Fabrication and microstructures of sequentially electroplated Au-rich, eutectic Au/Sn alloy solder

An Au-rich, eutectic Au/Sn alloy was fabricated by sequential electroplating of Au and Sn, and reflowing the as-deposited Au/Sn/Au triple-layer film at 320–350 °C. Microstructures and phase compositions for the as-deposited Au/Sn/Au triple-layer film and the reflowed Au-rich, eutectic Au/Sn alloys were studied. Two Si wafers, each with the Au-rich, eutectic Au/Sn alloy solder, were bonded together. For the deposited Au/Sn/Au triple-layer film, reaction between Au and Sn occurs at room temperature leading to the formation of AuSn and AuSn₄. After reflowing at 320 °C, two phases remain, AuSn and Au₅Sn, with the AuSn particles distributed randomly in the Au₅Sn matrix. There are also some micropores and microcracks in the reflowed alloy. If the annealing temperature is increased to 350 °C, the Au/Sn alloy is denser and contains fewer micropores. However, microcracks remain, forming preferentially along the Au₅Sn/AuSn interface. After reflowing at 320 °C under a pressure of 13 kPa, two Si wafers are joined using the Au-rich, eutectic Au/Sn alloy solder. The solder is in intimate contact with the Si wafers; however, there are some micropores within the solder. After reflowing at 350 °C, the bond is quite good, without microcracks or micropores at the Si wafer/solder interface or within the solder.     

Utilizing the thermodynamic nanoparticle size effects for low temperature Pb-free solder

Development of a Pb-free Sn nanosolder paste with an initial melting temperature near or below the melting temperature of eutectic Sn-Pb solder (183 °C) has been investigated using the size-dependent melting behavior of small particles. Three to five nanometer Sn nanoparticles were fabricated by sonochemical reduction and observed to melt at temperatures near or below 183 °C. Prototype nanosolder pastes were produced by combining the nanoparticles with flux and were characterized by differential scanning calorimetry (DSC) in terms of their melting, solidification, coalescence, and metal particle loading properties. The results indicate that, although target melting temperatures were achieved, nanoparticle coalescence was limited by low volume loading of the metal, due in part to the capping layer (an organic layer adsorbed on the metal surface during chemical synthesis).     

Low cycle fatigue behavior of a eutectic 80Au/20Sn solder alloy

The eutectic 80Au/20Sn solder alloy is widely used in high power electronics and optoelectronics packaging. In this study, low cycle fatigue behavior of a eutectic 80Au/20Sn solder alloy is reported. The 80Au/20Sn solder shows a quasi-static fracture characteristic at high strain rates, and then gradually transforms from a transgranular fracture (dominated by fatigue damage) to intergranular fracture (dominated by creep damage) at low strain rates with increasing temperature. Coffin-Manson and Morrow models are proposed to evaluate the low cycle fatigue behavior of the 80Au/20Sn solder. Besides, the 80Au/20Sn solder has enhanced fatigue resistance compared to the 63Sn/37Pb solder.     

Characterization of gold nanoparticle binding to microtubule filaments

Microtubule (MT) protein filaments were used as templates for fabricating Au nanowires as a bottom-up approach for fabricating building blocks for future integrated circuits. Photochemical reduction methods were employed to form Au nanoparticles which bind and uniformly cover the MT filaments. Synthesis of the MT-templated Au nanowires was characterized using UV/vis spectroscopy and transmission electron microscopy (TEM). In addition, binding between the MT filaments and Au nanoparticles was investigated using surface enhanced Raman spectroscopy (SERS) and X-ray photoelectron spectroscopy (XPS) to establish the nature of the binding sites. A variety of functional groups were identified by SERS to interact with the Au including imidazole, sulfur, aromatic rings, amine, and carboxylate. The imidazole ring in the histidine is the most prominent functional group for Au binding. The results from these studies provide better understanding of the binding between Au and the biotemplate and give insight concerning methods to improve Au coverage for MT-templated Au nanowires.     

