Growth Mechanism Of Whiskers Research Articles

<i>In Situ</i> Growth of TiB<sub>2</sub> Whiskers in TiB<sub>2</sub>-TiC<sub>x</sub> Matrix Powder

The TiB2 whiskers were fabricated in TiB2/TiCx composites by the in-situ synthesis process. The phase composition, thermodynamics, weight loss and lattice parameters of the obtained phases were studied. The obtained TiB2 whiskers showed an aspect ratio of 10-20 and a diameter of 0.5-1.5 μm. The synthesized TiC was a non-stoichiometric phase with a lattice parameter of 4.274 Å. Most of the whiskers had the cylinder shape, whereas, a few whiskers showed a bead-necklace morphology. The growth mechanisms of TiB2 whiskers were governed by the solid-liquid-solid (S-L-S) mechanism and the particle-to-particle connection mechanism.

Study on the synthesis and growth mechanisms of the refractory ZrC whiskers

ZrC whiskers, which can be used as a potential reinforcing and strengthening phase for ceramic composites, were successfully fabricated in an economical and efficient way by the carbothermal reduction technology. ZrC whiskers were synthesized from the mixture of ZrO2, C, Ni and NaF at 1500°C in an argon atmosphere. The length, diameter and aspect ratio ranged from 60 to 100μm, 1 to 3μm and 20 to 50, respectively. Whiskers exhibited two major morphologies which were prismatic-like and cylindrical-like shapes. Combined with thermodynamic calculation and the morphology, the solid–liquid–solid (S–L–S) mechanism and the Ostwald ripening were considered to govern the whisker growth.

Growth behaviors of tin whisker in RE‐doped Sn‐Zn‐Ga solder

PurposeThe purpose of this paper is to investigate the growth behavior and mechanism of Sn whisker induced by RE addition in Sn‐Zn‐Ga‐Pr solder at ambient condition.Design/methodology/approachBy means of aging treatment, FIB and SEM microstructure analysis, the whisker growth behavior was investigated.FindingsIt was found that the morphologies of tin whisker are changed during air exposure. After 60 days aging, the average length of the longest whiskers could reach up to 70 μm, some whiskers even can grow to a length of 100 μm. It was discussed that the oxidation of Pr‐Sn intermetallics provides driving force for whisker growth.Originality/valueTin whisker growth is a complex reliability issue for lead‐free solder. The current research can be helpful in re‐understanding the issue of tin whisker growth as well as an enriched understanding on the effects of REs on lead‐free solders.

リード線溶接における Sn ウィスカ発生メカニズムに関する研究

This paper describes the mechanism of Sn whisker growth in the welding leads of aluminum electrolytic condenser. Such whiskers are crystalline, have lengths of several microns to several millimeters, and can occasionally result in device failure. The leads used for aluminum electrolytic condensers are composed of Sn- and Cu-plated Fe and Al wire by arc-stud welding. Observations have shown that the Sn whiskers grow in Al-Sn alloy regions at the welding point, where a three-dimension network of Sn exists as a result of rapid solidification from the melt in Al-Sn alloy. This network is microstructure of rapidly solidified from the melt. This one for the most part is occluded by Al, and has slightly opening to surface. It is considered that the driving force for whisker formation is the compressive stress that is present in the Sn network as a result of the different shrinkage rates of Al and Sn during solidification.

<i>In Situ</i> Synthesis of SiC and β-Si<sub>3</sub>Al<sub>3</sub>O<sub>3</sub>N<sub>5</sub> Whiskers in Alumina Carbon Based Materials

SiC whiskers are the common reinforced phases in alumina carbon refractories. In this paper, the effect of transition metal catalyst on the evolution of matrix phase and the formation of β-Si3Al3O3N5 whiskers were studied. It was found that the whiskers of both SiC and β-Si3Al3O3N5 in alumina carbon materials with the catalyst in reducing atmosphere were generated. The existence of catalyst can promote the nitriding process of Si metal and facilitate the formation of β-Si3Al3O3N5 whiskers. The growth mechanism of both SiC and β-Si3Al3O3N5 whiskers agrees with the vapor-liquid-solid model.

