Growth Of Metal Whiskers Research Articles

Tin whisker growth from titanium-tin intermetallic and the mechanism

• Tin (Sn) whiskers were observed to grow out of Ti 6 Sn 5 intermetallic particles. • The critical role (as tin atom source) of Ti 6 Sn 5 intermetallic compound in Sn whisker growth is identified. • An atomic diffusion-based whisker growth model is proposed, and a new preparation method of metal nanowires is expected. Spontaneous growth of metal whiskers, represented by tin whiskers, has haunted tin-based platings and solder joints for decades and caused huge losses to the electronics industry. Despite numerous efforts, the underlying growth mechanism has been resisting interpretation, and the whiskering phenomenon even continues to expand its territory. Here, we report the growth of tin whiskers from a Ti 6 Sn 5 intermetallic. These tin whiskers share similar characteristics with those found on the platings or solder joints, but grow more and faster, with finer diameters. After tin whisker growth, Ti 6 Sn 5 retains its crystal structure, implying a dealloying process. Combining experimental and first-principles calculation results, we analyzed the growth mechanism of tin whiskers in detail, and proposed a diffusion-based growth model. The strain energy stored in Ti 6 Sn 5 during deformation provides a driving force for whisker growth, and the short-circuit diffusion paths generated by such deformation accelerate whisker growth. These findings identify the critical role of intermetallic substrate in the whiskering phenomenon, shedding new light for comprehensively understanding the whisker growth mechanisms. Furthermore, the plenty and rapid growth of tin whiskers also means a new method for the preparation of one-dimensional metallic materials.

Resistive Switching Memristor: On the Direct Observation of Physical Nature of Parameter Variability.

Ion-based memristive switching has attracted widespread attention from industries owing to its outstanding advantages in storage and neuromorphic computing. Major issues for achieving brain-inspired computation of highly functional memory in redox-based ion devices are relatively large variability in their operating parameters and limited cycling endurance. In some devices, volatile and nonvolatile operations often replace each other without changing operating conditions. To address these issues, it is important to observe directly what is happening in repeated operations. Herein, we use a planar device that enables direct capturing of microscopic behaviors in the nucleation and growth of metal whiskers under repeated switching to verify the microscopic origin of the large parameter variability. We report direct observations that reveal the physical origin for the large cycle-to-cycle and device-to-device variability in memristive switching, which was achieved using planar polymer atomic switches with a gap >1 μm. We find that the deposition location of metal atoms is closely related to the crystallinity of the ion transport layer (solid polymer electrolyte, SPE). The filament variability (shape, position, quantity, etc.) during different cycles and devices is indeed the main reason for the observed variability in the operating characteristics. The results shed unique light on the complexity of the operation of the ion device, that is, the evolution of the dielectric layer and metal filament must be considered.

Extraordinary Nanocrystalline Pb Whisker Growth from Bi-Mg-Pb Pools in Aluminum Alloy 6026 Moderated through Oriented Attachment.

The elucidation of spontaneous growth of metal whiskers from metal surfaces is still ongoing, with the mainstream research conducted on Sn whiskers. This work reports on the discovery of Pb whisker growth from Bi-Mg-Pb solid pools found in common machinable aluminum alloy. The whiskers and hillocks display unique morphologies and complex growth that have not been documented beforehand. In contrast to typical understanding of whisker growth, the presented Pb whiskers show a clear nanocrystalline induced growth mechanism, which is a novel concept. Furthermore, the investigated whiskers are also found to be completely composed of nanocrystals throughout their entire length. The performed research gives new insight into nucleation and growth of metal whiskers, which raises new theoretical questions and challenges current theories of spontaneous metal whisker growth. Additionally, this work provides the first microscopic confirmation of recrystallization growth theory of whiskers that relates to oriented attachment of nanocrystals formed within an amorphous metallic matrix. The impact of mechanical stress, generated through Bi oxidation within the pools, is theoretically discussed with relation to the observed whisker and hillock growth. The newly discovered nanocrystalline growth provides a new step towards understanding spontaneous metal whisker growth and possibility of developing nanostructures for potential usage in sensing and electronics applications.

