Sn Whisker Growth Research Articles

Kinetics of Sn whisker growth from Sn thin-films on Cu substrate

The kinetics of Sn whisker growth was investigated on vacuum-evaporated Sn thin-films. Sn film layers were deposited on a Cu substrate with 0.5 and 1 µm thicknesses. The samples were stored in room conditions (22 ± 1 °C/50 ± 5RH%) for 60 days. The Sn whiskers and the Cu–Sn layer structure underneath them were investigated with both scanning electron and ion microscopy. Fast Cu–Sn intermetallic formation resulted in considerable mechanical stress in the Sn layer, which initiated intensive whisker growth right after the layer deposition. The thinner Sn layer produced twice many whiskers compared to the thicker one. The lengths of the filament-type whiskers were similar, but the growth characteristics differed. The thinner Sn layer performed the highest whisker growth rates during the first 7 days, while the thicker Sn layer increased the growth rate only after 7 days. This phenomenon was explained by the cross-correlation of the stress relaxation ability of Sn layers and the amount of Sn atoms for whisker growth. The very high filament whisker growth rates might be caused by the interface flow mechanism, which could be initiated by the intermetallic layer growth itself. Furthermore, a correlation was found between the type of the whiskers and the morphology of the intermetallic layer underneath.

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.

Tin whisker growth on immiscible Al–Sn alloy

Spontaneous growth of Sn whiskers has seriously affected the reliability of electronic systems and caused many major accidents. Its growth mechanism and mitigation strategy have been the focus of research in the past decades. In this paper, a large number of Sn whiskers were found to grow quickly on Al–Sn alloys prepared by mechanical alloying when treated at 300 °C. Different from the large randomness of the whisker growth on metallic coatings and platings as in major research work of this topic, the Sn whisker growth from ball-milled Al–Sn substrate shows good repeatability. To demonstrate the compressive stress in the samples, X-ray diffraction residual stress analysis was detected. An average compressive stresses in Al phase after heat treatment was 81.2 MPa, which may have been responsible for the whiskers growing directly on the surface. Meanwhile, the interfacial energy caused by high-energy ball milling may have played a positive role in the growth of whiskers. Sn whiskers formed on Al–Sn alloy samples have shorter incubation period and faster growth rate, which makes it much more convenient to research the mechanism.

Tin (Sn) whisker growth from electroplated Sn finished

Electroplated tin (Sn)Limeited finishes are widely used in the electronics industry due to their excellent solderability, electrical conductivity and corrosion resistance. However, since prohibiting the use of lead (Pb) in electronics, the issue of tin (Sn) whisker 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 device. This paper investigates the effect of electroplating current density (10, 30 and 50 mA/cm2) on whisker growth at room temperature. A simple micro- indentation test was used to accelerate continuously the growth of Sn whisker at room temperature storage for 24 h. Morphology analysis of Sn whisker growth was done using scanning electron microscope (SEM) and image-J Software. Axial length of the Sn whisker was calculated by using JEDEC Standards (JESD22-A121A).

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.

Morphology Analysis of Spontaneous Tin (Sn) Whisker Growth on Lead-Free Solder

Due to the transition of the electronics industry to lead-free solders, the issue of tin (Sn) whisker 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 whisker is a conductive tin structure that grows spontaneously from tin finished surfaces, which can lead to well-documented system failures in electronics industries. Assessment of the Sn whisker growth is difficult because of the slow and unpredictable nature of the Sn whisker formation. This paper presents the results of morphology analysis of Sn whisker growth in terms of Sn whiskers shape, average density and maximum length of Sn whisker on Sn-Cu solder finished using scanning electron microscope (SEM). Micro-indentation test was used to accelerate the growth of Sn whisker at room temperature storage.

Effect of Sn Film Grain Size and Thickness on Kinetics of Spontaneous Sn Whisker Growth

The effects of the grain size and thickness of Sn films on the kinetics of spontaneous Sn whisker growth have been quantitatively analyzed separately. The results reveal that the length and diameter of the whiskers are related to the parameters of the Sn film. A thicker film has a stronger influence on the whisker morphology than does the grain size. When the biaxial stress in the film reached a steady state, the growth rate was dependent on the grain size and thickness of the film. An equation is proposed to calculate the spontaneous growth rate of Sn whiskers in correlation with the dimension and microstructure of the Sn film. Spontaneous Sn whisker growth can be suppressed by increasing the thickness and grain size of the film.

