Low-thermal-expansion copper composites via negative CTE metallic elements

Thermal management is an important issue in electronic packaging due to the increasing complexity, miniaturization, and high density of components in modern devices. In high-power-dissipation packages, heat sinks are essential for preventing thermal damage to heat-sensitive components on the silicon chip. However, commonly used heat-sink materials (e.g., copper and aluminum alloys) have a much higher coefficient of thermal expansion (CTE) than silicon. CTE mismatch between the various materials in an electronic package can lead to stresses that can trigger complex failure mechanisms like component distortion, stress rupture, thermomechanical fatigue, and creep, thereby seriously degrading device reliability and lifetime. Therefore, it is highly desirable to minimize the CTE mismatch by developing new heat-sink materials having CTEs that are close to the CTE of silicon. In this work, low-thermal-expansion copper composites with CTEs as low as 4 ppm/°C have been fabricated by employing a negative thermal-expansion alloy—equiatomic Ni-Ti, which has a CTE of approximately −21 ppm/°C. The use of negative CTE elements, especially those with very large negative CTE values, offers an attractive route for controlling the thermal-expansion behavior of various metallic and nonmetallic materials. H. Mavoori earned his Ph.D. in materials science and engineering from Northwestern University in 1996. He is a post-doctoral member of the technical staff of the Applied Materials and Metallurgy Research Group at Bell Laboratories, Lucent Technologies. Dr. Mavoori is also a member of TMS. S. Jin earned his Ph.D. in materials science at the University of California at Berkeley in 1974. He is currently technical manager at Bell Laboratories, Lucent Technologies. Dr. Jin is also a member of TMS. Author’s Note: Unless otherwise indicated, compositions are in weight percent.     

Forming and fractographical characteristics of Copper-Nickel-Beryllium sheets under tension and bending

Formability and its fracture characteristics are critical factors in fabricating spring components from sheet base materials. Copper-nickel-beryllium (C17510) alloys provide excellent formability for producing highly reliable connectors used in electrical and electronic applications. The formability of a commercially available CuNiBe alloy was studied with emphasis on springback evaluations. Experiments were conducted to investigate the forming related tensile and bending properties. Fractographical examinations were conducted to identify the characteristics of failure and predict its initiation. Several analytical formulas for predicting springback are presented herein to provide a simple tool for computer-assisted design of spring components. To verify their reliability, the analytical predictions were first compared with the experimental data. Then predictions based on different formulations were compared with each other to identify an appropriate formula to be used in design of the highly reliable spring components.     

Painting of Zinc Surfaces

Abstract

All types of zinc-coated steels have been painted in commercial practice for several decades; for the majority of these surfaces, simple general recommendations can be made and these have been incorporated in this paper For zinc-sprayed steel, however, there is still some research needed before completely generalised recommendations can be given. However commercial schemes which have been proved satisfactory in various environments are appropriate for all current uses and have been tabulated in paper.

The rapid increase in the use of painted zinc-coated steel and an appreciation of its cost advantages is stimulating great interest in new combinations of organic materials and zinc coatings: the benefits of developments in each technology are likely to be enhanced when the materials are used in combination, and some examples are given.     

Materials issues in area-array microelectronic packaging

The important issues in advanced area-array electronic packaging for semiconductor devices are materials driven. Some of the processing-driven materials issues include the effect of introducing a silicon device interface with copper pads and a low-κ dielectric, the effect of decreasing pitch and feature size on the package interconnects, the development and implementation of organic substrates, and advanced underfills for fine-pitch flip-chip applications. From a materials reliability aspect, important materials issues include enhanced solder interconnect reliability, α-particle-induced soft errors, and the introduction of lead-free solder alloys. Darrel R. Frear earned his Ph.D. in materials science at the University of California at Berkeley in 1987. He is currently manager of the low-cost flip-chip project at Sematech. Dr. Frear is also a member of TMS.     

