In early electronic technologies, circuit components were attached to circuit boards by mechanical means. The electrical leads were either twisted together or mechanically interlocked to a board prior to soldering. The possibility of an unreliable solder joint causing any kind of circuit failure was remote. Interconnections were made intrinsic to the board by applying solder to increase electrical and thermal conductance. Technological advances and the need for high-density electronics have since eliminated the luxury of mechanical interlocks. Soldering in advanced applications, like surface mount technology (SMT), provides electrical, thermal, and mechanical interconnections between the board and its electrical components. In SMT, solder joints are the only mechanical features on the board and must hold components in place in a wide range of environments. The solder joints themselves are decreasing in size as increased chip functionality and clock frequencies become available. The failure of a single solder joint can render a device, or an entire electrical system, inoperable. Therefore, as insignificant and innocuous as they may seem, solder joints have become a critical aspect of electronic circuit reliability.

History is replete with examples of the enormous impact of data compilations in all fields of science and technology. The advantages and opportunities such aggregations confer include:

• convenience of access,

• condensation and homogenization of raw data,

• formats tailored for the application,

• perception of patterns,

• detection of errors,

• definition of gaps and inconsistencies, and

• basis for formulating and testing theories.

The opportunities in materials science, and specifically in the subfield of alloy design, are no fewer than in other fields of science. Indeed, as I have remarked in a review of the history of the development of understanding and application of intermetallic compounds, breakthroughs in this field have usually come about not from “a new technique, a unique experimental observation, or an abstruse theory… Rather it was the amassing of a critical volume of experimental data in the literature which permitted an individual with great insight to perceive an underlying pattern not previously apparent.”

Oxide-based ceramics have long been used as linings in containment vessels for hot materials (metals, glasses, cement, etc.) and hot gases, at temperatures often in excess of 1500°C, because of their chemical compatibility with these hot materials and with the process ambient—conditions where metals and polymers simply can't perform. However, their low thermal conductivities and generally high thermal expansivities cause poor thermal shock resistance. In addition, their creep resistance (resistance to permanent deformation under load) is generally poorer than the more covalently bonded ceramic materials such as nitrides and carbides which also have excellent thermal shock resistance.

The best present-day single-bandgap solar cells have efficiencies around 20–25%. However, the Carnot efficiency of the earth-sun system is 95%, so there is considerable potential for improvement. The fundamental efficiency limitation in a conventional solar cell results from the tradeoff between a low bandgap which maximizes light absorption and hence output current and a high bandgap which maximizes output voltage. As a result, the maximum theoretical efficiency of a conventional solar cell is around 30% in unconcentrated sunlight at a bandgap close to that of GaAs.

The quantum-well solar cell is a novel approach to higher efficiency. In its simplest form, shown in Figure 1, it consists of a multiquantum-well (MQW) system in the undoped region of a p-i-n solar cell. For light with energy greater than the band-gap Eg, the quantum-well cell behaves like a conventional cell. However, light with energy below Eg can be absorbed in the quantum wells. Our studies show that if the material quality is good, the electrons and holes escape from the wells and contribute to a higher output current at a voltage between that of the barrier and well material. In AlGaAs/GaAs test devices, we have obtained efficiency enhancements of a factor of more than two when cells with quantum wells are compared with identical cells without wells.

The structure in Figure 1 is, of course, essentially similar to the MQW photodiode or modulator structure that operates in reverse bias, and the quantum-well laser that operates in forward bias beyond flat band.

The term “macro-defect-free” refers to the absence of relatively large voids (or defects) that are normally present in conventional cement pastes due to entrapped air or inadequate mixing. A decade ago, Birchall and co-workers developed a novel processing method that avoids the formation of these strength-limiting defects. This method, outlined schematically in Figure 1, consists of mixing hydraulic cement powder, a water-soluble polymer, and a minimal amount of water under high shear to produce a macro-defect-free (MDF) cement composite. Several cement/polymer systems can be processed by this flexible technique, although the calcium aluminate cement/polyvinyl alcohol-acetate (PVA) copolymer system is most common: MDF cements display unique properties relative to conventional cement pastes. For example, the flexural strength of MDF cement is more than 200 MPa as compared to values on the order of 10 MPa for conventional pastes. One can view MDF cements as a type of “inorganic plastic.” As is the case with plastic processing, fillers such as alumina, silicon carbide, or metal powders can be added to MDF cement to modify its performance properties (e.g., abrasion resistance, thermal or electrical conductivity, and hardness). The combined attractiveness of inexpensive raw materials and flexible, low-temperature processing has generated great interest in this new class of advanced cement-based materials.