Effects of thermal cycling on rare earth (Pr)-induced Sn whisker/hillock growth

The addition of rare earth (RE) into Pb-free solders has been shown to induce spontaneous Sn whisker growth on RE-rich intermetallics (IMCs). In this research, whisker growth behaviors under ambient and thermal cycling conditions were investigated to address the RE-induced Sn whisker growth mechanism. Unlike the needle-like whiskers grown mainly in ambience, it was found that coarse Sn hillocks were the dominant growth morphology under thermal cycling condition. It was discussed that the CTE mismatch between RE-rich PrSn3 IMCs and surrounding solder matrix induces compressive stress within the IMCs during thermal cycling, which contributes to the driving force for hillock growth.

Synthesis of ZnO whiskers via EDTA-assisted hydrothermal routes

Uniform ZnO whiskers with a length of 3.5–4.0 μm and a diameter of 180 nm have been synthesised via the EDTA–assisted hydrothermal route, using ZnO nanoparticles as the raw material and NaOH solution as the reaction medium. In order to convert the ZnO nanoparticles to ZnO whiskers via the dissolution–hydrothermal precipitation route, ZnO nanoparticles were firstly dissolved in concentrated NaOH solutions at room temperature and then diluted with water to form the super–saturated meta–stable solution before hydrothermal treatment. The presence of minor amount of EDTA favoured the 1D growth of ZnO whiskers. The possible growth mechanism of ZnO whiskers and the effect of EDTA were discussed from the viewpoint of thermodynamic analysis.

Mitigation and Verification Methods for Sn Whisker Growth in Pb-Free Automotive Electronics

This work describes mitigation methods against Sn whisker growth in Pb-free automotive electronics using a conformal coating technique, with an additional focus on determining an effective whisker assessment method. We suggest effective whisker growth conditions that involve temperature cycling and two types of storage conditions (high-temperature/humidity storage and ambient storage), and analyze whisker growth mechanisms. In determining an efficient mitigation method against whisker growth, surface finish and conformal coating have been validated as effective means. In our experiments, the surface finish of components comprised Ni/Sn, Ni/SnBi, and Ni/Pd. The effects of acrylic silicone, and rubber coating of components were compared with uncoated performance under high-temperature/humidity storage conditions. An effective whisker assessment method during temperature cycling and under various storage conditions (high temperature/humidity and ambient) is indicated for evaluating whisker growth. Although components were finished with Ni/Pd, we found that whiskers were generated at solder joints and that conformal coating is a useful mitigation method in this regard. Although whiskers penetrated most conformal coating materials (acrylic, silicone, and rubber) after 3500 h of high-temperature/humidity storage, the whisker length was markedly reduced due to the conformal coatings, with silicone providing superior mitigation over acrylic and rubber.

Microstructure and growth mechanism of tin whiskers on RESn3 compounds

An exclusive method was developed to prepare intact tin whiskers as transmission electron microscope specimens, and with this technique in situ observation of tin whisker growth from RESn3 (RE=Nd, La, Ce) film specimen was first achieved. Electron irradiation was discovered to have an effect on the growth of a tin whisker through its root. Large quantities of tin whiskers with diameters from 20nm to 10μm and lengths ranging from 50nm to 500μm were formed at a growth rate of 0.1–1.8nms−1 on the surface of RESn3 compounds. Most (>85%) of these tin whiskers have preferred growth directions of 〈100〉, 〈001〉, 〈101〉 and 〈103〉, as determined by statistics. This kind of tin whisker is single-crystal β-Sn even if it has growth striations, steps and kinks, and no dislocations or twin or grain boundaries were observed within the whisker body. RESn3 compounds undergo selective oxidation during whisker growth, and the oxidation provides continuous tin atoms for tin whisker growth until they are exhausted. The driving force for whisker growth is the compressive stress resulting from the restriction of the massive volume expansion (38–43%) during the oxidation by the surface RE(OH)3 layer. Tin atoms diffuse and flow to feed the continuous growth of tin whiskers under a compressive stress gradient formed from the extrusion of tin atoms/clusters at weak points on the surface RE(OH)3 layers. A growth model was proposed to discuss the characteristics and growth mechanism of tin whiskers from RESn3 compounds.

