Growth Mechanism Of Whiskers Research Articles

Evidence for a High Temperature Whisker Growth Mechanism Active in Tungsten during In Situ Nanopillar Compression.

A series of nanopillar compression tests were performed on tungsten as a function of temperature using in situ transmission electron microscopy with localized laser heating. Surface oxidation was observed to form on the pillars and grow in thickness with increasing temperature. Deformation between 850 °C and 1120 °C is facilitated by long-range diffusional transport from the tungsten pillar onto adjacent regions of the Y2O3-stabilized ZrO2 indenter. The constraint imposed by the surface oxidation is hypothesized to underly this mechanism for localized plasticity, which is generally the so-called whisker growth mechanism. The results are discussed in context of the tungsten fuzz growth mechanism in He plasma-facing environments. The two processes exhibit similar morphological features and the conditions under which fuzz evolves appear to satisfy the conditions necessary to induce whisker growth.

Surface synthesis of aluminum borate whiskers on the ZTA ceramics and its application to joining

In this study, aluminum borate whiskers were prepared on the zirconia-toughened alumina (ZTA) substrates for the first time. This result was achieved by calcining a mixture of B2O3 powders with and without adding the MnO2 flux in air. The morphologies of as-prepared whiskers were carefully characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The length-diameter ratio of whiskers prepared by adding MnO2 flux is larger than those prepared by using only B2O3 powders, and the growth mechanisms of whiskers follow solid–liquid–solid theory. In addition, aluminum borate whiskers were successfully used to join ZTA ceramics. By adding the MnO2 flux in the interlayer, the strength of the joints can be significantly improved. The highest flexural strength for the ZTA/whiskers/ZTA joints bonded using B2O3 + 5 wt% MnO2 powders reached 162 ± 11 MPa at 1100 °C after holding for 4 h.

Tin Whisker Growth on Electronic Assemblies Soldered with Bi-Containing, Pb-Free Alloys

With the introduction of environmental legislation, lead (Pb)-free materials have made their way into the electronics supply chain. As a result, tin whiskers may initiate and grow under specific conditions. Sufficiently long whiskers may cause in-field short circuit failures, which are a serious concern in many high-reliability applications such as aerospace, automotive, and medical. There are efforts in developing third generation Pb-free solder alloys to replace tin-silver-copper (Sn-Ag-Cu, or SAC). Bismuth (Bi) has been considered for inclusion in SAC alloys to reduce melting point and improve reliability. In this study, we examine whisker formation on small outline transistor (SOT) components assembled with SAC-Bi alloys after ambient temperature (25°C) or high temperature (85°C), high humidity (85% RH) storage. Four alloys were considered—Senju M42 (Sn-2.0Ag-0.7Cu-3.0Bi), Violet (Sn-2.25Ag-0.5Cu-6.0Bi), Sunflower (Sn-0.7Cu-7.0Bi), and SAC305 (Sn-3.0Ag-0.5Cu). The assemblies were finished with immersion tin (ImmSn) or electroless nickel immersion gold (ENIG), and some were intentionally contaminated prior to exposure. It was found that SAC305 tended to grow longer whiskers than SAC-Bi alloys, though more whiskers were found on the latter. Particle Stimulated Nucleation (PSN) of recrystallization of the β-Sn matrix, influenced by Bi precipitation, appears to influence dynamic recrystallization (DRX), the mechanism for whisker growth.

Whisker Growth in Sn Coatings: A Review of Current Status and Future Prospects

Whiskering is a spontaneous filamentary growth of material, and it is a major long-term reliability issue affecting microelectronic packages comprising Sn plating and Sn-rich solders. In particular, whisker growth out of Sn-plated surfaces has been studied extensively in recent years due to the advent of next-generation, environment-friendly, Pb-free microelectronic packaging. Here, we review this scientifically challenging and technologically important problem, especially in the light of relatively new insights gained in the recent past, intending to provide a quick overview of the important results and stimulating future studies. In particular, we discuss the mechanisms of whisker growth by critically examining the roles of stress and its regeneration, oxide layer, diffusion conduits, and crystal anisotropy in creating conditions conducive for whiskering. We also discuss the recent proposals for effectively mitigating whisker growth in Sn coatings. Finally, an outlook is provided, with details of important unresolved issues related to whisker growth.