Formation of Sn–Bi solder alloys by sequential electrodeposition and reflow

Eutectic Sn–Bi alloy is gaining considerable attention in the electronic packaging industry because of its favorable properties such as low melting temperature, good wettability, and good mechanical properties. Miniaturization of electronic devices requires small solder bumps, a few tens of micrometers in diameter. Electrodeposition is a reliable technique for the deposition of small volume of solder. This work focuses on the formation of eutectic Sn–Bi solder by reflowing a metal stack containing sequentially electrodeposited Sn and Bi layers. The effects of layering sequence on the composition and microstructure of the resulting alloy is investigated. Irrespective of the layering sequence, a homogeneous microstructure is achieved after reflow. The microstructure of the reflowed samples is the same as that of a metallurgically processed Sn–Bi alloy. Near-eutectic alloy with the composition Sn–54.6 wt% Bi is obtained by the sequential electrodeposition method.     

A simple process for electrodeposition of Sn-rich, Au–Sn solder films

The Sn-rich eutectic alloy (90 wt% Sn) in the Au–Sn system offers a potentially cheaper alternative to the Au-rich eutectic alloy (20 wt% Sn) for optoelectronic and microelectromechanical systems device packaging and may be applicable as a Pb-free solder for microelectronic packaging. A simple electrodeposition method was utilized to fabricate Sn-rich, Au–Sn solder films, including the eutectic composition for this purpose. The electrolyte consisted of a solution of Sn chloride and ammonium citrate. Gold was added to the electrolyte in the form of either a Au nanoparticle (<20 nm) suspension, prepared with Na citrate, or by directly adding Au powder (500–800 nm particles). The resultant suspensions were used to electrodeposit eutectic and near-eutectic alloy films. Uniform thicknesses and compositions were obtained with the latter approach, i.e., direct addition of Au powder. Gold content in the deposits increased with increasing Au particle loading in the electrolyte and increasing current density. Room temperature aging led to the formation of AuSn₄ at the Au particle-Sn matrix interface. Reflow of deposits with near-eutectic compositions resulted in the formation of the two eutectic phases, Sn and AuSn₄.     

Highly enhanced electrocatalytic oxygen reduction performance observed in bimetallic palladium-based nanowires prepared under ambient, surfactantless conditions

We have employed an ambient, template-based technique that is simple, efficient, and surfactantless to generate a series of bimetallic Pd(1-x)Au(x) and Pd(1-x)Pt(x) nanowires with control over composition and size. Our as-prepared nanowires maintain significantly enhanced activity toward oxygen reduction as compared with commercial Pt nanoparticles and other 1D nanostructures, as a result of their homogeneous alloyed structure. Specifically, Pd(9)Au and Pd(4)Pt nanowires possess oxygen reduction reaction (ORR) activities of 0.49 and 0.79 mA/cm(2), respectively, which are larger than the analogous value for commercial Pt nanoparticles (0.21 mA/cm(2)). In addition, core-shell Pt~Pd(9)Au nanowires have been prepared by electrodepositing a Pt monolayer shell and the corresponding specific, platinum mass, and platinum group metal mass activities were found to be 0.95 mA/cm(2), 2.08 A/mg(Pt), and 0.16 A/mg(PGM), respectively. The increased activity and catalytic performance is accompanied by improved durability toward ORR.     

The effect of Au on the Pb-1.5% Ag-1% Sn solder

During soldering, gold present, for example, as a plating on components is rapidly dissolved by the high melting point Pb-1.5% Ag-1 wt% Sn solder. One major effect is that the solidus temperature is significantly lowered from 311°C at O wt% Au to that of a postulated quaternary eutectic at 211°C, formed by additions of greater than 4.8 wt% Au. The liquidus of the solder is reduced from 311 to 272°C by additions of 10 wt% Au. The dissolved Au also reduces the strength of the solder, and drastically so at elevated temperatures at which the solder would otherwise be expected to possess reasonable strength. The deleterious effects of gold absorption can largely be avoided by controlling the solidus temperatures of the AuPbAgSn alloy formed during the soldering operation. This is accomplished by regulating the proportions of Au and solder able to react within each joint.     

