Significance of Nucleation Kinetics in Sn Whisker Formation

Sn whiskers are believed to form in response to stress in layers used as protective coatings. However, what makes them form at specific sites on the surface is not known. We have used thermal expansion mismatch to induce stress and observe the resulting whisker formation. Cross-sectional measurements of the region around whiskers show that there are oblique grain boundaries under the whiskers that are not seen in the as-deposited columnar structure. The kinetics also suggest that the whiskering sites may be formed by a nucleation process. Based on these results, we propose a nucleation mechanism in which the boundaries of the surrounding grains migrate due to strain energy differences and create oblique boundaries at which whiskers can form. A simple model is developed to predict the stress-dependence of the nucleation rate.     

Ti-25 Nb-25 Ta合金拉伸变形过程中｛332｝〈113〉变形孪晶的EBSD研究

本文利用EBSD技术对亚稳β型Ti?25Nb?25Ta合金室温拉伸变形过程中｛332｝〈113〉变形孪晶的演变趋势及交叉现象进行了研究。结果表明：随着变形量的增大，孪晶宽度不断增加，孪晶形态由变形初期的平直细条带状逐渐变为宽窄不一的条带状，至变形末期部分孪晶呈破碎状，孪晶界严重扭曲变形。多数晶粒内产生两种以上｛332｝〈113〉孪晶变体，部分孪晶能穿越晶界继续生长，某些初次孪晶内部有二次｛332｝〈113〉孪晶产生。不同变体孪晶间易发生交叉现象。不同｛332｝〈113〉孪晶变体的交叉作用导致交叉区域出现较大的局部晶格畸变，晶格畸变主要集中于孪晶内部，且交叉界面处的晶格畸变程度最为严重。交叉作用的结果是交叉区域的晶体相对于不同孪晶变体和基体的取向都发生一定程度的改变。     

低碳冲压钢板静态再结晶行为的研究

利用背散射电子衍射(EBSD)技术和X射线衍射(XRD)对SPCD冷轧钢板缓慢升温退火工艺下的再结晶取向特征、织构的形成规律及与形变织构的关系进行研究,并与快速加热退火工艺下的IF钢再结晶取向特点进行对比.结果表明:宏观织构显示冷轧态下{111}〈112〉形变织构稳定存在,随后的再结晶过程中γ线上存在{111}〈112...     

Investigation of Sn Whisker Growth in Electroplated Sn and Sn-Ag as a Function of Plating Variables and Storage Conditions

Sn whiskers are becoming a serious reliability issue in Pb-free electronic packaging applications. Among the numerous Sn whisker mitigation strategies, minor alloying additions to Sn have been proven effective. In this study, several commercial Sn and Sn-Ag baths of low-whisker formulations are evaluated to develop optimum mitigation strategies for electroplated Sn and Sn-Ag. The effects of plating variables and storage conditions, including plating thickness and current density, on Sn whisker growth are investigated for matte Sn, matte Sn-Ag, and bright Sn-Ag electroplated on a Si substrate. Two different storage conditions are applied: an ambient condition (30°C, dry air) and a high-temperature/high-humidity condition (55°C, 85% relative humidity). Scanning electron microscopy is employed to record the Sn whisker growth history of each sample up to 4000 h. Transmission electron microscopy, x-ray diffraction, and focused ion beam techniques are used to understand the microstructure, the formation of intermetallic compounds (IMCs), oxidation, the Sn whisker growth mechanism, and other features. In this study, it is found that whiskers are observed only under ambient conditions for both thin and thick samples regardless of the current density variations for matte Sn. However, whiskers are not observed on Sn-Ag-plated surfaces due to the equiaxed grains and fine Ag₃Sn IMCs located at grain boundaries. In addition, Sn whiskers can be suppressed under the high-temperature/high-humidity conditions due to the random growth of IMCs and the formation of thick oxide layers.     

Sn Corrosion and Its Influence on Whisker Growth

The microstructure and crystal structure of condensation-induced corrosion products, vapor phase induced oxidation products, Cu-Sn intermetallics, and Sn whiskers that formed on electroplated matte Sn on Cu-alloy after exposure 2500 h in a 60 degC/93%RH ambient were characterized with scanning electron microscopy, (SEM), focused ion beam (FIB) microscopy, energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and selected area electron diffraction (SAD). The corrosion product was identified as crystalline SnO₂. The oxidation of Sn in condensed water was at least four orders of magnitude larger than that in moist vapor at 60 degC. All Sn whiskers were found to be within 125 mum of the corrosion product. Based on these observations, a theory was developed. The theory assumes that oxidation leads to the displacement of Sn atoms within the film. Because the grain boundaries and free surfaces of the film are pinned, the oxidation-induced excess Sn atoms are constrained within the original volume of the Sn-film. The trapped excess Sn atoms create localized stress, excess strain energy, in the Sn-film. If and when the pinning constraint is relaxed, as for example would occur when the surface oxide on the film cracks, then the Sn atoms can diffuse to lower energy configurations. When this occurs, whisker nucleation and growth begins. The theory was tested by detailed measurements and comparison of the corrosion volume and the whisker volume in two different samples. The volume comparisons were consistent with the theory     