Crystallographic Characterization of an Electroplated Zinc Coating Prone to Whiskers

A Zn-electroplated steel prone to Zn whiskers has been investigated in order to obtain information about the microstructure of the Zn coating at the root of a whisker. Indeed, the characterization of the Zn coating is essential to understand the Zn whisker growth mechanism. Care was taken to prepare a sample at a whisker root by using a focused-ion beam in a dual-beam scanning electron microscope. The sample was analyzed using energy dispersive X-ray spectrometry and electron backscattered diffraction. Results show three different regions: 1) an inclusion enriched with Ca, Al, and C; 2) the Zn coating with columnar grains; and 3) the root of the whisker. An important point is that recrystallized grains are found at the whisker root. This observation supports recent whisker growth models based on recrystallization.

Synthesis of Mg2B2O5 whiskers via coprecipitation and sintering process

Mg2B2O5 whiskers with high aspect ratio were synthesized by coprecipitation and sintering process using MgCl2·6H2O, H3BO3, and NaOH as raw materials and KCl as a flux. Their formation process was investigated by thermogravimetry and differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), and scanning electron microcopy (SEM). It is found that the products synthesized at 832°C are monoclinic Mg2B2O5 whiskers with a diameter of 200–400 nm and a length of 50–80 μm. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) analyses show that the whiskers obtained at 832°C are single crystalline and grow along with the [010] direction. The growth mechanism of Mg2B2O5 whiskers was also presented.

Mg3Cd: A model alloy for studying the destabilization of magnesium hydride

We prepared an ordered Mg3Cd alloy by high energy ball milling of elemental powders. The synthesized alloy exhibited good hydrogenation kinetics and reversibly absorbed about 2.8 wt. % of hydrogen. The temperature dependence of hydrogenation kinetics of the alloy measured in the range of temperatures covering the order-disorder phase transformations in the Mg3Cd and MgCd phases did not exhibit any anomalies and could be fitted with a single Arrhenius line. The measured apparent activation energy (69 ± 2 kJ/mol) hinted that hydrogenation process was controlled by diffusion of Cd in metallic phase. The pressure-composition isotherms exhibited negligible pressure hysteresis and sloping pressure plateau. Based on microstructural evidence obtained with the aid of X-ray diffraction and scanning electron microscopy, we built a thermodynamic model predicting the plateau hydrogen pressure for partially hydrogenated alloy. The predictions of the model were in a good agreement with the experimental data. Finally, we discussed the origins and the growth mechanisms of Cd whiskers observed in the alloys after full hydrogenation cycle.

Preparation and growth mechanism of α-Bi2O3 crystal whiskers

Precursor of BiOCl was synthesized using BiCl3 and NaOH. Bi2O3 whiskers with high aspect ratio were prepared through reversing-titration method, i.e., dispersing BiOCl in Na2SO4, then dropped it in NaOH solution. The formation of monoclinic α-Bi2O3 crystal whiskers was characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). SEM and TEM results revealed the aspect ratios of Bi2O3 whiskers were from 10 to 40, and the morphology of α-Bi2O3 evolved from single-needle-like to multi-needle-like crystal whiskers with the increase of Na2SO4 concentration. Selected area electronic diffraction saed (SAED) patterns showed that the Bi2O3 whiskers were single crystalline grown along [210] direction. Furthermore, we discussed the mechanism of crystal growth.