The effect of Bi addition on the formation of metal whiskers in Ti2SnC/Sn-xBi system

The spontaneous growth of tin whiskers on tin-rich solders and platings has haunted the electronics industry for more than 70 years. Reliable mitigation strategies are long-waited with the proceeding of lead-free movement. However, the development of mitigation strategies has been retarded for lack of a clear whisker growth mechanism. Herein, a new research platform of Ti2SnC is developed, in which fast tin whisker growth with excellent repeatability is realized. The sample of Ti2SnC/Sn-xBi is prepared using ball-milling method, to reveal the effect of Bi addition on the growth behavior of Sn whiskers. Compared with the Ti2SnC/Sn system, an extended nucleation period is identified in Ti2SnC–Sn5Bi, Ti2SnC–Sn15Bi, Ti2SnC–Sn20Bi, which mitigates the growth of Sn whiskers in the initial stage; however, when cultivated at proper parameters, the density of Sn whiskers is found to increase with the rising of Bi content. Finally, the growth kinetics was studied using ESEM. A linear relationship between whisker length and square root of the holding time is obtained, suggesting a diffusion-controlled mechanism for the growth of Sn whiskers in this work.

A comparative study on the growth behaviors of Sn whiskers and hillocks in a Sn-Al alloy coating under different environments

In this study, the effects of factors such as temperature, solution and surface roughness on the growth of whiskers in a 100 μm thick Sn-Al alloy coating were comparatively investigated under different environments. The results indicated that whiskers did not grow after nucleation at 30 °C in oil. With the increase in aging temperature, whiskers started to grow after nucleation. The morphologies of whiskers showed the filament-type at 50 °C and the hillock-type at 100 °C. The residual compressive stress in the coating gradually decreased with the growth of whiskers and hillocks. Whisker nucleation was not found on the coating kept in water solution. The residual stress in the coating was rapidly released through the micro-cracks at the coating/substrate interface instead of growing whiskers. The oxide layer on the coatings with rough surfaces was easier to crack due to the higher thermal stress. The thermal stress on the oxide layer was in directly proportional to the depth of indentation and inversely proportional to the grain size. Whiskers preferentially grew on the (001)-oriented Sn grains in the coating. The finite element simulation results indicated that the grains with c-axis perpendicular to the coating surface had higher thermal strain and tended to grow whiskers. During creep deformation, the stress exponent increased from 7.0 to 11.5 with the increase in temperature. The mechanism for whisker growth was dislocation slip. Grain boundaries were observed in the hillocks. The power-law breakdown was the mechanism for the growth of hillocks.

Carbothermal reduction synthesis of near-stoichiometric ZrC whiskers via V-L-S and S-L-S growth mechanism

Near-stoichiometric zirconi um carbide (ZrC) whiskers, which were efficiently synthesized from the mixture of ZrO2, C, Ni and NaF by carbothermal reduction at 1500 °C in Ar atmosphere, are about 1 μm in diameter and 30–50 μm in length with smooth surfaces. Near-stoichiometric ZrC whiskers were prepared by regulating the concentration of precursors (carbon black, catalyst Ni), subsequently vapor-liquid-solid (V-L-S) and solid-liquid-solid (S-L-S) growth mechanisms of ZrC whiskers were clearly revealed and related models were established.

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.

Mineralization of simulated flue gas to prepare CaCO3 whisker using suspended CaSO4

AbstractCaCO3 whiskers were synthesized controllably by introducing simulated flue gas containing CO2 and N2 into a CaSO4 suspension. The effects of solution pH, reaction temperature, and simulated flue gas on the formation of CaCO3 whiskers were studied. The growth mechanism and growth model of CaCO3 whiskers had also been provisionally recommended. The reaction product was characterized by scanning electron microscopy analysis, thermogravimetric analysis, and X‐ray diffraction analysis. CaCO3 whiskers with 97% purity were synthesized at pH 7.5, 80°C, 0.1 L/min CO2 flow rate, and 16.7% CO2 purity, with a length between 15 and 20 μm and an aspect ratio of about 12.

