Understanding and predicting metallic whisker growth and its effects on reliability : LDRD final report.

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

Electrodeposition of High Magnetostrictive Cobalt Iron Alloy Films for Smart Tags and Sensor Applications

Magnetostrictive CoFe films were investigated for use as magnetoelastic tags or sensors. The ability to electrodeposit these films enables batch fabrication processes to pattern a variety of geometries while controlling the film stoichiometry and crystallography. In current research looking at CoFe, improved magnetostriction was achieved using a co-sputtering, annealing, and quenching method1. Other current research has reported electrodeposited CoFe films using a sulfate based chemistry resulting in film compositions that are Fe rich in the range of Co0.3-0.4Fe0.7-0.6 and have problems of co-deposition of undesirables that can have a negative impact on magnetic properties2, 3. The research presented here focused on maximizing magnetostriction at the optimal stoichiometry range of Co0.7-0.75Fe0.3-0.25, targeting the (fcc+bcc)/bcc phase boundary, and using a novel chemistry and plating parameters to deposit films without being limited to line of sight\u27 deposition1. To obtain the desired compositional range, a chemistry was selected to allow for a higher ratio of Co while maintaining stability and limiting the oxidation of the Fe2+ to Fe3+. As suggested by Osaka et al., Fe(OH)3 is formed and included into the film resulting in a decrease of the saturation magnetic flux density (Bs) value as the Fe cation is oxidized2. This led to a deviation from the traditional sulfate based chemistry used to deposit CoFe alloy thin films and the inclusion of additives acting as oxygen scavengers to stabilize deposition. The characteristics of the deposited films were controlled through the additives, temperature, agitation, concentrations, current density, and duty cycle of the pulsing regime. After initial chemistry characterization to determine the kinetics and mass transfer limitations, samples were plated across a range of current densities and duty cycles onto copper tuning fork substrates that enabled magnetic testing to be performed. The samples were then analyzed with EDS to determine the composition. Magnetic testing was performed using super conducting quantum interference device measurements (SQUID), as well as visual inspection of the displacement on a deposit stress analyzer as a magnetic field was applied to the films. The magnetostriction was then correlated to stoichiometry and the plating parameters to characterize magnetostriction performance. Electrochemical studies were conducted to examine the kinetic rate for the reduction of the cobalt iron alloy as a function of additive concentrations. The oxygen scavenger additives were found to increase the kinetics while anodically shifting the reduction peak for the alloy. The leveling and brightening agents shifted the reduction peak cathodically and decreased the standard rate constant. Adjusting the concentration of ascorbic acid minimized the cathodic shift and decrease in the kinetic rate caused by the brightening additives.\u2

Electrochemistry of Gadolinium-Polyoxometalate Complexes in Concentrated Salt Solutions

Lanthanide-polyoxometalate complexes have been recently shown to possess unique magnetic, luminescent, and catalytic properties. While their electrochemical behavior has been explored in literature to some degree, the role of metallic spe-cies within the POM and nature and concentration of counter-cations in solutions has not been fully explained. The cur-rent work seeks to address this by studying the Gd(OTf)3 – [H2W12O40]6-(MT) – Si(W12O40)-4 system in concentrated LiCl in N,N-dimethylformamide (DMF) using electrochemical and spectroscopic techniques. It was found that Gd3+ did not chem-ically bond to the POM species but interacted electrostatically through the Ot bonds of the MT POM but not Si based POM. Electrochemically, Gd3+ showed more reversible electrochemistry but was able to gain an additional electron in the pres-ence of the MT- species. The Si-POM did not show much electrochemical behavior in the presence of Li+ and Gd3+ likely due to its non-interaction with both species, found using FTIR. The current work presents a step forward in the under-standing of the role of metallic and cationic species in the electrochemical behavior of lanthanide-POM complexes

Understanding and predicting metallic whisker growth and its effects on reliability : LDRD final report.

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