Improved adhesion of polyurethane-based coatings to tin surface

Polyurethane (PU)—based conformal coatings which protect electronic components and boards from harsh environmental exposures were further developed to mitigate the tin (Sn) whisker growth in lead (Pb)—free electronics. As a means of achieving this goal, the adhesion mechanics of PU-based conformal coating on tin surfaces was investigated. We attempted to improve the interfacial adhesion between PU-based conformal coatings and tin (Sn) as it slows down the formation of tin whisker growth. A multi-layer sandwiched specimen made between two Si beams was employed to create an interface between PU coating and Sn for four-point bending testing (4PBT). The result shows that PU has strong adhesion to Sn while polyurethane acrylate (PUA) has much weaker adhesion to Sn. The PUA adhesion, however, increased by introducing a silane agent ((3-Aminopropyl)triethoxysilane) on the tin surface, which is covalently bonded to PUA. The adhesion improvement of a modified tin surface was also confirmed by a simple cross-cut tape peeling test. Post fracture analysis suggested that blocking the reaction between bare Sn surface and the acrylate component contribute to improved adhesion.

The influence of the crystallographic structure of the intermetallic grains on tin whisker growth

In this paper, the relationship between the crystallographic structure of Cu-Sn intermetallic grains and Sn whisker growth was investigated. In order to prevent the influence of the elements in the alloy composition and the effect of the soldering process on the formation of the intermetallic layer, 99.99% pure Sn was vacuum evaporated onto Cu substrates. The Sn layer thickness was sub-micron region (∼400 nm in average) to reach considerable and rapid compressive stress on the tin layer originated by the intermetallic formation. The samples were stored at room temperature for 1 month. Different types of whiskers (nodule and filament) and the layer structure underneath were studied with a scanning ion microscopy and transmission electron microscopy. It was found that not only the thickness of the intermetallic layer and shape of the intermetallic grains affects the whisker growth but the crystallographic structure of the intermetallic grains as well. The susceptibility of the Sn layer to whisker development is higher in those regions where the intermetallic layer is composed of monocrystalline grains instead of those regions, where it is composed of polycrystalline grains. This effect can be explained by the higher compressive stress generated by the monocrystalline intermetallics compared to the polycrystalline ones.

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.

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).

Critical Evaluation of Relative Importance of Stress and Stress Gradient in Whisker Growth in Sn Coatings

Here, we study the role of stress state and stress gradient in whisker growth in Sn coatings electrodeposited on brass. The bulk stress in Sn coatings was measured using a laser-optics based curvature setup, whereas glancing angle X-ray diffraction was employed to quantify the surface stress; this also allowed studying role of out-of-plane stress gradient in whisker growth. Both bulk stress and surface stress in Sn coating evolved with time, wherein both were compressive immediately after the deposition, and thereafter while the bulk stress monotonically became more compressive and subsequently saturated with aging at room temperature, the stress near the surface of the Sn coating continually became more tensile with aging. These opposing evolutionary behaviors of bulk and surface stresses readily established a negative out-of-plane stress gradient, required for spontaneous growth of whiskers. The importance of out-of-plane stress gradient was also validated by externally imposing widely different stress states and stress gradients in Sn coatings using a 3-point bending apparatus. It was consistently observed that whisker growth was more in the coatings under external tensile stress, however, with higher negative out-of-plane stress gradient. The results conclusively indicate the critical role of negative out-of-plane stress gradient on whisker growth, as compared to only the nature (i.e., sign and magnitude) of stress.

