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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.

Four different types of stress relaxation responses have been observed in terms of local microstructural changes along grain boundaries (GBs) in large-grained high-Sn (Sn-3.0Ag-0.5Cu) solder films after thermal cycling. The grain boundaries were characterized using scanning electron microscopy, electron back scattered diffraction, and focused-ion beam (FIB) cross-section imaging. While the anisotropic coefficient of thermal expansion of Sn plays an important role in determining which boundaries have high local stresses relative to the film plane and trace of the grain boundary plane during thermal cycling, the four different relaxation behaviors of specific boundaries (no observable changes, surface defect/whisker formation, GB sliding, or a combination of GB sliding and whisker/defect formation) were determined by a combination of Sn anisotropy, the GB geometry, and crystallographic orientation relative to the film plane. The ability to separate GB sliding from surface defect formation provides insights into how long, straight whiskers form in polycrystalline thin films, particularly with respect to grain rotation at the early stages of their growth.

There is a rising interest in developing functional electronics using additively manufactured components. Considerations in materials selection and pathways to forming hybrid circuits and devices must demonstrate useful electronic function; must enable integration; and must complement the complex shape, low cost, high volume, and high functionality of structural but generally electronically passive additively manufactured components. This article reviews several emerging technologies being used in industry and research/development to provide integration advantages of fabricating multilayer hybrid circuits or devices. First, we review a maskless, noncontact, direct write (DW) technology that excels in the deposition of metallic colloid inks for electrical interconnects. Second, we review a complementary technology, aerosol deposition (AD), which excels in the deposition of metallic and ceramic powder as consolidated, thick conformal coatings and is additionally patternable through masking. Finally, we show examples of hybrid circuits/devices integrated beyond 2-D planes, using combinations of DW or AD processes and conventional, established processes.

Aerosol deposition (AD) is a solid-state deposition technology that has been developed to fabricate ceramic coatings nominally at room temperature. Sub-micron ceramic particles accelerated by pressurized gas impact, deform, and consolidate on substrates under vacuum. Ceramic particle consolidation in AD coatings is highly dependent on particle deformation and bonding; these behaviors are not well understood. In this work, atomistic simulations and in situ micro-compressions in the scanning electron microscope, and the transmission electron microscope (TEM) were utilized to investigate fundamental mechanisms responsible for plastic deformation/fracture of particles under applied compression. Results showed that highly defective micron-sized alumina particles, initially containing numerous dislocations or a grain boundary, exhibited no observable shape change before fracture/fragmentation. Simulations and experimental results indicated that particles containing a grain boundary only accommodate low strain energy per unit volume before crack nucleation and propagation. In contrast, nearly defect-free, sub-micron, single crystal alumina particles exhibited plastic deformation and fracture without fragmentation. Dislocation nucleation/motion, significant plastic deformation, and shape change were observed. Simulation and TEM in situ micro-compression results indicated that nearly defect-free particles accommodate high strain energy per unit volume associated with dislocation plasticity before fracture. The identified deformation mechanisms provide insight into feedstock design for AD.

A strategy for identifying the preferred sites and overall propensity of a Sn film to form whiskers has been developed based on film textures, local grain orientations, and elastic strain energy densities (ESEDs), with preferred sites predicted to be grains with local high ESEDs. Using β-Sn films with various textures, ESED distributions were simulated for elastic and thermoelastic stresses depending on isothermal aging or thermal cycling conditions. Local high ESEDs are preferentially induced in (110) or (100) oriented grains with c-axes nearly parallel to the film plane; films with overall low ESEDs have strong (100) textures for elastic stresses and strong (001) textures for thermoelastic stresses, suggesting low propensities to form whiskers. This work establishes a model for understanding the effect of the β-Sn anisotropy on whisker formation and provides guidelines for testing whether engineering specific film textures will reduce a film’s propensity to form whiskers.

