For demanding environments such as automotive and defense, high-reliability solder alloys are required to maintain performance across a wide temperature range, from -65 °C up to 200 °C. Under these conditions, the wide temperature range induces a high strain-range cycling profile on solder joints, further exacerbated by large-size packages, necessitating careful fatigue life assessment of these alloys. In this study, we investigate the joint-scale solder fatigue and fracture behavior for three different high-reliability solder alloys namely, QSAC201, Innolot2 as well as another commercial alloy labeled here as Alloy 10, assembled on circuit board coupons with Organic Solderability Preservative (OSP) pad surface finish. We first carried out monotonic tests to failure at a strain rate of 7.7 × +10−4 s−1 followed by strain-based fatigue tests at four strain ranges under the same strain rate and with a 200 s dwell. As a baseline, Sn3.0Ag0.5Cu alloy (SAC305) was also tested and used for comparison against the candidate alloys. Using experimental test results, we discuss the following: 1) A comparison of all candidate solder alloys’ strain-to-fracture results from monotonic tests 2) fatigue tests at increasing strain ranges of 2%, 5%, 10% and 15% covering intermediate-cycle fatigue to low-cycle fatigue 3) numerical simulations to highlight the relation between experimental fatigue test results, thermal cycling, and package size. Previously reported fatigue test on high-temperature solder alloys results often claim superior performance relative to SAC305. Through our extensive isothermal testing, we elucidate the range of strains under which the claims are valid.

While previous studies linked whisker formation to various factors in Sn films, the influence of local grain boundary (GB) characteristics on nucleation mechanisms needed further investigation. This study examines how GB geometry and misorientation changes influence whisker nucleation by combining experimental characterization and crystal plasticity simulations. We establish a comprehensive whisker nucleation model where: (1) GB yielding occurs under local compressive stresses, influenced by GB misorientation; (2) normal displacements accumulate at GBs when shear stresses exceed a threshold; (3) localized grain rotation compensates shape changes; and (4) subgrain formation occurs as rotation intensifies, leading to the formation of new grain boundaries. Our results reveal that GB geometry controls this process by determining normal displacement differences through coefficient of thermal expansion mismatches and dictating whether newly nucleated grains achieve necessary configurations for whisker growth.

Molecular crystals are subjected to mechanical stress that can alter their slip planes which are structural features that play a critical role in governing mechanical behaviors such as milling, that requires fundamental insight for the successful manufacturing and performance of solid oral dosage forms. Milling is a critical unit operation in the manufacturing of pharmaceutical dosage forms. During this process, slip planes may become disrupted and dislocations can form, potentially inducing plastic deformation and significantly altering the physicochemical properties of active pharmaceutical ingredients (API). While slip, a dominant mechanism of plasticity, is well characterized in inorganic crystals, its behavior in organic molecular crystals remains poorly understood due to their low symmetry and anisotropic nature. In this study, we investigate plastic deformation in a model organic crystal, acetaminophen, using atomic force microscopy (AFM) nanoindentation. Distinct pile-up patterns forming parallel facets around indentation sites were observed and systematically analyzed. These pile-ups were experimentally confirmed to align with specific crystallographic slip planes. Based on this, a geometry-based strategy was developed to automatically identify, and index slip planes from indentation pile-up features. This method may offer new insights into the deformation behavior of organic crystals and serves as a broadly applicable tool for investigating slip systems across a wide range of pharmaceutical and molecular materials.

Low-melting-point Sn-Bi solder joints (melting point: 139°C) show remarkable resistance to damage accumulation during aggressive thermal cycling. In this study, we used isothermal aging at 85°C of near eutectic Sn-Bi solder joints to determine the effect of Sb in solid solution and Ag3Sn intermetallic on microstructural evolution and the resulting mechanical properties as a way to explain the thermal cycling behavior. Most importantly, the Sb in solid solution in these alloys resulted in higher strength and improved creep resistance when compared to eutectic Sn-Bi. In contrast to Sn-Pb and Sn-Ag-Cu Pb-free alloys, all the near-eutectic Sn-Bi alloys tested showed significant age hardening. In both the unaged and aged conditions, both Sb and Ag additions individually increased the saturation stress of the eutectic Sn-Bi solder joint, but Ag had a more significant effect. However, when both Sb and Ag were added to eutectic Sn-Bi, the saturation stress was lower than when 1 wt.