The Role of Silver in Mitigation of Whisker Formation on Thin Tin Films

The mitigating effect of alloying Sn thin films with Ag on the formation of Sn whiskers was investigated by time-resolved investigations employing x-ray diffraction for phase and stress analyses and focused ion beam microscopy for morphological characterization of the surfaces and cross-sections of the specimens. The investigated Sn-6 wt.%Ag thin films were prepared by galvanic co-deposition. The results are compared with those obtained from investigation of pure Sn films and discussed with regard to current whisker-growth models. The simultaneous deposition of Sn and Ag leads to a fine-grained microstructure consisting of columnar and equiaxed grains, i.e. an imperfect columnar Sn film microstructure. Isolated Ag3Sn grains are present at the Sn grain boundaries in the as-deposited films. Pronounced grain growth was observed during aging at room temperature, which provides a global stress relaxation mechanism that prevents Sn whisker growth.     

Microstructure Development of Mechanical-Deformation-Induced Sn Whiskers

The development of the microstructure of mechanical-deformation-induced Sn whiskers on electroplated films has been examined using a focused ion beam system (FIB). The 6-μm-thick matte Sn films were compressed by using a ZrO₂ ball indenter under ambient conditions. After compression, tin whiskers and small nodules were found adjacent to, and several grains further away from, the indents. The cross-sectional microstructures of the indents and whiskers indicate that the lateral boundaries of the newly created grains caused by recrystallization are the main routes for stress relaxation.     

Microtexture and Grain Boundaries in Freestanding CVD Diamond Films: Growth and Twinning Mechanisms

Three groups of free‐standing chemical vapor deposition (CVD) diamond films formed with variations in substrate temperature, methane concentration, and film thickness are analyzed using high‐resolution electron back‐scattering diffraction. Primarily {001}, {110}, and {111} fiber textures are observed. In addition, corresponding primary and higher order twinning components are found. As interfaces, high angle, low angle, primary twin, and secondary twin boundaries are observed. A growth and a twinning model are proposed based on the sp³ hybridization of the bond in the CH₄ molecule that is used as the deposition medium.     

冷轧无取向硅钢再结晶退火过程的EBSD分析

采用EBSD手段分析了低碳低硅（0.003%C、0.3%Si）冷轧硅钢试样在740℃加热保温6s～360s退火过程中微区取向的演变规律。结果表明：{111}〈110〉取向的再结晶晶粒在变形的{111}〈112〉取向晶粒的晶界处形核,同时{111}〈112〉取向的再结晶晶粒在变形的{111}〈110〉取向晶粒的晶界处形核,并在{111}〈112〉取向冷轧剪切带上形成{011}〈100〉取向再结晶晶粒。在一定的退火温度下,合理控制保温时间有利于提高{100}面织构的占有率。     

N80 ERW套管调质前后焊缝组织演变的EBSD研究

本文采用SEM和EBSD等显微分析技术研究了一种N80油套管调质处理（Q＆T）前后焊缝组织的演变规律。实验结果表明,经调质处理N80套管焊缝中心区的{101}〈101〉织构明显增强,而热影响区从较弱的{101}〈101〉织构变为较强的近{111}〈101〉织构。并且,焊缝和热影响区组织中取向差角小于10°的小角度晶界和大于50°的大角度晶界数量显著增加,而介于10°-50°之间的晶界数量下降。调质处理前焊缝和热影响区中晶粒尺寸分布较为均匀,而调质处理后主要以小尺寸晶粒为主。     

Investigation of relation between intermetallic and tin whisker growths under ambient condition

The relation between the whisker growth and intermetallic on various lead-free finish materials that have been stored at ambient condition for 2 yrs (6.3 × 10⁷ s) is investigated. The matte Sn plated leadframe (LF) had the needle-shaped whisker and the nodule-shaped whisker was observed on the semi-bright Sn plated LF. Both the Sn plated LFs had a same columnar grain structure and both whiskers were grown in connection with the scalloped intermetallic compound (IMC) layer. The morphology of the IMC layer is similar, regardless of the area which has whisker or not. On the Sn–Bi finish and bright Sn plated LF, hillock-shaped and sparsely grown branch-shaped whiskers were observed, respectively. The IMC grew irregularly under both the areas with or without whisker. The IMC growth along the Sn grain boundaries generated inner compressive stress at the plating layer. Atomic force microscopy (AFM) profiling analysis is useful for characterization the IMC growth on the Sn and Cu interface. The measured root mean square (RMS) values IMC roughness on semi-bright Sn, matte Sn, and bright Sn plated LF were 1.82 μm, 1.46 μm, and 0.63 μm, respectively. However, there is no direct relation between whisker growth and the RMS value. Two layers of η′-Cu₆Sn₅ were observed using field emission transmission electron microscopy (FE-TEM): fine grains and coarse grains existed over the fine grains.     