Isotope study reveals atomic motion mechanism for the formation of metal whiskers in MAX phase

Spontaneous growth of metal whiskers has been studied for around 70 years, but still resists interpretations. Herein, the extrusion of whiskers from MAX phases, a group of nanolaminate ceramics, is surveyed using a combination of experiment and simulation methods. It was once believed that the excess A in MAX substrate provided element source for whiskers, while in this work, however, direct evidence from isotope experiment of Ti2SnC/120Sn demonstrates that the extruded Sn whiskers are composed of the atoms from both the excess Sn and the lattice Sn. Moreover, the Ti2AC/A’ (A, A’ = Sn, Ga) crossover experiment indicates that the extrusion of A whiskers is always companied with the substitution process of excess A (or A’) for lattice A atoms, with the driving force of which concluded to be the chemical potential gradient (or reduced formation enthalpy). Accordingly, a lattice diffusion based atomic motion mechanism is proposed in this work, to explain the whiskering behaviors in MAX phase.

Isotope Study Reveals Atomic Motion Mechanism for the Formation of Tin Whiskers

Spontaneous growth of metal whiskers has been studied for around 70 years, but still resists interpretations. Herein, the extrusion of whiskers from MAX phases, a group of nanolaminate ceramics, is surveyed using a combination of experiment and simulation methods. It was once believed that the excess A in MAX substrate provided the element source for whiskers, while in this work, however, results of isotope experiments suggest that the whiskers are composed of both atoms from lattice and excess. In addition, a selective extrusion phenomenon of A element from the Ti 2 AC substrate is determined in cross-over experiments. Combined with the DFT results, the reducing formation enthalpy is proposed to provide driving force for the spontaneous growth of whiskers from MAX substrate. Accordingly, a new atomic motion-based mechanism relating with the interaction between MAX substrate and excess A element is proposed. The present work also sheds new light on the study of tin whiskers spontaneously grown on other materials, due to the shared underlying principles with respect to atomic-motion.

Selective growth of tin whiskers from its alloys on Ti2SnC

The spontaneous growth of metal whiskers has been investigated for more than 70 years. However, there is still no agreement on its growth mechanism, and moreover, new characteristics of this whiskering phenomenon continue to emerge. In this study, Ti2SnC is found to be capable of extracting Sn out of its alloys (SnBi, SnAg) by selectively growing Sn whiskers, and the Sn whiskers share the features of the traditional whiskers on platings and solders. Replacing the Ti2SnC substrate with TiC or SiC, under the same conditions, however, the selective growth of Sn whisker does not happen, which means Ti2SnC plays a critical role in it. Based on the unique crystal structure of Ti2SnC, active Sn atoms diffusing through the basal planes of Ti2SnC is proposed to explain the selectivity. The driving force is suggested to be the high interfacial energy between Ti2SnC and tin. This study is of importance to further understand the growth mechanism of metallic whiskers, and it may be also possible to be harnessed to develop paradigm-shifting technologies of metal purification and metallic whisker/nanowire preparation.

The stochastic growth of metal whiskers

The phenomenon of spontaneously growing metal whiskers (MWs) raises significant reliability concerns due to their related arcing and shorting in electric equipment. The growth kinetics of MWs remains poorly predictable. Here, we present a theory describing the earlier observed intermittent growth of MWs as caused by local energy barriers related to variations in the random electric fields generated by surface imperfections. We find the probabilistic distribution of MW stopping times, during which MW growth halts, which is important for reliability projections.