Effect of Substrate Composition on Whisker Growth in Sn Coatings

Whisker growth was studied in Sn coatings deposited on three different substrates, namely pure Cu, brass (Cu-35 wt.% Zn) and pure Ni. Additionally, the effect of a Ni under-layer (electro- or sputter-deposited and placed between the Sn coating and the substrate) on whisker growth was also studied. It was observed that the substrate composition and placement of under-layers significantly affected the whisker growth in Sn coating by altering the growth rate and the morphology of the interfacial intermetallic compounds (IMC). Whisker propensity was the highest when Sn coatings were deposited directly on the brass substrate, while it was completely inhibited for at least a year when the coatings were deposited on either pure Ni or brass with a Ni under-layer. Bulk and surface stress measurements revealed that the surface of the Sn coatings on Ni, irrespective of whether it was in bulk or under-layer form, remained more compressive as compared to the bulk, throughout the observation period. Therefore, a negative out-of-plane stress gradient, which is crucial for whisker growth, could never be established in these samples. Interestingly, a phenomenon of through-thickness columnar voiding (reverse of whiskering) was observed in the Sn coatings deposited on Ni. The origin of this phenomenon is discussed.

Preliminary Study on Deformation and Recrystallization Behavior of Pure Tin for Mitigation of Whisker Growth

Since the mechanism of Sn-whisker growth is closely related with the behavior of deformation and recrystallization, understanding of the behavior is very important to establish the measure for mitigation of whisker growth. In this work, microstructural changes after heavy deformation by scratching were characterized by EBSD for a single crystal of β-Sn, and the following results were obtained. Three types of crystal grains appear immediately after the deformation; small grains in aggregation, large serrated grains and rim-grains. The small grains are considered to be formed by dynamic recrystallization. They continue to grow at a room temperature over a lengthy period of time. The large grain has a certain crystallographic relationship with the matrix where <100> axes of the large grain and the matrix are almost parallel to each other. The serrated boundaries of large grain are so stable that the large grain does not show a grain growth process. The stable boundary is considered to promote a continuous growth of whiskers.

The Effects of Oxidation Layer, Temperature, and Stress on Tin Whisker Growth: A Short Review

In order to reduce the Tin (Sn) whisker growth phenomenon in solder alloys, the researcher all the world has studied the factor of this behaviour. However, this phenomenon still hunted the electronic devices and industries. The whiskers growth were able to cause the electrical short, which would lead to the catastrophic such as plane crush, the failure of heart pacemaker, and the lost satellite connection. This article focuses on the three factors that influence the whiskers growth in solder alloys which is stress, oxidation layer and temperature. This findings were allowed the researchers to develop various method on how to reduce the growth of the Sn whiskers.

Comparative studies on microelectronic reliability issue of Sn whisker growth in Sn-0.3Ag-0.7Cu-1Pr solder under different environments

In this study, comparative studies on Sn whisker growth in Sn-0.3Ag-0.7Cu-1Pr solder under different environments were conducted to investigate factors like ambient temperature, oxygen level, and 3.5wt% NaCl solution on whisker growth. The experimental results revealed that ambient temperature and oxygen level are two important factors that could determine the oxidation rate of PrSn3 phase, thus indirectly affecting the growth rate of Sn whiskers. In addition, mechanisms of whisker growth under these three environments were established from the perspective of atom diffusion based on the “compressive stress-induced” theory. Although whiskers under different environments were all squeezed out from Pr oxides (hydroxides), the forms of their driving forces were different. For whiskers squeezed out in air whether at room temperature or 150°C, the driving force is the compressive stress produced by lattice expansion due to the oxidation of PrSn3 phase. The representative example was whiskers' growth at 150°C, which could be simplified as three stages: (1) squeezing out, (2) cracking and (3) bursting out. For whisker growth in 3.5wt% NaCl solution, the driving force for much fewer whiskers' growth was proposed to come from lateral stress provided by interfacial IMC layer growth. Moreover, Sn nanoparticles and their agglomerations were also found to form under the driving force of the potential difference between Sn atoms and Sn crystals. Their morphologies could also be affected by factors of ambient temperature, oxygen level and Cl− ions in corrosive liquid.

Sn whisker growth mitigation by using NiO sublayers

The potential of NiO sublayers for whisker growth mitigation has been examined. A thin NiO film was applied on a Cu-coated substrate before the deposition of a thicker Sn layer. The growth of Sn whiskers was then followed by optical and scanning electron microscopy and was compared with the whisker growth on a control sample without the NiO sublayer. No whiskers were observed on the sample with the NiO layer even after 12 months, whereas the control sample developed whiskers of size and density that is generally expected, based on the vast amount of published work on the topic. The mechanisms of whisker growth and whisker growth suppression are briefly discussed as well.