放电等离子体烧结镍的形变组织研究

利用放电等离子体烧结(SPS)技术制备了粗晶(13μm)和细晶(2μm)两种镍样品.EBSD分析表明,两种烧结样都没有织构,且它们的晶粒都为完全再结晶组织.对此烧结样进行单轴压缩,研究晶粒尺寸对形变中微观组织演变的影响.在样品坐标系中,用每个像素点相对于平均取向的旋转轴来构图,结果显示在一些大晶粒中有扩展的平直界面.滑...     

Tin Whisker Growth Induced by High Electron Current Density

The effect of electric current on the tin whisker growth on Sn stripes was studied. The Sn stripes, 1 μm in thickness, were patterned on silicon wafers. The design of the Sn stripes allowed the simultaneous study of the effect of current crowding and current density. Current stressing was performed in ovens set at 30, 50, or 70°C, and the current density used ranged from 4.5 × 10⁴ A/cm² to 3.6 × 10⁵ A/cm². It was found that the stress induced by the electric current caused the formation of many Sn whiskers. A higher current density caused more Sn whiskers to form. Of the three temperatures studied, 50°C was the most favorable one for the formation of the Sn whiskers. In addition, the current-crowding effect also influenced whisker growth.     

Structural characterization of low temperature Epi-silicon grown on {100} and {111} Si substrates using ultrahigh resolution cross-sectional TEM

Low defect-density epitaxial silicon was grown at 550°C, but it became polysilicon or amorphous silicon when the substrate was submitted to bombardment of ECR argon plasma prior to growth. Through carefully characterizing the interface and structure of low temperature epitaxial silicon films using ultrahigh resolution cross-sectional transmission electron microscopy (UHRXTEM), defects were found to have different features in silicon epitaxial layers grown on {100} and {111} silicon substrates. Twinning was more likely to generate in the epitaxial layer grown on the {111} silicon substrate while stacking faults had priority in forming in the epitaxial layer grown on the {100} substrate. The probable causes of different defect formation mechanisms were analyzed and discussed with the help of UHRXTEM lattice images. The atom model of the twin boundary in the epitaxial silicon film was analyzed in detail.     

Effects of POSS-Silanol Addition on Whisker Formation in Sn-Based Pb-Free Electronic Solders

Previous studies have indicated that silanol in the form of polyhedral oligomeric silsesquioxane (POSS) trisilanol could form strong bonds with solder matrix without agglomeration, and inhibit diffusion of metal atoms when subjected to high ambient temperature and/or high current density. Addition of POSS-trisilanol has also been shown to improve the comprehensive performance of Sn-based Pb-free solders, such as shear strength, resistance to electromigration, as well as thermal fatigue. The current study investigated the whisker formation/growth behaviors of Sn-based Pb-free solders (eutectic Sn-Bi) modified with 3 wt.% POSS-trisilanol. Solder films on Cu substrates were aged at ambient temperature of 125°C to accelerate whisker growth. The microstructural evolution of the solder films’ central and edge areas was examined periodically using scanning electron microscopy. Bi whiskers were observed to extrude from the surface due to stress/strain relief during growth of Sn-Cu intermetallic compounds (IMCs). Addition of POSS-trisilanol was shown to retard the growth of Bi whiskers. The IMCs formed between POSS-modified solders and the Cu substrate showed smoother surface morphology and slower thickness growth rate during reflow and aging. It was indicated that POSS particles located at the phase boundaries inhibited diffusion of Sn atoms at elevated temperatures, and thus limited the formation and growth of IMCs, which resulted in the observed inhibition of Bi whisker growth in POSS-modified solders.     