Spontaneous growth of Sn whiskers and a new formation mechanism

The mechanism for spontaneous tin (Sn) whisker growth has been elusive for nearly 70 years. This study is to understand this phenomenon based on a new substrate system rather than traditional ones (Sn and its alloys). On ball-milled Ti/Sn/C samples, spontaneous growth of Sn whiskers different from general ones was observed. Ti2SnC grains were found to form during ball milling process, and the Sn atom planes of such Ti2SnC grains served as nucleation sites for Sn whiskers formation. The findings and the new mechanism proposed herein might renovate the research of spontaneous growth of metal whiskers, and are of significance to uncovering the phenomenon which resisted interpretation.

SU‐E‐T‐447: Growth of Metal Whiskers Under External Beam Irradiation: Experimental Evidence and Implications in Medical Electronic Devices for Radiation Therapy Treatments

Purpose:Thin metallic protrusions, termed “whiskers,” have been identified as a cause of failure in devices ranging from satellites to pacemakers. For decades, lead was used in tin‐based soldering alloys to suppress whisker formation. With the adoption of the Restriction of Hazardous Substances act and the expiration of its exemption on medical devices, including implanted medical devices (IMDs), electronic circuits are required to be lead‐free as of July 2014. The effect of radiation on such soldering components remains unknown.Methods:We have irradiated a thin (150 nm) tin metal layer, deposited on a 3 mm thick glass substrate, with a 6 MeV medical linac (Varian TrueBeam) electron beam in five 2‐hour long sessions. After receiving ∼10 kGy, whisker growth on the sample was assessed with scanning electron microscopy and compared to a reference sample not exposed to radiation.Results:After 10 hours of irradiation, the sample was found to develop intense whisker infestation, while the reference sample remained in its pristine as‐deposited condition. Repeating the same irradiation schedule generated more and longer whiskers. The observed phenomenon can be explained through charge accumulation in the glass substrate, generating an electric field that promotes whisker growth. The observed substrate glass darkening under irradiation points towards development of color centers related to charge trapping. Experiments on the same type of samples with direct application of the external field in a capacitor‐like setting also resulted in intense whisker growth.Conclusion:Extreme care should be taken in dealing with all electronic devices, especially IMDs, produced with lead‐free solder and components, subject to radiation exposure. While in our experiments strong electric fields were intentionally generated to accelerate whisker growth over hours, in everyday use the circuit soldering may cause problems in a matter of days or months. Designated reliability testing under radiation must be conducted.This work is partially supported by NRC grant No. NRC‐HQ‐12‐G‐38‐0042

Spontaneous Growth of Metal Whiskers on Surfaces of Solids: A Review

Spontaneous metal whisker growth phenomenon has been studied for ∼70 years, but still resists interpretations, which has haunted the electronics-rich systems from the birth of the industry since whiskers may lead to short circuits and hence other disasters. In addition to reliability problems, threat of Pb whiskers to human health is also significant due to the toxicity of the element and its geometry as well as its nanometer size. This issue becomes more attention drawing and urgent due to the restrictions on Pb use in EU, Japan etc., which phases out the Pb-addition practice to mitigate Sn whiskers. This paper reviews the origin of the whiskering phenomenon and whisker-induced problems, ranging from early in the World War II to the present; details the development of this issue in the electronics-rich systems including the phenomenon, problems and remedies; pictures the long history of the developments of the growth mechanisms and models which are full of hardship and contradictions. New aspects on the spontaneous whisker growth in some different substrates, such as in brasses and MAX phase materials are reviewed in more details. The latter was found to be a promising system to understand the whisker growth phenomenon, with new growth models proposed that may shed new light to the intricate battle field of the metal whisker research in general.

