Publications

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This Part 2 study examined the microstructural characteristics of braze joints made between two KOVar^TM base materials using the filler metals, Ag-xAl, having x = 0, 2, 5, and 10 wt.% Al additions. Brazing processes had temperatures of 965°C (1769°F) and 995°C and brazing times of 5 and 20 min. All brazing was performed under high vacuum.

The run-out phenomenon was observed in Ag-Cu-Zr active braze joints made between the alumina ceramic and Kovar™ base material. Run-out introduces a significant yield loss by generating functional and/or cosmetic defects in brazements. A prior study identified a correlation between run-out and the aluminum (Al) released by the reduction/oxidation reaction with alumina and aluminum's reaction with the Kovar™ base material. A study was undertaken to understand the fundamental principles of run-out by examining the interface reaction between Ag-xAl filler metals (x = 2,5, and 10 wt-%) and Kovar™ base material. Sessile drop samples were fabricated using brazing temperatures of 965° (T769°F) or 995°C 0823°F) and times of 5 or 20 min. The correlation was made between the degree of wetting and spreading by the sessile drops and the run-out phenomenon. Wetting and spreading increased with Al content (x) of the. Ag-xAl filler metal, but was largely insensitive to the brazing process parameters. The increased Al concentration resulted in higher Al contents of the (Fe, Ni, Co)xAly reaction layer. Run-out was predicted when the filler metal has a locally elevated Al content exceeding 2-5 wt-%. Several mitigation strategies were proposed, based upon these findings.

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Many challenges must be overcome in order to create reliable electrochemical energy storage devices with not only high energy but also high power densities. Gaps exist in both battery and supercapacitor technologies, with neither one satisfying the need for both large power and energy densities in a single device. To begin addressing these challenges (and others), we report a process to create a self-assembled array of electrochemically active nanoparticles bound directly to a current collector using extremely short (2 nm or less) conductive tethers. The tethered array of nanoparticles, MnO in this case, bound directly to a gold current collector via short conducting linkages eliminates the need for fillers, resulting in a material which achieves 99.9% active material by mass (excluding the current collector). This strategy is expected to be both scalable as well as effective for alternative tethers and metal oxide nanoparticles.

The transient transport of electrolytes in thermally-activated batteries is studied using electron probe micro-analysis (EPMA), demonstrating the robust capability of EPMA as a useful tool for studying and quantifying mass transport within porous materials, particularly in difficult environments where classical flow measurements are challenging. By tracking the mobility of bromine and potassium ions from the electrolyte stored within the separator into the lithium silicon anode and iron disulfide cathode, we are able to quantify the transport mechanisms and physical properties of the electrodes including permeability and tortuosity. Due to the micron to submicron scale porous structure of the initially dry anode, a fast capillary pressure driven flow is observed into the anode from which we are able to set a lower bound on the permeability of 10-1 mDarcy. The transport into the cathode is diffusion-limited because the cathode originally contained some electrolyte before activation. Using a transient one-dimensional diffusion model, we estimate the tortuosity of the cathode electrode to be 2.8 ± 0.8.

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Our study was performed to validate a first-principles model for whisker and hillock formation based on the cyclic dynamic recrystallization (DRX) mechanism in conjunction with long-range diffusion. The test specimens were evaporated Sn films on Si having thicknesses of 0.25 μm, 0.50 μm, 1.0 μm, 2.0 μm, and 4.9 μm. Air annealing was performed at 35°C, 60°C, 100°C, 120°C, or 150°C over a time duration of 9 days. The stresses, anelastic strains, and strain rates in the Sn films were predicted by a computational model based upon the constitutive properties of 95.5Sn-3.9Ag-0.6Cu (wt.%) as a surrogate for pure Sn. The cyclic DRX mechanism and, in particular, whether long whiskers or hillocks were formed, was validated by comparing the empirical data against the three hierarchal requirements: (1) DRX to occur at all: εc = A D o m Z n , (2) DRX to be cyclic: D o < 2D r, and (3) Grain boundary pinning (thin films): h versus d. Continuous DRX took place in the 2.0-μm and 4.9-μm films that resulted in short stubby whiskers. Depleted zones, which resulted solely from a tensile stress-driven diffusion mechanism, confirmed the pervasiveness of long-range diffusion so that it did not control whisker or hillock formation other than a small loss of activity by reduced thermal activation at lower temperatures. Furthermore, a first-principles DRX model paves the way to develop like mitigation strategies against long whisker growth.

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Time-lapse SEM studies of Sn whiskers were conducted to estimate growth kinetics and document whisker morphologies. For straight whiskers, growth rates of 3 to 4 microns per day were measured at room temperature. Two types of kinked whiskers were observed. For Type A kinks, the original growth segment spatial orientation remains unchanged, there are no other changes in morphology or diameter, and growth continues. For Type B kinks, the spatial orientation of the original segment changes and it appears that the whisker bends over. Whiskers with Type B kinks show changes in morphology and diameter at the base, indicating grain boundary motion in the film, which eliminates the conditions suitable for long-term whisker growth. To estimate the errors in the whisker growth measurements, a technique is presented to correct for SEM projection effects. With this technique, the actual growth angles and lengths of a large number of whiskers were collected. It was found that most whiskers grow at moderate or shallow angles with respect to the surface; few straight whiskers grow nearly normal to the surface. In addition, there is no simple correlation between growth angles and lengths for whiskers observed over an approximate 2-year period. © 2012 The Minerals, Metals & Materials Society and ASM International (outside the USA).

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Tin (Sn) whiskers are conductive Sn filaments that grow from Sn-plated surfaces, such as surface finishes on electronic packages. The phenomenon of Sn whiskering has become a concern in recent years due to requirements for lead (Pb)-free soldering and surface finishes in commercial electronics. Pure Sn finishes are more prone to whisker growth than their Sn-Pb counterparts and high profile failures due to whisker formation (causing short circuits) in space applications have been documented. At Sandia, Sn whiskers are of interest due to increased use of Pb-free commercial off-the-shelf (COTS) parts and possible future requirements for Pb-free solders and surface finishes in high-reliability microelectronics. Lead-free solders and surface finishes are currently being used or considered for several Sandia applications. Despite the long history of Sn whisker research and the recently renewed interest in this topic, a comprehensive understanding of whisker growth remains elusive. This report describes recent research on characterization of Sn whiskers with the aim of understanding the underlying whisker growth mechanism(s). The report is divided into four sections and an Appendix. In Section 1, the Sn plating process is summarized. Specifically, the Sn plating parameters that were successful in producing samples with whiskers will be reviewed. In Section 2, the scanning electron microscopy (SEM) of Sn whiskers and time-lapse SEM studies of whisker growth will be discussed. This discussion includes the characterization of straight as well as kinked whiskers. In Section 3, a detailed discussion is given of SEM/EBSD (electron backscatter diffraction) techniques developed to determine the crystallography of Sn whiskers. In Section 4, these SEM/EBSD methods are employed to determine the crystallography of Sn whiskers, with a statistically significant number of whiskers analyzed. This is the largest study of Sn whisker crystallography ever reported. This section includes a review of previous literature on Sn whisker crystallography. The overall texture of the Sn films was also analyzed by EBSD. Finally, a short Appendix is included at the end of this report, in which the X-Ray diffraction (XRD) results are discussed and compared to the EBSD analyses of the overall textures of the Sn films. Sections 2, 3, and 4 have been or will be submitted as stand-alone papers in peer-reviewed technical journals. A bibliography of recent Sandia Sn whisker publications and presentations is included at the end of the report.

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