Creep Properties of Sn-9Zn Solder Alloy

Creep behavior of the eutectic Sn-9Zn, Sn-9Zn-0.5Ag, and Sn-9Zn-0.5Al solder alloys was studied by impression testing under constant punching stress in the range of 60 MPa to 130 MPa and at temperatures in the range of 298 K to 370 K. Analysis of the data showed that, for all loads and temperatures, Sn-9Zn-0.5Al had the lowest creep rates and thus the highest creep resistance among all materials tested. The creep resistance of Sn-9Zn-0.5Ag was slightly lower than that of the Al-containing alloy. The enhanced creep behaviors of the ternary alloys are attributed to the presence of AgZn₃ and very fine Zn particles, which act as the main strengthening agents in the Sn-9Zn-0.5Ag and Sn-9Zn-0.5Al alloys, respectively. Assuming a power-law relationship between the impression rate and stress, average stress exponents of 6.9, 7.1, and 7.2 and activation energies of 42.1 kJ mol⁻¹, 42.9 kJ mol⁻¹, and 43.0 kJ mol⁻¹ were obtained for Sn-9Zn, Sn-9Zn-0.5Ag and Sn-9Zn-0.5Al, respectively. These activation energies are close to 46 kJ mol⁻¹ for dislocation climb, assisted by vacancy diffusion through dislocation cores in the Sn. This, together with the stress exponents of about 7, suggests that the operative creep mechanism is dislocation climb controlled by dislocation pipe diffusion.     

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.     

The correlation between stress relaxation and steady-state creep of eutectic Sn-Pb

This paper surveys and compares creep and stress relaxation data on finegrained eutectic Sn-Pb. It examines the consistency of the available data on this extensively studied solder material and studies whether stress relaxation offers a reasonable alternative to the more laborious conventional creep tests. The data survey reveals systematic differences between the creep behavior of material that is grain-refined by cold work and recrystallization (“recrystallized”) and that refined by rapid solidification (“quenched”). The recrystallized material has the conventional three regimes of creep behavior: a high-stress region with a stress exponent, n ∼ 4–7 and an activation energy Q ∼ 80 kJ/mole (a bit below that for self-diffusion of Pb and Sn), an intermediate region with n ∼ 2 and Q ∼ 45 kJ/mole (near that for grain boundary diffusion), and a low-stress region with n ∼ 3 and Q ∼ 80 (suggesting a reversion to a bulk mechanism). The quenched material shows only two regions: a high-stress creep with a stress exponent, n ∼ 3–7, and a low-stress region with n ∼ 3. The mechanisms in both regimes have activation energies intermediate between bulk and interface values (50–70 kJ/mole). With minor exceptions, the stress relaxation data and the creep data are in reasonable agreement. Most of the exceptions seem to be related to the difficulty of capturing the full details of grain boundary creep in stress relaxation tests.     

Constitutive relations on creep for SnAgCuRE lead-free solder joints

Taking the most promising substitute of the Sn-3.8Ag-0.7Cu solder as the research base, investigations were made to explore the effect of rare earths (REs) on the creep performance of the Sn-3.8Ag-0.7Cu solder joints. The SnAgCu-0.1RE solder with the longest creep-rupture life was selected for subsequent research. Creep strain tests were conducted on Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints in the intermediate temperature range from 298 K to 398 K, corresponding to the homologous temperatures η=0.606, 0.687, 0.748, and 0.809 and η = 0.602, 0.683, 0.743, and 0.804, respectively, to acquire the relevant creep parameters, such as stress exponent and activation energy, which characterize the creep mechanisms. The final creep constitutive equations for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints were established, demonstrating the dependence of steady-state creep rate on stress and temperature. By correcting the apparent creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints from the experiments, the true creep-activation energy is obtained. Results indicated that at low stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the lattice self-diffusion activation energy, so the steady-state creep rates of these two solder joints are both dominated by the rate of lattice self-diffusion. While at high stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the dislocation-pipe diffusion activation energy, so the steady-state creep rates are dominated by the rate of dislocation-pipe diffusion. At low stress, the best-fit stress exponents n of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are 6.9 and 8.2, respectively, and the true creep-activation energy of them both is close to that of lattice self-diffusion. At high stress, it equals 11.6 and 14.6 for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints, respectively, and the true creep-activation energy for both is close to that of the dislocation-pipe diffusion. Thus, under the condition of the experimental temperatures and stresses, the dislocation climbing mechanism serves as the controlling mechanism for creep deformation of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints. The creep values of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are both controlled by dislocation climbing. Dislocation glide and climb both contribute to creep deformation, but the controlling mechanism is dislocation climb. At low stress, dislocation climbing is dominated by the lattice self-diffusion process in the Sn matrix and dominated by the dislocation-pipe diffusion process at high stress.     