Study of Thermal Aging Effects on Intermetallic Growth Mechanisms and Shear Strength Behavior of Wafer-Level Solder Bumps

The effect of wafer-level solder bump size on solder bump shear strength and intermetallic growth as a function of time, at different storage temperatures is studied, using solder bumps of two different heights: 95 $mu{hbox {m}}$ (SB95) and 142 $mu{hbox {m}}$ (SB142). Smaller solder bumps (SB95) lead to a faster decrease in bump shear strength. Increasing aging temperature and time brings about an increase in the final fracture area of the solder, from a ductile to a quasi-brittle failure mode. It is observed that the smaller solder bump (SB95) exhibits a faster rate of increase in final fracture area, than the bigger bump. A linear decrease of solder bump shear strength with intermetallic thickness is established for the systems studied. Degradation of solder bump shear strength is found to be affected by the relative volume of the solder bump with respect to Ni/Au, such that a lower solder bump to Ni/Au volume ratio leads to faster shear strength degradation, due to the consumption of Cu in the bulk solder bump and the formation of brittle intermetallics. The overall intermetallic growth activation energies for the two solder bump sizes are found to be comparable, at 0.85– 0.86 eV. For this study, $({hbox {Cu}},{hbox {Ni}})_{6} {hbox {Sn}}_{5}$, is initially formed, at the solder bump-to-under bump metallization (UBM) interface, followed by the formation of $({hbox {Ni}},{hbox {Cu}})_{3} {hbox {Sn}}_{4}$, between the UBM and the $({hbox {Cu}},{hbox {Ni}})_{6} {hbox {Sn}}_{5}$ intermetallic layer.      

Influence of substrate on high-temperature behavior of copper film studied in situ by electron backscatter diffraction

Change in the in-plane orientation of (111) grains in the copper film was studied in situ by electron backscatter diffraction (EBSD) during its thermal treatment inside the scanning electron microscope (SEM). Two separate investigations were carried out each at different locations of the film. Both of the investigations showed the presence of (111) fiber texture with increased strengths of {111}〈112〉 and {111}〈110〉 orientations. During the first investigation, the {111}〈110〉 component became sharper relative to {111}〈112〉, while in the second investigation the sharpness decreased relative to {111} 〈112〉 with increasing temperature. No such changes in the in-plane orientation of the (111) grains were observed during the similar experiment carried out on the Cu film in freestanding condition. The role of silicon subtrate on influencing these changes has been proposed based on dislocation activity within the grains. The increase in the inclination of (111) planes to the specimen surface in {111}〈110〉 and {111}〈112〉 grains as a function of temperature was linked to the stress relaxation. The inclination of the (111) planes to the specimen surface leads to decrease in the sharpness of the (111) texture components. Finally, similar transformation in the texture of (111) grains in Cu damascene interconnects was investigated.     

La对SnAgCu/Cu及Ni界面金属间化合物的影响

利用扫描电镜、能谱分析仪对Sn0.3Ag0.7Cu-xLa/Cu(x=0～0.25)和Ni界面金属间化合物(IMC)形成及长大规律进行了研究.结果表明:微量La的添加使钎焊与时效后焊点/Cu界面生成的Cu<,6>Sn<,5>晶粒明显细化,当X超过0.10时,Cu<,6>Sn<,5>晶粒的上方出现大量的粒状Ag<,3>S...     

Interface Driven Pseudo‐Elasticity in a‐Fe Nanowires

Molecular dynamics simulations of bent [100] α‐Fe nanowires show the nucleation of twins and nanoscale interfaces that lead to pseudo‐elasticity during loading/unloading cycles. The new type of interfaces along {110} stems from the accumulation of individual <111>/{112} twin boundaries and stores high interfacial energies. These nonconventional interfaces provide a large part of the driving force for shape recovery upon unloading, while the minimization of surface energy is no longer the dominant driving force. This new pseudo‐elastic effect is not much affected by surface roughness, and can be extended over a wide range of wire diameters, if the sample is seeded with conventional twin boundaries, which will transform to the desired {110} interfaces under bending.     

Influence of crystallographic orientation of Sn-Ag-Cu on electromigration in flip-chip joint

The influence of the crystallographic orientation of Sn-3.0 wt%Ag-0.5 wt%Cu flip-chip joints and underfill on electromigration was investigated. The current density applied in our tests was 15 kA/cm² at 160 °C. Various times to failure of the test samples show a clear dependence of the electromigration behavior on the Sn grain orientations. Different microstructural evolutions were observed in all solder bumps in correlation with the crystallographic orientations of the Sn grains after an electromigration test. The primary failure of the solder joints was caused by dissolution of the Cu electrode at the cathode interface. Rapid dissolution of the Cu electrode occurred when the c-axis of the Sn grains was parallel to the direction of electron flow. On the other hand, slight dissolution of the Cu electrode was observed when the c-axis of the Sn grains was perpendicular to the direction of electron flow. Some grain boundaries interrupt the migration of Cu and the trapped Cu atoms form new grains of intermetallic compounds at the grain boundaries. In addition, underfill inhibits serious deformation of solder bumps during current stressing.