Zinc whisker growth from electroplated finishes – a review

Abstract

Electroplated zinc finishes have been associated with the electronics industry for many years as a result of their excellent corrosion resistance and relatively low cost. They are normally applied onto ferrous products to provide corrosion protection in a range of different environments. However, the formation of spontaneously grown whiskers on zinc-electroplated components, which are capable of resulting in electrical shorting or other damaging effects, can be highly problematic for the reliability of long life electrical and electronic equipment. The growth of zinc whiskers has been identified as the cause of some electrical and electronic failures in telecommunications and aerospace-based applications, with consequences ranging from mild inconvenience to complete system failures. Investigators have been striving to address the problems induced by whisker growth since 1940s. However, most research effort has been focused on tin whiskers, especially following European Union environmental legislation that restricted the use of lead (Pb), which when alloyed with tin (3–10% by weight) provided effective tin whisker mitigation. Compared with tin whisker research, much less attention has been paid to zinc whiskers. A number of mechanisms to explain zinc whisker growth have been proposed, but none of them are widely accepted and some are in conflict with each other. The aim of this paper is to review the available literature in regard to zinc whiskers, to discuss the reported growth mechanisms, to evaluate the effect of deposition parameters and to explore potential mitigation methods. This paper presents a chronologically ordered review of zinc whisker-related studies from 1946 to 2013. Some important early research, which investigated whisker growth in tin and cadmium, as well as zinc, has also been included.     

Intelligent pattern recognition and diagnosis of ultrasonic inspection of welding defects based on neural network and information fusion

Abstract

The detection of defects in real manual metal arc welds using ultrasonic non-destructive testing has been investigated. Twenty-six features, extracted from three domains, were applied for recognition of defect type. To increase the reliability and accuracy of identification and classification, statistical analysis was used to evaluate the features extracted from ultrasonic defect echoes. The subset of optimum feature was then selected using the method of discriminant analysis. An intelligent defect evaluation method derived from the study is presented. The results show that statistical analysis is an effective method for feature evaluation. The uncertainty of defect diagnosis can be decreased by the information fusion method, and for three specific defect types, defects were correctly identified in approximately 93% of cases.     

Determination of laser weld properties for finite element analysis of laser welded tailored blanks

Abstract

The present paper details two techniques that were employed to determine laser weld material properties, and outlines how the weld properties were used in the finite element analysis simulation of simple small scale axisymmetric tests. From these simulations, an understanding of some fundamental laser welded tailored blank formability issues was gained, which would otherwise have required substantial practical tests to be conducted. The materials studied in the present paper were high strength steels, including a relatively newly developed ultrahigh strength steel, for application in structural body in white components.     

Mechanical properties of welded joints in components of VVER-1000 reactors

Abstract

The use of non-destructive techniques for the testing of metallic materials, and in particular of welded joints, for the presence of internal defects is well known. The increasing use in industry of ceramic and glass components now requires that a non-destructive examination of these types of material also be carried out. This article reports on the collected experiences in this field to date.     

The use of XPS, FTIR, SEM/EDX, contact angle, and AFM in the characterization of coatings

Coatings are applied to surfaces for a variety of reasons: to enhance their appearance, to protect the substrate, to augment the adhesion to other layers, or to functionalize them for further reactions. To evaluate the efficacy of the coating, it is often necessary to analyze the substrate and the coating to ensure that the needed characteristics are present. To this end, the use of x-ray photoelectron spectroscopy (XPS), contact angle, and atomic force microscopy (AFM) can provide information about the surface composition, its morphology, and its ability to be wetted with various solvents. Scanning electron microscopy with energy dispersive x-ray analysis (SEM/EDX) and Fourier transform infrared spectroscopy (FTIR) can provide a clear picture of the near surface components as well as the continuity of coatings. All of these aspects are valuable in evaluating a coating and essential when problems are encountered. The application of these techniques to the analysis of coatings is discussed.     

Computational tools for alloy design: Solid solutions in Gamma-TiAl

Since the early days of quantum mechanics and computer science, computationally based materials design has been the dream of the materials community. Computational methods have become an integral part in the design of drugs, optical, and electronic devices. While computational tools have been developed to study specific structure-property relationships in structural materials, the overall materials problem has remained, for the most part, in the domain of empirical metallurgy. Computational methods can help to identify and understand basic technical factors controlling and limiting the performance of high-temperature structural materials. We have used several computational methods to study the influence of alloy chemistry on the flow behavior of monolithic γ-TiAl. Here, the results of several of these studies and how these insights have or may impact the alloy design process are reviewed. C. Woodward earned his Ph.D. in solid state physica at the University of Illinois, Champaign-Urbana in 1986. He is currently a senior scientist at UES. S.I. Rao earned his Ph.D. in materials science and engineering at Virginia Polytechnic Institute and State University in 1984. He is currently a research scientist at UES. Dr. Rao is a member of TMS. D.M. Dimiduk earned his Ph.D. in materials science and engineering at Carnegie Mellon University in 1989. He is currently a materials engineer at the Air Force Research Laboratory, Wright-Patterson Air Force Base. Dr. Dimiduk is also a member of TMS.     