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.     

Anomalously high tensile creep rates from thin cast Sn3.9Ag0.6Cu lead-free solder

The present paper compares the creep and microstructural changes during creep behavior of bulk and thin cast forms of Sn3.9Ag0.6Cu. The processing parameters of the thin cast material were selected to result in a very fine microstructure analogous to what occurs in very small size solder electronic interconnections. We found that the thin cast material is less creep resistant than the bulk material. A comparison of Ag element maps between as-crept bulk and thin cast material shows that the relevant climb process occurs in a very different environment in the bulk material as compared to the thin cast material. In the bulk material, the relevant climb process occurs within a finely dispersed intermetallic compound (IMC) eutectic, which covers broad areas within the material. In the thin cast material, the relevant climb process occurs primarily in the beta-Sn grains that continuously surround isolated, coarse IMC particles. This resulted in the activation energy of the bulk material being larger than that for the thin cast material. Finally, it is important to note that the strength deficiency of the thin cast material is persistent—once the material is cast in thin cast form, it will remain weak in comparison to the bulk material. Therefore, using data obtained from bulk material samples for the construction of thermomechanical models of very small scale solder interconnections is likely to result in significant, intrinsic errors.     

Enhancing the microstructure and tensile creep resistance of Sn-3.0Ag-0.5Cu solder alloy by reinforcing nano-sized ZnO particles

Sn-Ag-Cu lead-free solders are regarded as a potential substitute for Pb-Sn solder alloys. In the current study, the non-reacting, non-coarsening ZnO nano-particles (ZnO NPs) were successfully incorporated into Sn–3.0Ag–0.5Cu (SAC305) lead-free solder by mechanical mixing of ZnO powders and melting at 900 °C for 2 h. Tensile creep testing was performed for plain SAC305 solder and SAC305-0.7 wt% ZnO NPs composite solders and a Garofalo hyperbolic sine power-law relationship was created from the experimental data to predict the creep mechanism as a function of tensile stress and temperature. Based on the tensile creep results, the creep resistance of SAC305 solder alloy was improved considerably with ZnO NPs addition, although the creep lifetime was increased. From microstructure observation, reinforcing ZnO NPs into SAC305 solder substantially suppressed the enlargement of Ag₃Sn and Cu₆Sn₅ intermetallic compound (IMC) particles and decreased the spacing of the inter-particles between them, reduced the grain size of β-Sn and increased the eutectic area in the alloy matrix. The modification of microstructure, which leaded to a strong adsorption effect and high surface-free energy of ZnO NPs, could result in hindering the dislocation slipping, and thus provides standard dispersion strengthening mechanism. Moreover, the average activation energy (Q) for SAC305 and SAC305-0.7ZnO alloys were 50.5 and 53.1 kJ/mol, respectively, close to that of pipe diffusion mechanism in matrix Sn.     

A grain structure based statistical simulation of electromigration damage in chip level interconnect lines

Reliability assessment of chip level interconnects is based on accelerated testing at a higher temperature and a larger current density than expected in service conditions. The critical parameters needed to extrapolate accelerated test data to service conditions are the activation energy, Q, and the current density exponent, n. Although current density exponents and activation energies are well known for the elemental processes (like void nucleation due to electromigration (EM) generated stress), there is no consensus on which apparent activation energy or current density exponent values would be applicable in reliability estimates for realistic line structures. Here, we first review our EM simulation tool. We then apply the EM simulation tool to statistical life time analysis of realistic-like line structures generated by a Monte-Carlo algorithm. For a given grain structure distribution, the stress evolution along the line is simulated, letting voids nucleate at the sites where the stress exceeds a critical level, and the nucleated voids are then allowed to grow till the largest one reaches the preset critical size, resulting in a ‘failure’ of the particular line. By repeating this process for various current densities and temperatures, it becomes possible to extract the apparent activation energies and current density exponents from the simulation data.     