Lead-free solders: Reliability, an ongoing saga

To read more about him, turn to page 9. To join TMS, visit www.tms.org/Society/Membership.aspx. It has been half a decade since the legislated ban on lead in electrical and electronic equipment throughout the European Union according to Restriction of the use of certain Hazardous Substances in Electrical and Electronic Equipment directives.1 In response, “Management Methods for Prevention and Control of Pollution from Production of Electronic and Information Products”2 was drafted by China, while in the United States, California legislated electronic waste recycling.3 In response, the Electronic Packaging/ Interconnection Materials Committee of the TMS Electronic, Magnetic & Photonic Materials Division organized a series of symposia on lead-free and lead-bearing solders. The results were published4–17 in JOM and special issues of the Journal of Electronic Materials. Continuing the trend, the “Pb-Free Solders and Other Materials for Emerging Interconnect and Packaging Technologies” symposium was held February 27–March 3, 2011 at the TMS Annual Meeting. At the symposium organized by Idranath Dutta et al., more than 60 papers dealing with next generation packaging, whisker growth, alloy and microstructural development, thermomechanical behavior, electromigration, processing and reliability for lead-free solders were presented. The four papers selected for this issue deal with tin whiskers, three-dimensional (3-D) interconnects, and polymeric thermal interfaces. Pure Sn fi nish is fast becoming the most economical way of replacing SnPb solder on lead frames. However, plated pure Sn can nucleate and grow whiskers that are detrimental to the operation of the devices and printed circuit board assembly. In the fi rst paper J.W. Osenbach presents the variety of different Sn whisker morphologies reported in literature since late 1940s when the whisker growth was fi rst identifi ed and published. The paper by E. Chason et al. throws light on the main driving force for whisker formation, namely, compressive stresses from intermetallic compound growth. To better understand the whisker growth mechanism, the authors studied the effect of Sn layer properties such as grain size, thickness, microstructure, etc. on the evolution of whiskers. In the quest for miniaturization and faster clock speeds electronic packaging has gone vertical by stacking die to maximize real estate. Through-silicon Via (TSV) is one of the enabling technology trends in 3-D packages. However, these vertically stacked packages are fraught with reliability concerns due to contrasting properties of Cu fi lled TSVs and rigid silicon die. I. Dutta et al. discuss the results that illustrate the reliability challenges facing the 3-D packages with TSVs. The last paper from D.F. Rae et al. focuses on the thermal interface materials that improve the heat dissipation from devices to improve reliability and extend device life. In this paper the effect of metal fi ller material properties and processing on the overall thermal properties of a polymeric thermal bond-line are discussed. References

Sn whisker growth during thermal cycling

Pure Sn plating on ceramic chip capacitors was tested by thermal cycling both in air and in vacuum for up to 3000 cycles and the whisker growth mechanism was clarified. A thin crystalline SnO layer is formed on the surface of Sn plating and whiskers, which exhibits a high level of cracking. Thermal cycling whiskers exhibit two distinct features: fine striation rings on the whisker side face vertical to the whisker growth axis; and deep grooves at the root of the whiskers. One ring of the fine striations corresponds to each thermal cycle. The formation of the grooves can be attributed to thermal cycle cracking along grain boundaries of Sn followed by oxidation and growth of whiskers from the root grains. The characteristic winding feature observed for thermal cycling whiskers can be attributed to the formation of root grooves with severe oxidation. Whisker growth in vacuum is faster than in air. Whiskers grown in vacuum are thinner and longer than whiskers grown in air, while the whisker density is not influenced by atmosphere. The interval of striation rings is wider for vacuum-grown whiskers as compared with air-grown whiskers.

The mechanism of oxide whisker growth and hot corrosion of hot-dipped Al–Si coated 430 stainless steels in air–NaCl (g) atmosphere

The mechanisms of oxide whisker growth and hot corrosion of 430 stainless steel (430SS) and aluminide 430 stainless steel hot-dipped in a Al–10 wt.%Si molten bath (430HDAS) were studied at 750 and 850 °C in air mixed with 500 and 990 vppm NaCl (g). The results showed that the loose Cr 2O 3 scale which formed on the 430SS could not prevent the corrosion of 430SS in a 500 vppm NaCl (g) atmosphere, resulting in the formation of Fe 2O 3 scale. Fe 2O 3 whiskers grew at the grain boundary of the Fe 2O 3 scale. However, no Fe 2O 3 whiskers formed on the Fe 2O 3 scale while 430SS was exposed in a 900 vppm NaCl (g) atmosphere. During the initial high-temperature corrosion of 430HDAS in a 500 vppm NaCl (g) atmosphere, a dense Al 2O 3 scale formed on the surface of the specimens. Also, Al 2O 3 whiskers grew on the Al 2O 3 scale. As exposure time increased, cyclic chlorination/oxidation degraded the protective aluminide layer and caused the formation of Fe 2O 3 scale and Fe 2O 3 whiskers. The morphology of Fe 2O 3 whiskers formed at 750 °C is more slender than those formed at 850 °C. The formation and growth of both Fe 2O 3 and Al 2O 3 whiskers may be attributed to the chloridation of both the steel substrate and aluminide layer, accelerating the diffusion rate of metallic ions in the oxide scales.