This study examines the effect of tensile stress on the ferroelectric properties of Pb(Zr0.4Ti0.6)O3 thin film, with a focus on Barkhausen noise, observed for the first time under such conditions. Tensile stress significantly alters domain wall motions, affecting Barkhausen noise more than average polarization. Frequency analysis identifies grain boundaries as primary pinning sites, consistent across stress levels. A nonlinear relationship between stress, domain wall mobility, and polarization is found, where increased stress initially enhances pinning and polarization changes, but this effect diminishes at higher stress levels, indicating a shift in behavior.

As a result of thermal cycling, polycrystalline tin films with a large in-plane grain size relative to film thickness on copper substrates show grain boundary (GB) sliding, creep, yielding, and nucleation of shallow grains that become whiskers. These microstructural changes occur near GBs, with different types of changes for different GBs. To understand these different phenomena, it is important to identify local microstructural changes and signatures of deformation mechanisms on free surfaces as deformation progresses, particularly in how they affect the formation of shallow grains. This study does that by characterizing the evolution of morphological, crystallographic, and curvature changes near GBs in large-grained Sn films during rapid thermal cycling. The observed responses include (1) nucleation of new grains accompanied by local yielding as indicated by slip band formation and grain misorientation changes, (2) GB sliding and diffusion, and (3) localized GB migration, with GB sliding and near-GB rotation occurring earlier than other phenomena. The angles between slip planes and observed slip traces suggested that dislocations moved along activated slip planes to form new GBs. These results on large-grained bicrystal films provide insights into the complex stress relaxation behavior of columnar polycrystalline tin films under the same thermal cycling conditions.

In this study, we report the mechanical behavior of two low-temperature solder joints formed with Sn58Bi and Alloy-4 on Organic Solderability Preservative (OSP) surface finish. The comprehensive testing is aimed at developing constitutive models for a variety of low-temperature solder (LTS) alloys including Sn58Bi, Sn57Bi-1Ag, 42SnBi-0.5Sb, and Alloy-4. Towards this end, we report preliminary test results on Sn57Bi-1Ag 42SnBi-0.5Sb alloys. The investigation utilizes double lap-shear mechanical tests conducted using a custom-made closed-loop capacitively controlled sub-micron precision mechanical tester. In total, nine monotonic tests and nine creep tests were conducted for each material at three different temperatures. Monotonic tests were carried out at three different strain rates, ranging from 7.7 * 10 −5 s −1 to 7.7 * 10 −3 s −1 , while creep tests were conducted at three stress levels, ranging from 5.17 MPa to 20.69 MPa. The data obtained from these tests will be utilized to derive viscoplastic constitutive models including the Anand model, the power-law creep model, and the time-hardening primary-secondary creep model. Using the experimental data, we discuss the following aspects: 1) creep and monotonic testing results under various conditions to understand the constitutive behavior of four alloys 2) aging effect on the tested alloys to understand how their properties evolve over time.

This study examines the changes in strain rate dependence and the deformation behavior of near-eutectic Sn-Bi alloys as a function of Sb concentration using nanoindentation. Alloying near-eutectic Sn-Bi solder with Sb has been shown to increase the strain to failure under tensile and shear conditions in solder ball geometries, with Sb additions remaining in solid solution up to 0.5 wt.

The use of Low Temperature Solders (LTS) as an alternative to high temperature Sn-Ag-Cu (SAC) alloys has gained increasing interest owing to the benefits associated with low reflow temperatures such as decrease in warpage during as-sembly, nontoxicity, and lower operating and energy costs. Sn-Bi alloy system, with reduced reflow temperature of approximately 183°C, is a popular LTS alloy system. In addition to the existing literature on microstructural properties, there is a need to study and compare the mechanical properties of the new LTS solders. As part of this study, a baseline mechanical characterization of Sn3.0Ag0.5Cu and Sn58Bi solder compositions is first performed. Several alloying elements have been suggested to improve the reliability of Sn-Bi system, of which Ag, In and Sb have been studied in this work. Mechanical behavior of Sn57BilAg, HRL3 (by MacDermid Alpha), L27 and L29 (by Senju Metal Industry) alloys are presented here. In addition, the effect of pretest aging (after reflow and aged to 10 days at 85°C/125°C) on the mechanical behavior is studied. A comprehensive comparison of the mechanical behavior of the four new LTS alloys against the baseline alloys is presented. A custom designed micro-precision mechanical tester is used to perform isothermal creep, monotonic and fatigue testing of the solder test specimen under shear at room temperature and an elevated temperature of 70° C.