Identification of common sensory features for the control of CNC milling operations under varying cutting conditions

The main focus of this study is to identify the most influential and common sensory features for the process quality characteristics in CNC milling operations—dimensional accuracy (bore size tolerance) and surface roughness—using three different material types (6061-T6 aluminum, 7075-T6 aluminum, and ANSI-4140 steel). The materials were machined on a vertical CNC mill, retrofitted with multiple sensors and data acquisition systems, to investigate the effects of variations in material types and machining parameters. The sensor data include cutting force measurements, spindle quill vibration, and acoustic emission, each of which further divided into measurable components, such as x, y, and z components in cutting force, x and y spindle quill vibration, DC, AC, and Count Rate for acoustic emission signals. Those components were filtered and analyzed to determine the sensory features that best correlate with process quality characteristics. Tool wear rate and machining characteristics appeared differently, depending on the material types, yet some components of the sensory data were found to be significant with relation to the variations in bore size and surface roughness for all three types of materials. This suggests that even under the varying cutting conditions involving different materials, the identified sensory features can be used for the reliable and accurate control of milling operations.     

Surface nanocrystallisation engineering: scientific and industrial potential

Abstract

By means of surface mechanical attrition treatment (SMAT), a nanostructured surface layer with a graded grain size distribution ranging from nano-to micrometres can be synthesised on various metallic materials. In this paper, the grain refinement mechanism, mechanical and diffusion properties, and chemical reactivity of the nanostructured surface layer, are reviewed. In addition, effects of the nanostructured surface layer on the mechanical performance and surface thermochemical treatment processes of engineering materials are described. Previous investigations have indicated that the nanostructured surface layer synthesised by means of SMAT on metallic materials provides many unique opportunities in both basic scientific research and technological applications.     

Surface characterization methods— XPS,TOF-SIMS, and SAM a complimentary ensemble of tools

X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and scanning auger microscopy (SAM) analytical techniques have played important roles in the characterization of the surface and the interfacial chemistry governing properties and performance of materials, and material interfaces. These techniques afford spatially resolved elemental and molecular analysis of the topmost atomic layers of solid surfaces and interfaces. Currently available instrumentation provides qualitative/quantitative analysis on molecularly complex materials with detection limits in the parts-per-billion (ppb) range and spatial resolutions approaching 30 nm. Each technique is unique in the information attained, therefore necessitating a multitechnique approach to achieve a complete surface characterization. Examples of coating/interfacial characterization by XPS, TOF-SIMS, and SAM are presented illustrating the functionality of these tools and the complimentary natures of them.     

Fracture toughness of ceramic moulds for investment casting with ice patterns

Abstract

Ice patterns can be used to make ceramic investment moulds for metal castings. Owing to the characteristics of ice, the ceramic mould must be made at subzero temperatures and consequently, requires a different formulation than shells built at room temperature. Success of this process depends greatly on the fracture toughness of mould materials. The present paper describes the experimental results of fracture toughness of mould materials processed from different compositions. The Taguchi method was used to reduce the trial runs. The parameters considered included the ratio of fibre containing fused silica and aluminosilicate powders, the volume of binder and the volume of catalyst. The microstructure and green fracture surface of test bars were also examined to understand the underlying mechanism. While conducting the four point bend test on ceramic mould samples, some samples had exceedingly low strengths appearing as outliers in the Weibull analysis. Examination of these low strength ceramic samples improved understanding of failure of mould materials. Sound moulds have been made for the investment casting process with ice patterns based on the analysis of experimental results. The casting of an M8 bolt is used to demonstrate that metal castings of complex geometry can be fabricated using ice patterns. The measured tolerances are within the required tolerance range.     

Tuning the electronic structure of solids with nanometer-sized microstructures

Most materials studied and/or used technologically today are electrically neutral,i.e. the positive and negative electric charges are balanced. Limited attention has been paid so far to the option of influencing the properties of materials by deviating from charge neutrality. In fact, solids with nanometer-sized microstructures may open the way to generate materials with an excess or a deficit of electrons or holes of up to about 0.3 electrons/holes per atom. Such deviations from charge neutrality may be achieved either by means of an externally applied voltage or by space charges at interfaces between materials with different chemical compositions (or combinations of both). As many properties of solid materials depend on their electronic structure, significant deviations from charge neutrality result in materials with new, yet mostly unexplored properties such as modified electric, ferromagnetic, optical etc. properties as well as alloys of conventionally immiscible components or materials with new types of atomic structures. Existing and conceivable new technological applications of solids deviating from charge neutrality are discussed.     