Effect of Electromigration and Isothermal Aging on the Formation of Metal Whiskers and Hillocks in Eutectic Sn-Bi Solder Joints and Reaction Films

Numerous electronic system failures have been attributed to short circuits caused by metal whiskers that bridged closely spaced circuit elements when they were maintained in a high-current-density environment. Typically, in single-phase interconnect materials, atoms are driven from the cathode to the anode and a compressive stress can build up at the anode end of the stripe, which can induce the formation of whiskers and hillocks. However, the electronic solders used in interconnects are multiphase materials where primary and secondary diffusion atoms/ions exist. In order to accelerate the growth of whiskers and hillocks, a high current density (10 4 A/cm 2 ) combined with high ambient temperatures (80C) was applied to eutectic Sn-Bi solder joints. Metal whiskers and hillocks were observed on the overflowed solder film above the Cu substrate. However, in the absence of the electric field, metal whiskers and hillocks could also be squeezed out from the solder reaction films after several days of isothermal aging (125C), demonstrating that the chemical reaction between Sn atoms and Cu atoms can provide the driving force for the formation and growth of metal whiskers and hillocks.

Vapor Phase Growth of Metal Nanowhiskers Induced by Glancing Angle Deposition at High Temperature

We demonstrate the growth of unusual metal nanowhiskers by glancing angle deposition on a high temperature (HT-GLAD) substrate, while the usual nanocolumnar structures completely disappear due to accelerated surface diffusion. HT-GLAD is essential for the nucleation of the nanowhiskers and efficient supply of metal atoms on the side surface of the vertically growing nanowhiskers. HT-GLAD will, for the first time, reveal the mechanisms for the vapor growth of metal whiskers and be useful to produce high-quality metal nanowires for novel device elements.

The origin of driving force for the formation of Sn whiskers at room temperature

Sn whiskers can form at room temperature on the agglomerated flakes produced by mechanical alloying (MA) of Ti, Sn, and C powders, whether the flakes are stored in air or water. The Sn whiskers forming in air are tens of micrometers to several centimeters in length and 0.5 to ∼10 μm in diameter. Whereas a large amount of Sn polyhedra forms on the flakes that are stored in water, a small amount of Sn whiskers forms on the polyhedra. The driving force for Sn whisker formation is the compressive stress induced by mechanical alloying (MA) and oxidation. The mechanism about the spontaneous growth of metal whiskers is discussed.

Analysis of the lattice diffusion-controlled growth of metallic whiskers

Whisker growth plays an important role in determining the performance and lifetime of micro/nano electronic devices. In this work, we analyse the contribution of lattice diffusion to the whisker growth in a bilayer structure. An analytical solution of the growth rate is obtained as a function of the average chemical stress and the whisker size. The growth rate is inversely proportional to the whisker size for the growth of a whisker over a semi-infinite crystal, while it is proportional to the thickness of the surface layer for larger ratio of the whisker size to the thickness of the surface layer. The analytical result provides us a rational basis to evaluate the growth behaviour of metallic whiskers and nanowires controlled by the lattice diffusion.

Vapor phase growth of Al whiskers induced by glancing angle deposition at high temperature

The authors demonstrate the growth of unusual Al whiskers by glancing angle deposition on a high temperature (HT-GLAD) substrate, while the usual columnar structures completely disappear due to accelerated surface diffusion. HT-GLAD is essential for the nucleation of the whiskers and efficient supply of Al atoms on the side surface of the vertically growing whiskers. HT-GLAD will, for the first time, reveal the mechanisms for the vapor growth of metal whiskers.

Pressure-Induced Growth of Metal Whiskers

Pressure-Induced Growth of Metal Whiskers

On the Growth of Metal Whiskers by Hydrogen Reduction of Halides

Observations have been made of the pressures of hydrogen and hydrogen halide gases prevailing during growth of a variety of metal whiskers by hydrogen reduction of volatile metal halides. The orders of magnitude of the maximum metal activities observed are only 1 to 100 rather than the maximum thermodynamically feasible values of 104 to 106 that might be attained in these reactions. This result is more in accord with current theories of whisker growth. It appears that the reduction of metal halide occurs primarily on the whisker surface and that metal halide vapor rather than metal vapor provides most of the vapor-phase mass transport during whisker growth by this process.