Tensile, creep, and ABI tests on sn5%sb solder for mechanical property evaluation

Sn5%Sb is one of the materials considered for replacing lead containing alloys for soldering in electronic packaging. We evaluated the tensile properties of the bulk material at varied strain-rates and temperatures (to 473K) to determine the underlying deformation mechanisms. Stress exponents of about three and seven were observed at low and high stresses, respectively, and very low activation energies for creep (about 16.7 and 37.7 kJ/mole) were noted. A maximum ductility of about 350% was noted at ambient temperature. Creep tests performed in the same temperature regime also showed two distinct regions, albeit with slightly different exponents (three and five) and activation energy (about 54.4 kJ/mole). Ball indentation tests were performed on the shoulder portions of the creep samples (prior to creep tests) using a Stress-Strain Microprobe^@ (Advanced Technology Corporation) at varied indentation rates (strain-rates). The automated ball indentation (ABI) data were at relatively high strain-rates; however, they were in excellent agreement with creep data, while both these results deviated from the tensile test data. Work is planned to perform creep at high stresses at ambient and extend ABI tests to elevated temperatures.     

Effects of cooling rate on creep behavior of a Sn-3.5Ag alloy

The effect of cooling rate on microstructure and creep behavior of bulk, eutectic Sn-3.5Ag solders was studied. The cooling rate is an important processing variable that significantly affects the microstructure of the solder and therefore determines its mechanical behavior. Controlled cooling rates were obtained by cooling specimens in different media: water, air, and furnace, which resulted in cooling rates of 24°C/s, 0.5°C/s, and 0.08°C/s, respectively. The cooling rate decreased the secondary dendrite arm size and the spacing of the Sn-rich phase, as well as the morphology of Ag₃Sn. The Sn-dendrite arm size and spacing were smaller at fast cooling rates, while slower cooling rates yielded a nearly eutectic microstructure. The morphology of Ag₃Sn also changed from relatively spherical, at faster cooling rates, to needlelike for slower cooling. The effect of cooling rate on creep behavior was studied at 25°C, 60°C, 95°C, and 120°C. Faster cooling rates were found to increase the creep strength of the solder due to the refinement of the solder microstructure. Stress exponents, n, indicated that dislocation climb was the controlling mechanism. Activation energies, for all cooling rates, indicated that the dominant diffusional mechanism corresponded to dislocation pipe diffusion of Sn. Grain boundary sliding (GBS) measurements were conducted, using both scanning electron microscopy (SEM) and atomic force microscopy (AFM). It was observed that GBS had a very small contribution to the total creep strain.     

Creep, stress relaxation, and plastic deformation in Sn-Ag and Sn-Zn eutectic solders

Because of the high homologous operation temperature of solders used in electronic devices, time and temperature dependent relaxation and creep processes affect their mechanical behavior. In this paper, two eutectic lead-free solders (96.5Sn-3.5Ag and 91Sn-9Zn) are investigated for their creep and stress relaxation behavior. The creep tests were done in load-control with initial stresses in the range of 10-22 MPa at two temperatures, 25 and 80°C. The stress relaxation tests were performed under constant-strain conditions with strains in the range of 0.3-2.4% and at 25 and 80°C. Since creep/relaxation processes are active even during monotonie tensile tests at ambient temperatures, stress-strain curves at different temperatures and strain rates provide insight into these processes. Activation energies obtained from the monotonic tensile, stress relaxation, and creep tests are compared and discussed in light of the governing mechanisms. These data along with creep exponents, strain rate sensitivities and damage mechanisms are useful for aiding the modeling of solder interconnects for reliability and lifetime prediction. Constitutive modeling for creep and stress relaxation behavior was done using a formulation based on unified creep plasticity theory which has been previously employed in the modeling of high temperature superalloys with satisfactory results.     

Creep resistance of tin-based lead-free solder alloys

This paper reports on the microstructure-creep property relationship of three precipitation-strengthened tin (Sn)-based lead (Pb)-free solder alloys (Sn-0.7Cu, Sn-3.5Ag, and Sn-3.8Ag-0.7Cu) in bulk samples, together with Sn-37Pb as the alloy for comparison at temperatures of 303 K, 348 K, and 393 K. The creep resistance of these three Sn-based Pb-free solders increases, i.e., the steady-state creep rates decrease, with increasing volume fraction of precipitate phases for the Pb-free solder alloys. Their apparent stress exponents (n_a ∼ 7.3-17), which are all higher than that of pure Sn, attain higher values with increasing volume fraction of precipitate phases at constant temperature, and with decreasing temperature for the same solder alloy.     