Microstructural study of co-electroplated Au/Sn alloys

Gold-tin eutectic solder (20 wt% Sn), because of its excellent mechanical and thermal properties, is utilized for flip chip and laser bonding in optoelectronic applications. Coelectroplating of Au and Sn has been investigated as an alternative to conventional methods for depositing Au/Sn alloys. Pulse current (PC) and direct current (DC) plating tests have been performed and compared using a suitably stable plating solution. Plating conditions, including current density and ON and OFF times (for PC plating), have been varied to optimize the process. Reproducibility tests have also been performed. It is shown that a range of alloy compositions can be deposited, including eutectic and near-eutectic compositions, with compositional and microstructural uniformity potentially suitable for microelectronic and optoelectronic solder applications.     

Coaxial metal-oxide-semiconductor (MOS) Au/Ga2O3/GaN nanowires

Coaxial metal-oxide-semiconductor (MOS) Au-Ga2O3-GaN heterostructure nanowires were successfully fabricated by an in situ two-step process. The Au-Ga2O3 core-shell nanowires were first synthesized by the reaction of Ga powder, a mediated Au thin layer, and a SiO2 substrate at 800 degrees C. Subsequently, these core-shell nanowires were nitridized in ambient ammonia to form a GaN coating layer at 600 degrees C. The GaN shell is a single crystal, an atomic flat interface between the oxide and semiconductor that ensures that the high quality of the MOS device is achieved. These novel 1D nitride-based MOS nanowires may have promise as building blocks to the future nitride-based vertical nanodevices.     

Intermetallic reactions in a Sn-3.5Ag-1.5In solder ball-grid-array package with Au/Ni/Cu pads

In this paper, the interfacial reactions between Sn-3.5Ag solder and Sn-3.5Ag-1.5In solder and Au/Ni/Cu pads in ball-grid-array (BGA) packages during solid aging were investigated by microstructural observations and phase analysis. During the solid aging, the intermetallic compound (IMC) layer in Sn-3.5Ag/Au/Ni/Cu solder joints evolved from the (Ni, Au)Sn₄ phase to the Ni₃Sn₄ phase, but the rate of growth of the IMC layer did not change significantly. While, in Sn-3.5Ag-1.5In/Au/Ni/Cu solder joints, the phases evolved from the (Ni, Au)Sn₄ and Ni₃Sn₄ phases into Ni₃(Sn, In)₄ phase. The distribution of In atoms in the solder alloy weakened interatomic force in the Sn-3.5Ag-1.5In solder alloy and the involvement of In atoms in the interfacial reaction generated more energy of distortion of the Ni₃(Sn, In)₄ and (Ni, Au)(Sn, In)₄ lattices. These both accelerated the diffusion of Sn atoms and the rate of growth of the whole IMC layer, but this effect reduced gradually after prolonged aging.     

激光加热条件下 SnPb共晶钎料与Au/Ni/Cu焊盘界面反应动力学

激光重熔在电子封装领域中SnPb共晶钎料凸点制作方面存在极大的优势。采用扫描电子显微镜（SEM）分析了激光加热条件下SnPb共晶钎料与Au/Ni/Cu焊盘之间的界面反应，探讨了钎料中的溶解与扩散动力学。结果表明：CnPb共晶钎料在激光加热瞬间与Au/Ni/Cu焊盘中的Au发生反应，生成Au-Sn金属间化合物，其形貌和分布与激光输入能量密切相关；随着激光输入能量的增加，Au-Su化合物由边境连续层状转变为针状，最后以细小颗粒弥散分布在钎料内部。     

Influence of Thermal Cycling on the Microstructure and Shear Strength of Sn3.5Ag0.75Cu and Sn63Pb37 Solder Joints on Au/Ni Metallization

The influence of thermal cycling on the microstructure and joint strength of Sn3.5Ag0.75Cu （SAC） and Sn63Pb37 （SnPb） solder joints was investigated. SAC and SnPb solder balls were soldered on 0.1 and 0.9 μm Au finished metallization, respectively. After 1000 thermal cycles between -40℃ and 125℃, a very thin intermetallic compound （IMC） layer containing Au, Sn, Ni, and Cu formed at the interface between SAC solder joints and underneath metallization with 0.1 μm Au finish, and （Au, Ni, Cu）Sn4 and a very thin AuSn-Ni-Cu IMC layer formed between SAC solder joints and underneath metallization with 0.9 μm Au finish. For SnPb solder joints with 0.1 μm Au finish, a thin （Ni, Cu, Au）3Sn4 IMC layer and a Pb-rich layer formed below and above the （Au, Ni）Sn4 IMC, respectively. Cu diffused through Ni layer and was involved into the IMC formation process. Similar interfacial microstructure was also found for SnPb solder joints with 0.9μm Au finish. The results of shear test show that the shear strength of SAC solder joints is consistently higher than that of SnPb eutectic solder joints during thermal cycling.     