Inhibition of tin whisker by electroplating ultra-thin Co-W amorphous barrier layer

Selecting suitable barrier layer material is one of the crucial issues for solder microbump in advanced interconnect metallization. In this work, the inhibiting effect of two barriers, Co-W and Ni, on tin whisker growth in high temperature and humidity environment (55 °C/85% RH) were studied. Low-cost electrodeposition was conducted to prepare the 200 nm amorphous Co-W barrier layer on Cu substrate. Compared with the Ni barrier layer, the amorphous Co-W barrier layer can effectively reduce the formation of intermetallic compounds during the storage process, suppressing the growth of tin whiskers and oxidative corrosion. Based on experimental results and inner stress calculation, the mitigation mechanisms of tin whisker growth within the two barrier layers were proposed, respectively, which are of great importance to further understand the behavior of tin whisker mitigation.

μ-CT investigation of tin whisker growth mechanisms

The current paper considers the applicability of micro-tomographic methods for the investigation of growth mechanisms of metallic tin whiskers. Tin whiskers are metallic fibers that grow spontaneously from lead-free tin-coated surfaces, causing short circuit related issues in electronic devices, therefore making this phenomenon an interesting topic for in-depth analysis. In order to investigate such minuscule structures by X-rays, a tomographic setup employing a direct-converting pixel large area detector based on the Timepix readout ASIC is used. Featuring an extraordinary contrast and high dynamic range, these detectors have proven to be powerful tools in the analysis of samples containing fine features of low radiographic absorption.Initial tomographic results reveal fully 3D morphological information on tin whiskers, albeit at lower spatial resolution than by scanning electron microscope (SEM), which is the commonly used method to investigate this phenomenon. However, the additional morphological information obtained by micro-tomography gives additional means of analysis, likely to help understand the underlying growth mechanisms.

Recent Advances in Mitigation of Whiskers from Electroplated Tin

Tin whiskers pose an electrical reliability risk in the form of potential short circuits. This problem was solved in the past by adding a few percent of Pb during Sn electroplating, but with the ban on Pb in Europe and elsewhere, it has resurfaced. While various mitigation strategies have been formulated based on known whisker growth mechanisms, the electronic industry is still awaiting a satisfactory solution to the whisker growth problem in Pb-free electronics. Recent advances in the understanding and mitigation of whiskers are reviewed herein. The effects of various alloying elements/dopants are discussed in terms of the mechanisms that impact whisker growth kinetics and mitigation. Most recently, indium addition to Sn has been found to eliminate whiskers with even better mitigation performance than Pb addition. The mechanisms are discussed, and the important role of indium on the surface oxide and subsurface enrichment is highlighted.

Pressure Field Assisted Polycondensation Nonaqueous Precipitation Synthesis of Mullite Whiskers and Their Application as Epoxy Resin Reinforcement.

Mullite whiskers were novelty prepared via pressure field assisted polycondensation nonaqueous precipitation method. The precipitate phase transition in heating process, phase compositions and microstructure of samples calcined at different temperatures, effect of pressure field on precursors polycondensation and AlF3 amount on sample morphology, the structure and the growth mechanism of whiskers were investigated. The results indicate that pressure field caused by kettle treatment promotes the polycondensation reaction between AlF3 and tetraethyl orthosilicate (TEOS), the excess aluminum fluoride coordinates with the precipitate skeleton of the =Al–O–Si≡, which brings about the low mullitization temperature (900 °C). The sample prepared with the optimal amount of aluminum fluoride (1.3 of the theoretical amount) calcined at 1100 °C presents high yield and aspect ratio (>15, 100 nm in diameter) of mullite whiskers. Growth of whiskers prepared via pressure field assisted polycondensation nonaqueous precipitation method is attributed to a vapor-solid (VS) mechanism with the inducement of screw. These mullite whiskers with the structure of multi-needle whiskers connected in the same center can be distributed evenly in epoxy resin, which greatly improves the mechanical properties of epoxy resin.