Whisker and hillock formation in thin films is well known as a highly local mechanism for stress relaxation, where in many cases, only a few whiskers form out of thousands of grains in a film. In this article, the microstructural characteristics for specific grains to form whiskers in β-Sn films are discussed in light of our recent whisker growth model, establishing a relationship among grain boundary sliding limited Coble creep, surface grain geometry, and film stress for different stress conditions, including for thermal cycling. Through our recent finite-element simulations of stresses induced by room-temperature aging and thermal cycling of textured microstructures, the role of elastic and thermoelastic anisotropy in creating preferred whisker formation sites and the general propensity of a film to form whiskers have been proposed for a range of β-Sn film textures. Taken together, these models suggest a strategy for identifying the effects of local microstructure and β-Sn anisotropy on whisker formation. If these predictions are accurate, then whisker growth risk may be effectively reduced by engineering film microstructures and textures for specific applications and stress conditions.

Direct evidence was obtained for the nucleation of surface grains through recrystallization and their growth into whiskers and hillocks during thermal cycling of Sn-alloy films. Shallow grains with low dislocation densities nucleated at pre-existing film grain boundaries and grew at the expense of deformed parent grains. Micro-diffraction analysis showed that parent grains have a high grain orientation spread, subgrain formation, and high dislocation density. The whisker and hillock grains have different orientations from the parent grains. A crystallographic orientation relationship between a surface defect grain and one of the parent grains was demonstrated with the misorientation axis lying in the active slip planes of the parent grain.

A model incorporating the effects of grain geometry and size and grain boundary properties on the growth of whiskers and hillocks has been developed based on coupling between localized Coble creep and grain boundary sliding. For both whiskers and hillocks accretion of atoms by Coble creep on grain boundary planes normal to the growth direction is limited by grain boundary sliding on planes parallel to the direction of whisker growth. If the accretion-induced shear stresses are not coupled to grain boundary migration a whisker forms when sliding occurs. In the case of hillocks an additional coupling between grain boundary sliding and shear-induced grain boundary migration leads to the observed lateral growth. By incorporating grain size and geometry, a structure-dependent grain boundary sliding coefficient and measured film stresses the local conditions for whisker growth, including the growth rates, can be calculated. As described here, other commonly observed whisker and hillock morphologies and geometries are consistent with this model, as are the effects of a surface oxide film and thermal cycling.

A whisker and hillock growth model based on local film microstructure, grain misorientation, and elastic strain energy density (ESED) as the driving force for growth was developed to predict preferred sites for growth. Local grain orientations and strains measured by synchrotron microdiffraction in nine regions containing whiskers or hillocks were compared with elastic finite element analysis simulations including Sn elastic anisotropy. Whisker and hillock grains were observed to have higher crystallographic misorientations with neighboring grains than generally observed in the microstructure. While elastic simulations predicted higher local out-of-plane elastic strains and ESEDs at those locations with high misorientations before growth, synchrotron measurements of out-of-plane strains of whisker and hillock grains after growth showed relaxation, with correspondingly low ESEDs calculated from measured strains. Hence, highly localized out-of-plane elastic strains and ESEDs of grains with high relative misorientations with their neighbors determined, at least in part, which grains became whiskers or hillocks.

In this paper, the concept of a defect phase diagram is introduced which quantifies the effects of Cu and Pb additions to electrodeposited Sn films on surface defect formation, including but not limited to the formation of Sn whiskers. Transitions were observed in both the defect densities and the morphologies of hillocks and whiskers as Cu and Pb film compositions were systematically varied. Changes in crystallographic texture were also reported for a subset of the Sn-Cu-Pb alloys examined. The transitions between different defect types and the coexistence of certain defect types help to interpret the role of grain boundary pinning in hillock and whisker formation.

It is well known that nanostructured metals can exhibit significantly improved properties compared to metals with conventional grain size. Unfortunately, nanocrystalline metals typically are not thermodynamically stable and exhibit rapid grain growth at moderate temperatures. This severely limits their processing and use, making them impractical for most engineering applications. Recent work has shown that a number of thermodynamically stable nanocrystalline metal alloys exist. These alloys have been prepared as powders using severe plastic deformation (e.g. ball milling) processes. Consolidation of these powders without compromise of their nanocrystalline microstructure is a critical step to enabling their use as engineering materials. We demonstrate solid-state consolidation of ball milled copper-tantalum nanocrystalline metal powder using cold spray. Unfortunately, the nanocrystalline copper-tantalum powder that was consolidated did not contain the thermodynamically stable copper-tantalum nanostructure. Nevertheless, this does this demonstrates a pathway to preparation of bulk thermodynamically stable nanocrystalline copper-tantalum. Furthermore, it demonstrates a pathway to additive manufacturing (3D printing) of nanocrystalline copper-tantalum. Additive manufacturing of thermodynamically stable nanocrystalline metals is attractive because it enables maximum flexibility and efficiency in the use of these unique materials.