Inducing imperfections in germanium nanowires

Nanowires with inhomogeneous heterostructures such as polytypes and periodic twin boundaries are interesting due to their potential use as components for optical,electrical,and thermophysical applications.Additionally,the incorporation of metal impurities in semiconductor nanowires could substantially alter their electronic and optical properties.In this highlight article,we review our recent progress and understanding in the deliberate induction of imperfections,in terms of both twin boundaries and additional impurities in germanium nanowires for new/enhanced functionalities.The role of catalysts and catalyst-nanowire interfaces for the growth of engineered nanowires via a three-phase paradigm is explored.Three-phase bottom-up growth is a feasible way to incorporate and engineer imperfections such as crystal defects and impurities in semiconductor nanowires via catalyst and/or interfacial manipulation."Epitaxial defect transfer"process and catalyst-nanowire interfacial engineering are employed to induce twin defects parallel and perpendicular to the nanowire growth axis.By inducing and manipulating twin boundaries in the metal catalysts,twin formation and density are controlled in Ge nanowires.The formation of Ge polytypes is also observed in nanowires for the growth of highly dense lateral twin boundaries.Additionally,metal impurity in the form of Sn is injected and engineered via third-party metal catalysts resulting in above-equilibrium incorporation of Sn adatoms in Ge nanowires.Sn impurities are precipitated into Ge bi-layers during Ge nanowire growth,where the impurity Sn atoms become trapped with the deposition of successive layers,thus giving an extraordinary Sn content (＞6 at.％) in Ge nanowires.A larger amount of Sn impingement (＞9 at.％) is further encouraged by utilizing the eutectic solubility of Sn in Ge along with impurity trapping.     

Influence of alumina bubble particles on microstructure and mechanical strength in porous Cu–Sn–Ti metals

In order to obtain high porosity and satisfactory strength simultaneously for the porous metallic layer of the grinding tools, alumina bubble particles were added into Cu–Sn–Ti alloy powders to fabricate porous metals using a vacuum sintering method. The influence of the alumina bubble particles on the microstructure and the mechanical strength of the porous Cu–Sn–Ti metal blocks were investigated. Results show that adding alumina bubble particles into Cu–Sn–Ti alloy powder generate closed pore structures for the metal blocks. Good bonding interface between alumina bubble particles and Cu–Sn–Ti alloy is formed mainly dependent on the chemical resultants of TiAl and TiO. A relationship between the bending strength of the porous Cu–Sn–Ti metal blocks and the size and volume fraction of the alumina bubble particles is established.     

Bimetallic Nanostructures as Active Raman Markers: Gold‐Nanoparticle Assembly on 1D and 2D Silver Nanostructure Surfaces

It is demonstrated that bimetallic silver–gold anisotropic nanostructures can be easily assembled from various nanoparticle building blocks with well‐defined geometries by means of electrostatic interactions. One‐dimensional (1D) silver nanowires, two‐dimensional (2D) silver nanoplates, and spherical gold nanoparticles are used as representative building blocks for bottom‐up assembly. The gold nanoparticles are electrostatically bound onto the 1D silver nanowires and the 2D silver nanoplates to give bimetallic nanostructures. The unique feature of the resulting nanostructures is the particle‐to‐particle interaction that subjects absorbed analytes to an enhanced electromagnetic field with strong polarization dependence. The Raman activity of the bimetallic nanostructures is compared with that of the individual nanoparticle blocks by using rhodamine 6G solution as the model analyte. The Raman intensity of the best‐performing silver–gold nanostructure is comparable with the dense array of silver nanowires and silver nanoplates that were prepared by means of the Langmuir–Blodgett technique. An optimized design of a single‐nanostructure substrate for surface‐enhanced Raman spectroscopy (SERS), based on a wet‐assembly technique proposed here, can serve as a compact and low‐cost alternative to fabricated nanoparticle arrays.     