Since 1987, an intensive Portuguese-Spanish collaboration program has been developed on microstructural characterization of LiNbO3 by nuclear (ion-beam and hyperfine interaction) techniques. The collaboration involves the Department of Applied Physics of the Universidad Autónoma de Madrid, on the Spanish side, and the Center of Nuclear Physics and the Department of Physics of LNETI, on the Portuguese side. The combined effort of the two latter institutions is now channeled through the recently created Center for Materials Characterization and Processing.

LiNbO3 is an important material for optoelectronic applications, including light modulation, microwave filters, frequency doublers, and those applications based on the photorefractive effect. Photorefractive applications, still at the laboratory level, include holographic information storage, coherent beam amplification, beam deflectors, and applications relying on the production of phase-conjugated beams by four-wave mixing. LiNb03 is particularly useful for optical information processing, optical interconnects, associative memories, and neural net implementations. These applications involve bulk LiNbO3, but mostly waveguide structures fabricated in LiNbO3 substrates. Recently the development of efficient waveguide lasers on ion-implanted and proton-exchanged waveguides has opened a new field whose final objective is the fabrication of integrated laser modulators and frequency doublers on the same “chip.” The mastering and optimization of all these applications rely on an adequate microstructural characterization of the material with particular reference to both intrinsic and extrinsic point defects. The knowledge and control of this microstructure should permit the fabrication of well-characterized and reproducible devices.

The development of high-field magnets for pulsed-field applications is the subject of considerable research activity throughout the world. Induced field strengths in excess of 30 T can be produced only as pulses generated by the discharge of energy, e.g., from a capacitor bank into a resistive solenoid. A variety of electromagnetic design issues relate to the pulse duration, field strength, and volume of the coil; however, from the viewpoint of materials selection and coil design, there are two salient property requirements: (1) high electrical conductivity to minimize the temperature rise due to the flow of current in the coil, and (2) high mechanical strength to resist the Lorentz forces exerted during the pulse. Such forces can result in catastrophic failure, as illustrated in Figure 1, because the coil is, in essence, stressed in the manner of an internally pressured cylindrical vessel. At lower stresses, plastic distortion of the coil is observed.

Electronics packaging has always accounted for about 50% of reliability shortcomings in electronics systems. Materials, processes, and geometries used in microelectronics packaging are quickly evolving to meet the commercial challenge for more reliable, lighter, smaller, cheaper devices, and lower power consumption. Amid the rapid evolution, methodology and metrology to measure package reliability and materials aging properties must be standardized to allow quantitative technology comparisons.

What Is Packaging?

For most purposes packaging can be thought of as everything involved in making an electronics system except the wafer fabrication. For example, all the design, fabrication, assembly, and finishing that goes into interconnecting the integrated circuits, discrete devices, resistors, and capacitors and then interfacing the system to the user is traditionally called “packaging.”

In recent years, iron sulfide (FeS2), otherwise known as pyrite, has received some attention as a possible candidate for thin-film photovoltaic applications. This interest stems in part from the fact that it has a relatively large absorption coefficient of α > 105 cm−1 for photon energies larger than about 1.3 eV (see Figure 1), such that a thin layer of less than 1,000 Å should effectively absorb the visible part of the solar spectrum. The bandgap of pyrite of 0.95 eV is probably on the borderline of what might still be considered acceptable for a high-efficiency single-junction device, but can be considered ideal in combination with a 1.8 eV material for tandem structures. Another important advantage of this compound semiconductor is that it is composed of cheap, abundant, and non-toxic elements, the latter being of particular importance in times of growing environmental consciousness.

Pyrite is a naturally occurring mineral (also known as “fool's gold” because of its golden color) which shows both n- and p-type conduction. This could indicate possible electronic viability of this material. The study of synthetic crystals has been severely impeded by the fact that pyrite undergoes a peritectic decomposition into the more stable FeS and liquid sulfur at 743°C and that therefore the melt growth of high-purity large single-crystalline material is not possible. Nevertheless, it has been shown that polycrystalline pyrite layers could be prepared with different thin-film preparation techniques (see Table I), such that in principle a cost-effective large-scale application could be envisaged.