Phase transformation process and step growth mechanism of hydroxyapatite whiskers under constant impulsion system

Hydroxyapatite (HAP) whiskers were synthesized using urea as the precipitator by a phase transformation method, and their phase transformation process and growth mechanism were investigated. The results showed that with the decomposition of urea and the corresponding increase of pH value of the reaction system, dicalcium phosphate anhydrous (DCPA) and octacalcium phosphate (OCP) were precipitated at pH of 3.3–4.3; then Ca 2+ and HPO 4 2 − ions began to be released from DCPA at pH values greater than 4.5. Finally HAP whiskers heterogeneously nucleated and grew up into short column crystals along the surface of the OCP flakes. In the absence of the ionic resources, DCPA gradually dissolved and the OCP flakes transformed into HAP continuously and the short columnar HAP whiskers grew up. The aspect ratio of the HAP whiskers with length of 20–100 μm and diameter of 1–2 μm was about 25. The HRTEM and AFM images showed that HAP whiskers grew along the c-axis direction, the (1 0 0) steps were clearly observed at their heads and the straight step lines instead of helical Frank ones were present on the side face of the (1 0 0) steps. The calculation on the basis of the surface energy of the HAP crystal showed that the growth rate of the (0 0 1) plane was the fastest, the growth rate at the homogeneous twist sites was the second and that at heterogeneous twist sites could be the slowest, which were the main factors finally leading to the preferential growth of HAP whiskers along the c-axis direction as well as the formation of the growth steps.

Preparation and Growth Mechanism of Magnesium Borate Whiskers with High Aspect Ratio

Magnesium borate whiskers were prepared by molten salt method using MgCl2?6H2O and Na2B4O7?10H2O as raw materials,KCl and NaCl as fluxes.The lengths of the above whiskers varied from 200μm to 800 μm,and the maximum aspect ratio reached 250.The produced samples were characterized by X-ray diffraction(XRD),microscope and scanning electron microscope(SEM).The effects of n(Mg)/n(B),reaction temperature and reaction time on the whisker growth process and product quality were explored.It was found that the best operating parameters of n(Mg)/n(B),reaction temperature and reaction time were 3.0,900 ℃ and 6h,respectively.According to the XRD analysis,Mg3B7O13Cl is the intermediate product,and the growth of Mg2B2O5 whiskers needs a boron-enriched environment.The whisker growth process agrees to the liquid-solid(L-S) mechanism.

Effect of intermetallic growth rate on spontaneous whisker growth from a tin coating on copper

Intermetallic compound (IMC) growth at the interface between a Sn coating and a Cu substrate with or without a Ni underlayer and its related stress state was evaluated by real-time measurements using the flexure beam method. For the Sn coating without a Ni underlayer, pyramid-shaped IMC grains of Cu6Sn5 grew along the grain boundaries of Sn from the Cu substrate, especially at the triple grain boundary junctions and the IMC grains rapidly increased their volume during the initial 3 days, During this time, the IMC growth rate was 2.3 μm3/day and the compressive stress in the Sn coating rapidly developed and became saturated at about −11 MPa. In contrast, platelet IMC grains of Ni3Sn4 formed at the surface of the Ni underlayer and the IMCs grew at a rate of 1.0 μm3/day for the Sn/Ni coating. In addition, the stress in the Sn coating with a Ni underlayer remained low, slightly tensile and no whiskers were formed on the Sn plating. Therefore, the IMC growth rate and shape are key factors that affect the mechanism of Sn whisker growth during room temperature storage.

Vapor–solid–solid Si nano-whiskers growth using pure hydrogen as the source gas

This paper describes the growth mechanism and structure of Si whiskers grown on a Si substrate in a tungsten hot filament chemical vapor deposition reactor with pure hydrogen as source gas using a two step process. In the first step, atomic hydrogen etched the silicon surface, forming silicon hydrides that react with tungsten from the filament. The resulting silicide particles deposited on the silicon surface forming a mesh-like pattern. The particles work as an etching mask against hydrogen radical etching of the silicon surface and inverted-pyramids or V-groove-shaped surface texture were obtained. In the second step the filament current was reduced and whiskers grew onto the substrate due to the interaction of the silicon hydrides with the particles and subsequent precipitation of saturated Si. The whiskers were found to have tungsten silicide particles on their tip, suggesting the whisker growth was through the Vapor–Solid–Solid (VSS) mechanism. A balance between the hydrogen radical etching effect and the supply of silicon hydride from the etching reaction on the silicon surface is crucial for the growth of dense silicon whiskers.