Prediction of the failure mode of automotive steels resistance spot welds

ABSTRACT

In this study, the critical nugget size, at which the failure state in tensile shear test changed from the interfacial failure mode to the pull-out failure one, was estimated as a function of nugget and base metal hardness. The proposed approach could address the effect of various parameters involved in resistance spot welding process, such as sheet thickness, base metal chemical composition and physical properties of electrodes and sheets. The reliability of the present model was evaluated using independent experimental results. Based on the obtained results, the effect of steel composition on critical nugget diameter was found to be more important, especially for the sheets thicker than about 1?mm, whereas predicted nugget sizes by previous models could not guarantee the pull-out failure mode.     

Current–voltage (I–V) characteristics of the molecular electronic devices using various organic molecules

Organic molecules have many properties that make them attractive for electronic applications. We have been examining the progress of memory cell by using molecular-scale switch to give an example of the application using both nano scale components and Si-technology. In this study, molecular electronic devices were fabricated with amino-style derivatives as redox-active component. This molecule is amphiphilic to allow monolayer formation by the Langmuir–Blodgett (LB) method, and then this LB monolayer is inserted between two metal electrodes. According to the current–voltage (I–V) characteristics, it was found that the devices show remarkable hysteresis behavior and can be used as memory devices at ambient conditions, when aluminum oxide layer was existed on bottom electrode. The diode-like characteristics were measured only, when Pt layer was existed as bottom electrode. It was also found that this metal layer interacts with organic molecules and acts as a protecting layer, when thin Ti layer was inserted between the organic molecular layer and Al top electrode. These electrical properties of the devices may be applicable to active components for the memory and/or logic gates in the future.     

Current–voltage (I–V) characteristics of the molecular electronic devices using various organic molecules

Organic molecules have many properties that make them attractive for electronic applications. We have been examining the progress of memory cell by using molecular-scale switch to give an example of the application using both nano scale components and Si-technology. In this study, molecular electronic devices were fabricated with amino style derivatives as redox-active component. This molecule is amphiphilic to allow monolayer formation by the Langmuir–Blodgett (LB) method, and then this LB monolayer is inserted between two metal electrodes. According to the current–voltage (I–V) characteristics, it was found that the devices show remarkable hysteresis behavior and can be used as memory devices at ambient conditions, when aluminum oxide layer was existed on bottom electrode. The diode-like characteristics were measured only, when Pt layer was existed as bottom electrode. It was also found that this metal layer interacts with organic molecules and acts as a protecting layer, when thin Ti layer was inserted between the organic molecular layer and Al top electrode. These electrical properties of the devices may be applicable to active components for the memory and/or logic gates in the future.     

A two stage approach for the validation of welding and heat treatment models used in product development

Abstract

Many components used in the aerospace industry have complex shape and are manufactured from high strength materials. Performing large scale tests is costly and time consuming, therefore, simulation tools are needed to support an effective product development process. Using manufacturing simulations during product development requires a validated model of the material and manufacturing process. In this paper, a validation scheme is proposed for thermomechanical models of welding and post-weld heat treatment. The scheme was investigated by comparing simulations using shell elements with experimental results, which showed good agreement when predicting residual stresses after welding, but an overestimation of the out-of-plane deformations when simulating both welding and heat treatment. However, the simulations showed that the outof-plane deformation is strongly influenced by the initial geometry. It can be concluded that the simulation model is adequately accurate to be used in concept evaluation.     

Structure-Property Relationships in Sensors

Abstract

Various devices are described which record physical phenomena and which respond by transmitting signals via information processing units to actuators. For example, a thermistor senses temperature and transmits an electrical signal to an integrated circuit which may in turn actuate a furnace control. This review emphasizes the structure-property relationships of the sensor materials. An ionic conductor, such as stabilized zirconia, can be used to detect oxygen in air/fuel mixtures; metal oxides of large surface area can monitor humidity by surface conduction; and semiconducting transition-metal oxides can measure pH. The mechanism of vanadium dioxide critical temperature thermistors is described; as well as the operation of thermistors of negative and positive temperature coefficients (NTC and PTC) made from, for example, doped nickel oxide and barium titanate respectively. Composite sensors of two or more phases have properties which are the product of those of the individual components and have wide application. For example, combining the Hall effect with electrical conductivity will give magnetoresistance, a property which can be used in a composite magnetic field sensor. Two non-magnetoelectric materials can be cleverly combined to form a magnetoelectric composite. A composite material will exhibit only those symmetry elements that are common to its constituent phases and to their geometrical arrangement. The microstructures are given of composite thermistors and humidity sensors. A variety of integrated sensors can be built into single-crystal silicon chips to detect gases and to act as accelerometers, their fabrication involving micromachining and both isotropic and anisotropic etching.