The low melting temperature of eutectic Sn–Bi alloys (139 °C) makes eutectic Sn–Bi a suitable low temperature alternative to high-Sn, lead-free solders in electronics packaging. The low eutectic temperature allows for a peak reflow temperature of 180 °C rather than the 240 °C reflow temperature required for Sn–Ag–Cu alloys. Lower reflow temperatures reduce warpage-induced solder joint defects. However, the strain-rate sensitivity of Sn-Bi alloys results in lower drop-shock performance despite having higher reliability in thermal cycling. The literature shows evidence in bulk solders that microalloying with Sb and Ag can improve ductility and mitigate the strain-rate sensitivity of Sn–Bi alloys. Improved ductility and a low strain-rate sensitivity are known to enhance drop-shock performance and create a more reliable joint. Our results demonstrate how small changes in composition due to Cu-dissolution from substrates make the microstructure more heterogeneous and how those changes impact the mechanical properties of Sn–Bi solder joints.

Low temperature solders based on tin-bismuth (Sn-Bi) are used as substitutes for tin-silver-copper (Sn-Ag-Cu, SAC) alloys, reducing warpage-induced defects and reflow temperature. Understanding the effects of microalloying elements on the solder mechanical, microstructural, and thermodynamic properties constitute an essential part of alloy design. This study focuses on the changes in melting and solidification behavior of near-eutectic Sn-Bi alloys with antimony (Sb, 0.5–2 wt

A thermodynamically consistent phase field formulation was developed to describe what has been historically known as the premelted surface layer in Sn nanoparticles. Two interfacial phases were identified: 1) a disordered interfacial phase, i.e., the experimentally observed premelted surface layer; and 2) an ordered surficial phase displaying a remnant degree of order in fully melted particles. A volumetric partially disordered particle core is predicted to exist for small particle sizes, r < 7 nm. Four regimes of behavior are discussed: a) the classic premelting regime , for r >= 20 nm, in which the surface of the particle forms a liquid shell at temperatures below melting with clear delineation between the solid core and the premelted surficial liquid shell, as traditionally expected; b) the transition regime , for 5 < r < 20 nm, in which the volumetric chemical potential and interfacial forces balance each other out; c) the disordered volume phase regime , for r <= 5 nm, in which a disordered interphase dominates the thermodynamic equilibrium of the system resulting in a partially crystalline core, and d) the mesoscale regime , in which the nanoparticles are no longer considered crystalline and experience an associated decrease in latent heat.(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Cyclic bending of beta-Sn films with 1 mu m thickness on atomic force microscopy silicon cantilevers was performed at resonance in both first and second mode vibration. The stress response as a function of position along the cantilever at very small strain amplitudes was examined by changes in surface morphology. Hillock formation and growth, the dominant responses observed, were measured using scanning electron microscopy as a function of the number of cycles and the strain distribution along the cantilever. New hillocks formed with increasing numbers of cycles, with their morphologies similar to the pre-existing hillocks observed before cycling. However, all hillocks exhibited a constant diameter-to-height ratio observed of 1: (1-1.2), with both pre-existing and new hillocks reached a limiting height which did not grow with further cycling. A saturation in the number of hillocks was observed with increased cycling, with the cycles to saturation depending on the vibration mode. The hillock density along the cantilever was found to be proportional to the local strain amplitude in both the first and second vibration modes. These results demonstrate that even the small maximum strain amplitudes in these experiments are sufficient to drive the nucleation and growth of hillocks as a stress relaxation response. Comparisons between these experiments and prior studies of whisker and hillock formation on beta-Sn films due to CuSn intermetallic compound growth and thermal mismatch stresses in thermal cycling are made in terms of morphologies, densities, and time dependence of nucleation and growth.

Solid-state sintering is the bonding and densification of particles by the application of heat below the melting point of a material. During sintering, the free surface area of the compact decreases, and this is normally accompanied by an increase in the density. As such, it represents an essential part of powder processing as a high density is usually required for optimal material's properties. This article explains the driving force for the different stages of sintering along with the mass transport mechanisms that result in densification. Beyond this, the impact of heating rates, pressures, electric fields and mechanical constraints on densification are discussed and an overview of common sintering techniques is given.