Fabrication and magnetic study of Co/Pt multilayer nanowires and Co–Pt alloy nanowires electrodeposited into porous Si substrates

In this work, we grow composite structures consisting of magnetic and non-magnetic metal or alloy nanowires electrodeposited into the ion etched tracks previously created inside Si substrates. The holes are then filled by Co–Pt alloys and Co/Pt multilayers using electrodeposition technique making a large number of parallel nanowires. This process takes place in a single electrolyte containing Co⁺² and Pt⁺⁴ ions by applying a proper deposition potential using a computer control potentiostat. The magnetic properties of the sample were studied using vibrating sample magnetometer. Magnetoresistive behaviour of the nanowire samples was then studied by subjecting the samples to an external magnetic field. The results show that the Co/Pt multilayered nanowires exhibit a large magnetoresistance, while the Co–Pt alloys only show anisotropic magnetoresistance. This result could be of a great interest for the sensor fabrication community as they will provide a view on a very important direction of the development of the wide spread sensor industry, and more importantly for understanding the physical phenomena underlying the magnetic/non-magnetic nanostructures.     

Large‐Area 3D Hierarchical Superstructures Assembled from Colloidal Nanoparticles

Assembling nanosized building blocks into macroscopic 3D complex structures is challenging. Here, nanosized metal and semiconductor building blocks with a variety of sizes and shapes (spheres, stars, and rods) are successfully assembled into a broad range of hierarchical (nanometer to micrometer) assemblies of functional materials in centimeter size using butterfly wings as templates. This is achieved by the introduction of steric hindrance to the assembly process, which compensates for attraction from the environmentally sensitive hydrogen bonds and prevents the aggregation of nanosized building blocks. Of these materials, Au nanostar assemblies show a superior enhancement in surface‐enhanced Raman scattering (SERS) performance (rhodamine 6G, 1506 cm^?1) under 532, 633, and 780 nm excitation—this is 3.1–4.4, 3.6–3.9, and 2.9–47.3 folds surpassing Au nanosphere assemblies and commercial SERS substrates (Q‐SERS), respectively. This method provides a versatile route for the assembly of various nanosized building blocks into different 3D superstructures and for the construction of hybrid nanomaterials and nanocomposites.     

表面组装技术用焊锡粉末的制备

根据拉瓦尔喷管原理设计出一种新型低压超音速雾化器 ,用 6 3A焊锡和Sn Ag系无铅焊锡进行了雾化试验研究。试验结果表明 ,新型雾化器可获得微细的球形焊锡合金粉末 ,可以满足表面组装技术 (SMT)用焊锡粉的要求 ,焊锡熔化温度为 4 0 0～ 4 5 0℃ ,雾化压力为 0 .7～ 1.0MPa。影响微粉粒度形成的主要因素有合金过热度 ,雾化压力 ,雾化环境中的氧含量等     

Bottom-up assembly of large-area nanowire resonator arrays

Directed-assembly of nanowire-based devices will enable the development of integrated circuits with new functions that extend well beyond mainstream digital logic. For example, nanoelectromechanical resonators are very attractive for chip-based sensor arrays because of their potential for ultrasensitive mass detection. In this letter, we introduce a new bottom-up assembly method to fabricate large-area nanoelectromechanical arrays each having over 2,000 single-nanowire resonators. The nanowires are synthesized and chemically functionalized before they are integrated onto a silicon chip at predetermined locations. Peptide nucleic acid probe molecules attached to the nanowires before assembly maintain their binding selectivity and recognize complementary oligonucleotide targets once the resonator array is assembled. The two types of cantilevered resonators we integrated here using silicon and rhodium nanowires had Q-factors of approximately 4,500 and approximately 1,150, respectively, in vacuum. Taken together, these results show that bottom-up nanowire assembly can offer a practical alternative to top-down fabrication for sensitive chip-based detection.