Annotated tin whisker bibliography and anthology

Whisker publications from 1947 through 2004 are summarized and referenced in this document. The bibliography listings are chronological (by publication year) while the anthology section arranges references by topic. Almost all of the referenced documents deal with tin whiskers, but there are few that deal with zinc whiskers.     

Investigation of Sn-Cu intermetallic compounds by AFM: new aspects of the role of intermetallic compounds in whisker formation

It has been observed that different tin whisker growth properties can result from the same tin deposit on two copper leadframe materials, C19400 and C70250. It was also found that modification of the surface condition of identical substrates, through pretreatment prior to Sn-plating, affects whisker growth behavior. Here, we report an atomic force microscopy (AFM) investigation of intermetallic compounds (IMC) formed on samples with significantly different whisker growth tendencies. AFM data analysis in conjunction with the IMC growth rate, determined using the weight gain method, provides quantitative data of the IMC characteristics. It was found that the whisker propensity is not directly related to the uneven growth front of IMC as detected by AFM. Whisker growth tendency, with respect to whisker density and length, decreases when the unevenness of the IMC layer exceeds a certain level. The results of this work show evidence that the whisker propensity is associated with the copper substrate texture as well as the substrate surface topography.     

Statistical analysis of tin whisker growth

As the result of the global movement to lead-free electronics, companies which assemble semiconductor devices are switching from finishes incorporating lead to pure tin or high tin lead-free alloys. This transition has resulted in a reliability issue, concerning the formation of conductive tin whiskers which can grow across leads of a package and cause current leakage or short circuits. This paper presents the results of an experimental tin whisker growth study of bright tin on brass substrates. A probabilistic model is applied to describe the phenomenon of whisker growth in terms of whisker density, length and growth rate.     

Humidity sensor based on BiOBr synthesized under ambient condition

Flexible humidity sensors are effective portable devices for human respiratory monitoring. However, the current preparation of sensitive materials need harsh terms and the small production output limits their practicability. Here, we report a synthesis method of single-crystal BiOBr nanosheets under room temperature and atmospheric pressure based on a sonochemical strategy. A flexible humidity sensor enabled by BiOBr nanosheets deliver efficient sensing performance, a high humidity sensitivity (I_g/I₀ = 550%) with relative humidity from 40% to 100%, an excellent selectivity, and a detection response/recovery time of 11 and 6 s, respectively. The flexible humidity sensor shows a potential application value as a wearable monitoring device for respiratory disease prevention and health monitoring.     

The effect of tin grain structure on whisker growth

Tin whisker growth has become a major concern in the electronics industry as banning Lead application. In order to further understanding whisker growth with varied grain structures, three different grain structures (columnar, semi-columnar, and horizontal) and three mixed type deposits were prepared by changing the deposition conditions. The grain structure of deposited layer and intermetallics (IMC) were examined by focused ion beam (FIB) and chemical etching. After 4000 h of testing in 55 °C with 85% RH, whisker growth on the columnar sample was easily observed, and caused by the wedge type IMC. The semi-columnar tin grains formed small amounts of long straight whiskers accompanied with hillocks, and also seeing uneven IMC. The horizontal type tin grains formed only hillocks, accordingly finding the IMC more uniform than the others. Mixed grain structures were prepared, and consisted of different structures on the top and bottom layers. The top layer dominates the forms of whiskers or hillocks, owing to the grain boundary guiding the diffusion of tin atoms, and the bottom layer affects the density of whiskers or hillocks due to local stress building up from the formation of intermetallic phase.     

Effects of continuously applied stress on tin whisker growth

A simple four-point bending technique in conjunction with scanning electron microscopy (SEM) was employed to investigate the relationship between continuously applied mechanical stress and tin whisker growth on bright tin-plated copper specimens at room temperature. Measurements of whisker length and density were periodically taken for each specimen during a 7-month exposure to applied stress. Tin whiskers were found on the tin-plated specimens with or without the presence of mechanically applied stresses. A continuously applied compressive stress resulted in formation of longer and more dense tin whiskers, in comparison with the cases of no applied stress and applied tensile stress. However, an increase in the applied compressive stress level caused an increase in the whisker density but a decrease in the whisker length. On the other hand, a continuously applied tensile stress was found to reduce both the whisker density and length, compared to the case of no applied stress. Apparently, application of a continuous tensile stress could provide an effective means in retarding tin whisker growth.     

钎料稀土相表面Sn晶须生长的研究

为了解对钎料可靠性影响极大的Sn晶须的生长机理,系统研究了钎料稀土相CeSn3、LaSn3及ErSn3表面Sn晶须生长的影响因素.结果表明:稀土相的氧化倾向与时效温度共同影响其表面Sn晶须的生长.室温时效条件下,在稀土相CeSn3与LaSn3表面易出现小尺寸的线状Sn晶须,直径为0.1～0.2 μ.m,而在稀土相ErS...     