Preliminary Study of Electrical Current Stressing on Tin Whisker Formation

The growth of Sn whiskers on Cu substrate Pb-free solder is considered as one of the emerging problems in electric and electronic devices as well as in aerospace interconnects. Electrical current stressing have been identified as a major driving force for Sn whisker formation. In this paper, we investigate the mechanism of Sn whisker growth through the effect of electromigration of Sn on copper substrate. Test samples were prepared by dipping method before proceed with current testing, and the characterization of whisker growths were evaluated using the Scanning Electron Microscopy (SEM) on the deposits near the anode and cathode areas. Results demonstrate that the accumulation took place at the anode side forming whiskers and depletion occured at the cathode side leading to the formation of void with the effect of current stressing associated with different exposure of time.

Mechanism and mitigation of spontaneous Ga whisker growth on Cr2GaC

Spontaneous growth of A-element whiskers on Mn+1AXn (MAX for short) phase materials poses a barrier to their practical applications, since it casts doubts on their stability. In this study, Ga whisker growth on sintered Cr2GaC samples was investigated. The elemental source for spontaneous growth of Ga whiskers is identified as the free Ga contained in the Cr2GaC material, not the lattice atoms from Cr2GaC grains, which removes the doubts on the stability of Cr2GaC material. The growth behavior and morphologies of the Ga whiskers follow a new catalysis-based model, with cleavage planes of Cr2GaC grains involved as nucleation sites. This model explains and predicts well the growth behavior of the whiskers. The mitigation strategy based on this model is in principle simple: to prevent free Ga in Cr2GaC material or limit it to a certain level; to avoid cleavage plane of Cr2GaC grains; to achieve high density of the Cr2GaC material.

Simple and novel synthesis of zirconia whiskers from a phosphate flux

High quality zirconia whiskers have been successfully prepared by molten salt method, using zirconium oxychloride (ZrOCl2·8H2O) and sodium phosphate tribasic dodecahydrate (Na3PO4·12H2O) as precursor and molten salt, respectively. The effects of types of molten salt and heat treatment temperature on the formation of zirconia whiskers were characterized by XRD, Raman, DTA-TG, FE-SEM, TEM, SAED and HR-TEM. When Na3PO4·12H2O is utilized as molten salt and the heat treatment temperature is 900 °C, the as-prepared zirconia whiskers with length ranging from 4 µm to 8 µm show an average aspect ratio of 25. The obtained ZrO2 whiskers with monoclinic structure are elongated along [010] direction and exhibit a smooth surface with no distinct defects. The XRD and Raman results reveal that the phase transformation from tetragonal zirconia to monoclinic zirconia occurs with the increased crystal size and the water quenching treatment can significantly reduce the content of sodium zirconium phosphate [Na9–4×Zrx(PO4)3] in the final product. The growth mechanism of zirconia whiskers is supposed to be a dissolution-precipitation process. Since the sodium zirconium phosphate [Na9–4×Zrx(PO4)3] effectively promotes the dissolution of zirconia in liquid molten salt, zirconia can grow into zirconia whiskers according to its anisotropy.

Novel facile nonaqueous precipitation in-situ synthesis of mullite whisker skeleton porous materials

High strength, lightweight mullite whisker skeleton porous materials were prepared via novel facile nonaqueous precipitation method. The phase transition process of precipitate, microstructures and phase compositions of samples, and effects of aluminum fluoride amount on morphologies and properties of mullite whisker skeleton porous materials were investigated by DTA-TG, FE-SEM, XRD, the acid solubility and compressive strength tests. The growth direction of the whisker was determined. The growth mechanism of mullite whisker and formation mechanism of skeleton porous material were also investigated. The results show that the mullite phase forms at a low temperature of 900 °C via nonaqueous precipitation method, due to the formation of Al-O-Si bonds in the precipitate. The formation of mullite whiskers is controlled by gas-solid reaction and the whiskers grow along [001] direction. Well-developed mullite whiskers interlacedly grow into skeleton porous material at 1100 °C. The optimum mullite whisker skeleton porous material with the optimized aluminum fluoride amount of 1.3 of theoretical amount exhibits attractive properties of low density (1.21 g cm−3), high strength (158.6 MPa), low acid solubility (1.58 wt%), low thermal expansion coefficient (1.6 × 10−6 °C−1) and low thermal conductivity (0.28 W/(m K)). This material has the mean pore size of 542 nm and pore size distribution of 32–865 nm. It shows attractive application prospects in filtering and biomedical applications.