In this work, the fundamental mechanisms for ceramic particle deformation in aerosol deposition were investigated. We hypothesized that pre-existing defects affect ceramic particle deformation under compression. Preliminary results showed that near defect-free, sub-micron, single crystal alumina (Al2O3) particles exhibited dislocation nucleation and motion along with significant plastic deformation, shape change, and cracking in compression at room temperature. In contrast, highly defected, micron-sized alumina particles exhibit no observable change in shape before fracture and fragmentation. Particle deformation mechanisms, identified through this work, provide insight into feedstock design for solid state alumina deposition using the aerosol deposition process.

The ability to integrate ceramics with other materials has been limited due to high temperature (>800°C) ceramic processing. Recently, researchers demonstrated a novel process, aerosol deposition (AD), to fabricate ceramic films at room temperature (RT). In this process, sub-micron sized ceramic particles are accelerated by pressurized gas, impacted on the substrate, plastically deformed, and form a dense film under vacuum. This AD process eliminates high temperature processing thereby enabling new coatings and device integration, in which ceramics can be deposited on metals, plastics, and glass. However, knowledge in fundamental mechanisms for ceramic particles to deform and form a dense ceramic film is still needed and is essential in advancing this novel RT technology. In this work, a combination of experimentation and atomistic simulation was used to determine the deformation behavior of sub-micron sized ceramic particles; this is the first fundamental step needed to explain coating formation in the AD process. High purity, single crystal, alpha alumina particles with nominal sizes of 0.3 μm and 3.0 μm were examined. Particle characterization, using transmission electron microscopy (TEM), showed that the 0.3 μm particles were relatively defect-free single crystals whereas 3.0 μm particles were highly defective single crystals or particles contained low angle grain boundaries. Sub-micron sized Al2O3 particles exhibited ductile failure in compression. In situ compression experiments showed 0.3μm particles deformed plastically, fractured, and became polycrystalline. Moreover, dislocation activity was observed within these particles during compression. These sub-micron sized Al2O3 particles exhibited large accumulated strain (2-3 times those of micron-sized particles) before first fracture. In agreement with the findings from experimentation, atomistic simulations of nano-Al2O3 particles showed dislocation slip and significant plastic deformation during compression. On the other hand, the micron sized Al2O3 particles exhibited brittle fracture in compression. In situ compression experiments showed 3μm Al2O3 particles fractured into pieces without observable plastic deformation in compression. Particle deformation behaviors will be used to inform Al2O3 coating deposition parameters and particle-particle bonding in the consolidated Al2O3 coatings.

For electroplated Sn and Sn alloy finishes, one of the reliability concerns remains the risk of whisker growth. Results from recent work have suggested that whiskers are most likely to form in regions of the films where high stress or a stress gradient exists. If strain/stress distribution information can be collected at a grain-by-grain level, correlations between such information and the propensity of whisker growth can be further understood. In this work, we utilized a highly focused X-ray beam from a synchrotron source to perform micro-diffraction on a series of Sn and Sn-containing finishes. The high brightness and small beam size of the X-ray enabled the generation of grain-by-grain orientation map as well as the strain/stress levels in individual grains. The electroplated finishes analyzed included pure Sn, Sn-Cu, and Sn-Cu-Pb finishes with various concentrations of Cu and Pb. Plating current density was also varied for each finish composition and the textures of these finishes were compared. After plating, these finishes were stored at ambient condition and examined regularly for surface defect formation. Once hillock or whisker growth was observed, the areas surrounding the growth were scanned with the X-ray. Additionally, these samples were also analyzed with standard X-ray diffraction and inverse pole figures were generated to compare the texture of the samples. A finite element model was also generated to simulate the texture of the finishes. By implementing the stiffness matrix of the finishes, we were able to explicitly implement the variation of finish texture on a grain-by-grain basis, and thus assess the strain/stress distribution in the finish. The analytical and simulation results from this study suggest that plating process parameters such as current density have a significant impact on the crystallographic texture of the plated finishes. Under similar strain conditions, certain textures would generate higher stresses in the finishes and result in higher levels of whisker growth.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.