A relation between Kolmogorov-Prokhorov's condition and Ohya'sfractal dimensions

A computational method giving upper bounds of ∈-entropy of stochastic processes satisfying Kolmogorov-Prokhorov's condition is studied, in terms of Shannon's entropy of the probability measures on C[0,1] induced by these processes. Moreover, this method and Ohya's fractal dimensions are applied to investigating the asymptotic behavior of ∈-entropy of such processes     

Automatic characterisation method for statistical evaluation of tin whisker growth

In this paper, an automatic method was developed to characterise whisker growth quantitatively in SEM images. The key step of the automatic methods in this case is the determination of the optimal threshold value for image segmentation, i.e. separation of the objects (whiskers) from the background (substrate). A thresholding method was developed for this purpose and was compared to manual and to general purpose automatic methods as references. As it was proven in previous studies the vacuum deposited tin layers on copper substrates can produce numerous tin whiskers in various shapes and lengths in a short time. This layer deposition technology was therefore chosen for the comparison of the thresholding methods. Images of the produced whiskers were captured by a FEI Inspect S50 Scanning Electron Microscope. By executing the automatic methods in the captured images, the area density of the whiskers, and the maximum and the mean length of the whiskers were measured. Based on the results of area density, the automatic methods were compared to manual counting and the Mean Absolute Percentage Error (MAPE) was determined. Finally, the reference automatic methods were compared to the self-developed method from the maximum and mean length of whiskers point of view.     

The suppression of tin whisker growth by the coating of tin oxide nano particles and surface treatment

Tin oxide nano particles dispersed in water solution were sprayed on the tin-plated copper surface and served as coating layer in order to study its effect on the prevention of tin whisker formation. The results indicated that tin oxide nano particles could inhibit the growth of tin whiskers to certain extent. Many hillocks instead of long whiskers grew on the surfaces of samples that underwent 25, 40 and 60 °C annealing for 10 weeks. Furthermore, a strong etchant and polishing were applied to the tin-plated samples. XPS results showed that the surface oxide was removed by surface treatments; the surfaces were then coated by spraying nano particles. It is found that the coherency of the re-grown oxide during annealing was reduced, leading to the growth of hillocks instead of long whiskers. This approach seems to successfully enhance the relaxation of stress to prevent the growth of long whiskers.     

SnAgCuLa钎料合金表面Sn晶须的生长研究

在钎料中添加微量稀土可以显著改善钎料合金的综合性能。但添加过量的稀土时,将会出现Sn晶须的快速生长。研究发现,如果将Sn3.8Ag0.7Cu1.0La钎料内部的稀土相暴露于空气中,稀土相将发生氧化而产生体积膨胀,钎料基体对体积膨胀的抑制作用将使稀土相内部产生巨大的压应力从而加速Sn晶须的生长。晶须直径约1μm,长度可达300μm。     

The effect of electroplating parameters and substrate material on tin whisker formation

Electroplated tin finishes are widely used in the electronics industry due to their excellent solderability, electrical conductivity and corrosion resistance. However, the spontaneous growth of tin whiskers during service can result in localised electrical shorting or other harmful effects. Until recently, the growth of tin whiskers was successfully mitigated by alloying the tin with lead. However, restriction in the use of lead in electronics as a result of EU legislation (RoHS) has led to renewed interest in finding a successful alternative mitigation strategy.Whisker formation has been investigated for a bright tin electrodeposit to determine whether whisker growth can, at least partially, be mitigated by control of electroplating parameters such as deposition current density and deposit thickness. The influence of substrate material and storage at 55 °C/85% humidity on whisker growth have also been investigated.Whisker growth studies indicate that deposition parameters have a significant effect on both whisker density and whisker morphology. As deposition current density is increased there is a reduction in whisker density and a transition towards the formation of large eruptions rather than potentially more harmful filament whiskers. Increasing the tin coating thickness also results in a reduction in whisker density. Results demonstrate that whisker growth is most prolific from tin deposits on brass, whilst that from tin deposits on rolled silver is greater than that observed for tin deposits on copper.     

A physical model and analysis for whisker growth caused by chemical intermetallic reaction

The formation of intermetallic compound Cu₆Sn₅ gives rise to the internal stress in the lead-free coating, which causes the growth of Sn whiskers. This phenomenon is characterized with the expansion of inclusion in a plate perfectly bonded between two infinite solids. Based on the grain boundary diffusion mechanism, a model is established to evaluate the growth rate of Sn whiskers. The results show that the growth rate of whisker varies with the relative site between whisker and inclusion. When the distance between the whisker and inclusion exceeds a critical value, negative growth rate will appear, and it approaches zero as the distance increases. They explain some phenomena observed in experiments.     