Effect of Pr addition on properties and Sn whisker growth of Sn–0.3Ag–0.7Cu low-Ag solder for electronic packaging

In this study, trace amount of rare earth Pr was added into Sn–0.3Ag–0.7Cu low-Ag solder to enhance properties of solders. Experimental results indicated that optimal amount of Pr addition (~0.06 wt%) can improve properties of wettability, shear force and ductility of Sn–0.3Ag–0.7Cu low-Ag solder. This is because that solder with optimal Pr addition not only had a refined microstructure but also owned a regular and thin interfacial IMC layer after soldering on Cu substrate. Meanwhile, we have explained the change of morphology and thickness of interfacial IMC layer after Pr addition based on Kim and Tu’s kinetic model of interfacial IMC growth. However, it was found excessive Pr addition led to the formation of PrSn3 phase, which was easy to be oxidized and became a great deterioration on the properties referred above. Besides, the fracture mode of solder joint also gradually changed from ductile fracture to cleavage fracture. Moreover, this oxidized PrSn3 became the birthplace of Sn whisker since it provided abundant supply of Sn sources and compressive stress for Sn whisker growth. By the in-situ observation of Sn whisker growth in 0.5 wt% Pr-doped solder joint, we obtained that the incubation period of Sn whisker is very short and its growth rate may have a great decrease with time extending. Besides, it was found after 1 day at room temperature, spindly and rod-like Sn whiskers also grew in 0.5 wt% Pr-doped solder joint besides dot-shaped Sn whisker that grew in 0.5 wt% Pr-doped solder matrix. We suggest this may be related with the extra compressive stress provided by interfacial IMC layer growth in 0.5 wt% Pr-doped solder joint.

Spontaneous Sn whisker formation on Ti2SnC

Spontaneous Sn whisker growth on Sn-containing substrates has resisted interpretation for several decades. In this paper, Ti2SnC-Sn and Ti2SnC samples were prepared and then cultivated in the same conditions. Many Sn whiskers had appeared on the ball milled Ti2SnC-Sn sample after being stored in air for 40 days, while no whisker grew on the Ti2SnC-Sn samples without ball milling although the cultivation time had been extended to more than half a year. Moreover, even though it was ball milled, the Sn-free Ti2SnC sample did not grow any whisker. The whiskers formed on this system share typical features with the ones grown on Sn platings and solders, while they do not follow any of the existing growth models. A catalysis-based whisker formation model, in which the cleavage planes of Ti2SnC grains act as heterogeneous nucleation sites, is proposed, which well interprets Sn whisker formation in the ball milled Ti2SnC-Sn system.

Eliminating Whisker Growth by Indium Addition in Electroplated Sn on Copper Substrate

Whisker growth from Sn coatings is a reliability concern in electronic packages, until recently mitigated by Pb addition. Recently, it was demonstrated that doping with In dramatically reduces whisker growth in 1 μm thick Sn. Here, we present the results of In-doping on whisker growth from 3 μm and 6 μm thick Sn-films and explore the reasons behind this mitigation, and compare the results with a baseline sample of pure Sn and a control sample of tri-layer Sn-In-Sn, all subjected to identical thermal treatments. It is shown that In addition completely stops whisker growth from electroplated Sn. The impact of In addition on the film microstructure and the role of the surface oxide coating are investigated. Previous work had shown that while In addition reduces grain boundary diffusivity, it does not fully account for the observed dramatic reduction of whisker growth. In this work, it is shown by Auger electron spectroscopy and x-ray photoelectron spectroscopy that In is incorporated in the surface-oxide. Since whisker-growth is contingent on the presence of a tenacious surface-oxide, this suggests that the alteration of the oxide properties may be responsible for the observed reduction in whisker growth. Finite element modeling is utilized to demonstrate that a reduction of the elastic modulus of the surface oxide would reduce the driving force of Sn whisker growth, thus proffering a rationale for the effect of In incorporation.