Sn Whiskers Nucleation and Growth - Short Review

Sn whisker growth on Cu substrate Pb-free solder is a serious problem in electric and electronic devices and as well as in aerospace applications. Due to the concern on the toxicity of lead by Restriction of Hazardous Substances Directive (RoHS), new lead free materials have been developed, and this resulted in the resurfacing of Sn whisker. The compressive stress, corrosionand surface oxide have been identified as the driving force for Sn whisker formation induced by mechanical alloying and oxidation. In this paper, we report the study to understand the mechanism of Sn whisker growth that control whisker formation on Sn finished.Based on the review, a preliminary conclusion has been made, where the analysis of the topography and microstructural characterization can be determined by evaluating under various environmental influences.Furthermore, the whisker growth happening on lead-free soldered can be considerably reduced by controlling the compressive stress in the solder which initiates the growth of intermetallic compounds (IMCs).

Spontaneous Tin (Sn) Whisker Growth from Electroplated Tin and Lead-Free Tin Alloys Coatings: A Short Review

Since the environmental regulations of Reduction of Hazardous Substances (RoHS) directive came into effect in Europe and Asia on July 1, 2006, requiring the removal of any lead (Pb) content from the electronics industry, the issue of tin (Sn) whisker growth from pure Sn and SnPb-free alloys has become one of the most imperative issues that need to be resolved. Moreover, with the increasing demand for electronics miniaturization, Sn whisker growth is a severe threat to the reliability of microelectronic devices. Sn whiskers grow spontaneously from an electrodeposited tin coating on a copper substrate at room temperature, which can lead to well-documented system failures in electronics industries. The Sn whisker phenomenon unavoidably gives rise to troubles. This paper briefly reviews to better understand the fundamental properties of Sn whisker growth and at the same time discover the effective mitigation practices for whisker growth in green electronic devices. It is generally accepted that compressive stress generated from the growth of Cu6Sn5 intermetallic compound (IMC) is the primary driving force for Sn whisker growth during room temperature storage. It is, therefore, important to determine that the relationship between IMC growth and Sn whisker growth. Reduction of stress in the IMC layer can therefore reduce the driving force for whisker formation and be used as a means for whisker mitigation. To date, there are no successful methods that can suppress the growth of Sn whisker as efficient as Pb addition. It is hoped that the Sn whisker growth mechanisms are understood better in the future, with better measuring and monitoring methodologies and systems being developed, the real solutions may be eventually developed to eliminate or mitigate the Sn whisker problems of green reliability lead-free electronic assemblies.

Effect of Bi2O3 on phase formation and microstructure evolution of mullite ceramics from mechanochemically activated oxide mixtures

Mullite ceramics with hollow whisker structure have been synthesized firstly through ordinary sintering process. The effects of Bi2O3 and processing, on mullitization behavior and morphology development of mullite ceramics, derived from the mechanochemically activated mixture of Al2O3 and SiO2, were investigated in this paper. When the content of Bi2O3 was less than 10 mol%, the mullite grains show a short rod-like morphology, without the formation of whisker. As the content of Bi2O3 was increased to more than 10 mol%, the formation temperature of mullite was decreased from 1400 °C to 1100 °C. After sintering at 1400 °C, well-developed mullite whiskers with hollow structure were formed. The formation process and growth mechanism of hollow structural whiskers in mullite ceramic doped with high content of Bi2O3 were discussed in detail.