多径条件下阵列天线相位关系分析

针对多径条件下阵列天线阵元间的相位差,研究了散射体在发射台附近的有直达路径、无直达路径以及远端强反射等传播方式下的相位关系计算模型,通过仿真计算,分析了相位关系的统计特征.该计算模型可模拟多径环境下的矢量信道参数,可用于通过相位信息进行到达方位测量的测向算法的研究.     

Suppression of tin whisker formation on fine pitch connectors by surface roughening

In the electronics industry, lead-free solder processes such as the terminal plating of electronic components, fine pitch connectors, and flexible printed circuits (FPCs) are invariably hampered by the serious problem of tin whisker formation. Here, a new and simple method, the JVC Micro Island (JMI) process, is proposed for the prevention of tin whisker formation in fine pitch connectors. Briefly, the base copper terminal was acid etched to afford a roughened surface, which was then tin plated. The contact test with Knoop indentation proved the effectiveness of the present process. The maximum length of the as-formed tin whiskers was less than 50 μm. The solderability of the JMI FPCs was not influenced by the present process. Thus, the new JMI process is shown to have a great advantage for the prevention of tin whisker formation in fine pitch connectors.     

挠性电路板引脚嵌合部无铅镀层的锡须生长

以民用挠性印制电路板(FPC)引脚和连接器嵌合部无铅镀层为对象,通过研究引脚上的Ni/Sn无铅镀层的显微形貌和锡须尺寸,探讨了Ni/Sn无铅镀层的长期可靠性。结果表明,在25℃,RH为45%～55%的条件下,挠性印制电路板引脚和连接器嵌合部无铅镀层上生长的锡须呈现针状、柱状等多种不同的显微形貌,其中大部分是针状锡须,少量针状锡须的长度已超过了50μm临界值,很可能因锡须桥接引起电流泄漏和短路,对FPC互连可靠性产生威胁。抑制少量超长的针状晶须的生长,是防止风险的关键。     

Rapid method for testing efficacy of nano-engineered coatings for mitigating tin whisker growth

The risk of failure of electronic components due to tin (Sn) whiskers growth has become an issue with the current regulations limiting the use of lead in Sn solders. New strategies using engineered coatings for mitigating Sn whiskers are being developed. Typically, these coatings are evaluated by an aging process where whiskers are allowed to grow naturally. Unfortunately, this process can produce unreliable growth results and can take several years. Thus, faster, more reliable methods are needed. In this study, a simple, rapid (3–10 days), and cost-effective method was developed for testing the efficacy of nano-engineered coatings for mitigating the growth of Sn whiskers. This method consisted of a micro-indentation process using a ball-bearing adhered to a few hundred gram weight, which are placed in a stabilizing printed holder. For uncoated samples, Sn whiskers and hillocks were abundant near the indentation area, while only hillocks were found further outside the area (i.e., >0.2 mm). For samples coated with nano-engineered ceramic or polymeric coatings, the indentation method was observed to damage coatings only at the point of contact (e.g., no delamination), while still allowing Sn whiskers and hillocks to grow outside the indentation area.     

The impact of electrical current, mechanical bending, and thermal annealing on tin whisker growth

This paper presents the results of an eight-month design-of-experiment assessment of whisker growth on bright and matte tin-plated copper, mechanically deformed and unformed coupons, subject to a continuous 50 °C/50%RH environment, with and without the presence of a constant electrical current density magnitude of 0.48 × 10² A/cm². Whiskers were observed to grow both at the anode and cathode end. The distribution-based data showed a reduction in whisker density due to annealing and/or the application of electrical current for both bright and matte tin. However, the application of electrical current was observed to increase the standard deviation of the length distribution, and to generate longer whiskers.     

无铅焊点拉伸冲击疲劳损伤的频率特性研究

针对低频振动条件下无铅焊点损伤的频率特性问题,利用无铅焊点损伤测试系统在室温下实时测量不同循环拉伸频率下的焊点损伤。结果表明:0.1～5.0 Hz范围内焊点损伤率随循环拉伸频率的增加而增加,而在10～20 Hz焊点损伤率随频率的增加而减小;5～10 Hz焊点损伤率大于其他频率范围。在5～10 Hz存在一个导致焊点损伤最大、寿命最短的频率点(损伤特征频率)。最后证明了合金材料在低频振动条件下普遍存在损伤特征频率。     

Correlation between intermetallic thickness and roughness during solder reflow

The evolution of Cu−Sn and Ni−Sn intermetallic compound (IM) morphologies during isochronal reflow of a PbSn solder with Cu and electroless-Ni was investigated. The root mean square roughness of the Cu−Sn and Ni−Sn IM layers exhibited a linear dependence on average thickness, with gradients () of 0.33 and 0.65, respectively. The difference in values was attributed to different thickness distributions resulting from morphological dissimilarity of the IM layers. The apparent activation energies for growth and roughness evolution of both IM were found to be similar and suggest that both IM thickness and roughness evolution are controlled by the same mechanism.