Low reflow temperature solder interconnect technology based on Sn-Bi alloys is currently being considered as an alternative for Sn-Ag-Cu solder alloys to form solder interconnects at significantly lower melting temperatures than required for Sn-Ag-Cu alloys. Microstructural evolution after reflow and aging, especially of intermetallic compound (IMC) growth at solder/pad surface finish interfaces, is important to understanding fatigue life and crack paths in the solder joints. This study describes intermetallic growth in homogeneous solder joints of Sn-Bi eutectic alloy and Sn-Bi-Ag alloys formed with electroless nickel-immersion gold (ENIG) and Cu-organic surface protection (Cu-OSP) surface finishes. Experimental observations revealed that, during solid state annealing following reflow, the 50nm Au from the ENIG surface finish catalyzed rapid (Ni,Au)Sn4 intermetallic growth at the Ni-solder interface in both Sn-Bi and Sn-Bi-Ag homogeneous joints, which led to significant solder joint embrittlement during fatigue testing. Intermetallic growth of (Ni,Au)Sn4 was decreased by Ag alloying of eutectic Sn-Bi solder and was completely eliminated by changing the metallization from ENIG to Cu-OSP on the board side of the assembly. The reduction in (Ni,Au)Sn4 growth rate with Ag additions is attributed to changes in grain boundary wetting of the IMC by Bi with Ag alloying.

Two grain boundaries (GBs) in a Sn-alloy film exhibited different morphological changes during thermal cycling: whisker growth and GB sliding. Along GB-I, pervasive nucleation of high-angle and low-angle GBs occurred, with whisker growth of some shallow grains depending on sub-surface GB geometry. Most whiskers showed substantial changes of crystallographic orientation and surface normal relative to the parent grains. Along GB-II, GB sliding was characterized by mass accumulation, crystallographic rotation, and localized yielding. Results for two GBs suggest that grain crystallography and GB geometry, particularly GB inclination, are critical in determining the extent of new grain formation and subsequent whisker growth.

Whisker and hillock formation are a stress relaxation response in Sn thin films and have become a critical reliability problem in Pb-free electronics. Whiskers grow by diffusion of atoms driven by stress gradients and usually have shallow grains at their roots. Therefore, the local conditions under which surface grains form need to be understood to mitigate whisker growth in Sn thin films. Two possible mechanisms for the nucleation of shallow grains are investigated using finite element simulations: grain boundary migration and pinch-off, and subgrain formation by local lattice rotation. By incorporating the role of localized plasticity, we find that shallow grains do not form by motion and bowing of grain boundaries. The simulations show that deformation-induced lattice rotation creates subgrains close to the thin film surface. These shallow grains form by the local heterogeneity of the strain in Sn polycrystals leading to high dislocation densities that subdivide grains into subgrains that may be potential sites for whisker formation.

The first direct Barkhausen noise measurement in a ferroelectric thin film is presented. The Barkhausen noise energy loop is reconstructed from the measured Barkhausen noise and is closely related to the classic ferroelectric P vs E hysteresis loop. Grain boundaries act as a dominant ferroelectric domain wall pinning site in a polycrystalline thin film based on the calculated domain wall jump distance using the Barkhausen noise frequency. The technique is promising for the measurement of ferroelectric switching dynamics, and provides a physical insight for improving application performance.

The equilibrium crystal shape is a convex shape bound by the lowest energy interfaces. In many polycrystalline microstructures created by grain growth, the observed distribution of grain boundary planes appears to be dominated at low driving forces (after long grain growth times) by the planes present in the equilibrium crystal shape. However, at earlier stages of grain growth, it is expected that kinetic effects will play an important role in grain boundary motion and morphology. Analogous to the equilibrium crystal shape, the kinetic crystal shape of seed crystals growing from a liquid at higher supersaturations is bound by the slowest growing orientations. This study presents an equivalent construction for grain boundaries in polycrystals and uses it to determine the kinetic crystal shape for strontium titanate as a function of temperature. Relative grain boundary mobilities for strontium titanate for the low energy crystallographic orientations from seeded polycrystal experiments are used to calculate the kinetic crystal shapes as a function of temperature and annealing atmosphere. The kinetic crystal shapes are then compared to the morphologies and orientations of the interfaces of the growing seed crystals, and to the equilibrium crystal shapes, as well. The conclusions are that (1) the kinetic crystal shape is extremely anisotropic and displays significant transitions as a function of temperature that do not mirror changes in equilibrium crystal shape, (2) the kinetic shapes observed in the microstructures are dominated by the growing side of the interface (single crystal) and not by the dissolving side (polycrystalline matrix), and (3) faster growing orientations break up into macroscopic facets composed of slower growing orientations. The implications for grain growth underscore the applicability of crystal growth models to grain growth in polycrystals. In particular, in strontium titanate, the anisotropy of the grain boundary mobility as represented in the kinetic crystal shape is expected to be reduced from five macroscopic parameters to two (interface normal) allowing for incorporation of growth rate anisotropy in simulations of microstructure evolution at the earliest stages of grain growth, i.e. at the highest driving forces. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.