Isothermal Fatigue Behavior of the Near-Eutectic Sn-Ag-Cu Alloy between −25°C and 125°C

When near-eutectic Sn-Ag-Cu (SAC) alloys are used to make soldered ball-grid-array (BGA) assemblies, the grain size of the joints is very large. During thermomechanical cycling, the solder joint fatigue process is often initiated with recrystallization of the Sn grains, resulting in a smaller grain size in the deformed areas. Grain boundary sliding and increased grain boundary diffusion then results in intergranular crack nucleation and propagation along the recrystallized Sn grain boundaries. In this work, fatigue tests were used to study the initial stages of cyclic deformation damage in Sn-Ag-Cu alloy samples. To separate the solder properties from the constraints introduced by the substrate, the tests were done with free-standing solder specimens, instead of solder joints. The test samples were cast dog-bone specimens that have a cross-sectional diameter of 1 mm, which corresponds to a typical solder joint diameter in BGAs. Mechanical cycling was performed isothermally at several temperatures, from −25°C to 125°C. Typical test conditions were ± 1.5% strain and 15-minute hold at tensile peak stress to allow stress relaxation to take place. Optical microscopy, scanning electron microscopy, and electron backscattering diffraction were used to study the microstructures of the samples before and after fatigue testing in order to obtain insight into the nucleation and growth mechanisms of fatigue damage.     

Superplastic creep of eutectic tinlead solder joints

This paper presents experimental evidence that as-solidified eutectic Pb-Sn solder joints can exhibit superplastic behavior in shear creep loading. Stepped load creep tests of as-solidified joints show a change in the stress exponent from a high value typical of con-ventional creep at high stress and strain rate to a superplastic value near 2 at lower stress and strain rates. In addition, the change in stress exponent is accompanied by a change in the activation energy for creep from a value near that for bulk self-diffusion (20 kcal/mol) to a value near that for grain boundary diffusion (12 kcal/mol). The total shear deformation of joints in stress-rupture tests performed at 65° C are found to ex-ceed 150%. The concomitant observation that quenched solder joints creep faster than air-cooled ones is attributed to a grain, or phase, size dependence of the strain rate. The source of superplastic behavior is a fine, equiaxed microstructure. It is not yet clear whether the superplastic microstructure is present in the as-solidified joint, or develops during the early stages of plastic deformation.     

Growth nature of in-situ Cu6Sn5-phase and their influence on creep and damping characteristics of Sn-Cu material under high-temperature and humidity

This paper describes the morphology and growth nature of in-situ Cu₆Sn₅ intermetallic compound (IMC) and their impacts on material properties of an environmental-friendly Sn-0. 7Cu (wt%) material when exposed to high-temperature (85 °C) and relative humidity (85%) environments. A detail microstructural characterization is carried-out by electron microscopy e.g., SEM, EBSD and TEM techniques. In as-cast Sn-Cu material, along with the fine matrix Sn grains, the in-situ Cu₆Sn₅ IMC in grain boundary and interior grain appear with a dimension of submicron size and more consistently spread in the matrix. Such fine and uniform dispersed IMC acted as a pinning effect that obstacle the dislocation movement and enhanced their hardness and creep performance. In contrast, after exposing at harsh environment, such in-situ IMC phase morphologies are changed and seemed to a coarse elongated-shaped IMC and reduced their aspect-ratio. These morphological changes negatively impact on the mechanical reliability of Sn-Cu material. A contrast between the as-cast specimen and specimen exposed to high-temperature and relative humidity presents that the electrical resistivity reduces to 12%, where their hardness degrades about 18.5%. However, it is worthy noted that the coarse IMC positively impacts on damping property of Sn-Cu material at given a strain and temperature.     

Sn-3.7Ag-0.9Zn无铅钎料合金压入蠕变性能研究

采用自制的压入蠕变装置,研究了共晶型Sn-3.7Ag-0.9Zn无铅钎料合金在333~418K,压入应力为34.1~75.3MPa时的压入蠕变性能,并获得其稳态压入蠕变速率的本构方程;利用XRD和SEM对合金蠕变前后的成分和微观组织进行了分析。结果表明:Sn-3.7Ag-0.9Zn无铅钎料合金的应力指数n为4.6,蠕变激活能Qc为82.03kJ/mol,材料的结构常数A为1.74×10–5,其压入蠕变机制主要是由位错攀移运动控制的蠕变;金属间化合物Ag3Sn、AgZn提高了合金的抗压入蠕变性能。