Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

A review on the adsorption of heavy metals by clay minerals, with special focus on the past decade

Solder is widely used to connect chips to their packaging substrates in flip chip technology as well as in surface mount technology. At present, the electronic packaging industry is actively searching for Pb-free solders due to environmental concern of Pb-based solders. Concerning the reliability of Pb-free solders, some electronic companies are reluctant to adopt them into their high-end products. Hence, a review of the reliability behavior of Pb-free solders is timely. We use the format of “case study” to review six reliability problems of Pb-free solders in electronic packaging technology. We conducted analysis of these cases on the basis of thermodynamic driving force, time-dependent kinetic processes, and morphology and microstructure changes. We made a direct comparison to the similar problem in SnPb solder whenever it is available. Specifically, we reviewed: (1) interfacial reactions between Pb-free solder and thick metalliztion of bond-pad on the substrate-side, (2) interfacial reactions between Pb-free solder and thin-film under-bump metallization on the chip-side, (3) the growth of a layered intermetallic compound (IMC) by ripening in solid state aging of solder joints, (4) a long range interaction between chip-side and substrate-side metallizations across a solder joint, (5) electromigration in flip chip solder joints, and finally (6) Sn whisker growth on Pb-free finish on Cu leadframe. Perhaps, these cases may serve as helpful references to the understanding of other reliability behaviors of Pb-free solders.

Six cases of reliability study of Pb-free solder joints in electronic packaging technology

A new field of organic materials science that deals with amorphous molecular glasses has been opened up. In addition, amorphous molecular materials have constituted a new class of functional organic materials for use in various applications. This article is focused on the recent progress of studies on photo- and electroactive amorphous molecular materials, highlighting photochromic amorphous molecular materials, amorphous molecular resists, and amorphous molecular materials for use in devices such as organic EL devices. The molecular design concepts, syntheses, reactions, molecular and solid-state properties, functions, and device fabrication and performance are described.

Photo- and electroactive amorphous molecular materials—molecular design, syntheses, reactions, properties, and applications

The optical band gap of ZnO thin films deposited on fused quartz by metal-organic chemical-vapor deposition was studied. The optical band gap of as-grown ZnO blueshifted from 3.13to4.06eV as the growth temperature decreased from 500to200°C. After annealing, the optical band gap shifted back to the single-crystal value. All the ZnO thin films studied show strong band-edge photoluminescence. X-ray diffraction measurements showed that samples deposited at low temperatures (<450°C) consisted of amorphous and crystalline phases. The redshift of the optical band gap back to the original position after annealing was strong evidence that the blueshift was due to an amorphous phase. The unshifted photoluminescence spectra indicated that the luminescence was due to the crystalline phase of ZnO, which was in the form of nanocrystals embedded in the amorphous phase.

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Blueshift of optical band gap in ZnO thin films grown by metal-organic chemical-vapor deposition

Failure mechanisms of solder interconnects under current stressing in advanced electronic packages

Research advances in nano-composite solders

Growth kinetics of interfacial Cu‐Sn intermetallic (IMC) layer and its effect on the shear strength on practical LCCC surface mount solder joints were studied for isothermal aging at 70, 120, 155 and 170°C. Only normal Cu6Sn5 (h-phase) intermetallic was found in the interfacial IMC layer of as-soldered solder joint, whereas the duplex structure of both h-phase and o-phase Cu3Sn existed in all annealed joints. The growth kinetics of the overall interfacial IMC layer can simply be described by classical kinetic theory for solid-state diffusional growth with an activation energy of 1.09 eV and interdiffusion coefficient of 1.61 10 4 m 2 s 1 . The relatively higher activation energy, as compared with that found for bi-metallic couple of eutectic Sn‐Pb solder on copper, is attributed to the dissolution of Ni from the component metallization into the bulk Sn‐Pb solder. In addition, the shear fractures in all the solder joints investigated are shown to be ductile in nature and confined in the bulk solder rather than through the interfacial IMC layer. A linear reduction in shear joint strength was observed with an increase in intermetallic layer thickness up to 5.6 mm. Such a reduction in joint strength is due to a continuous removal of Sn from the bulk solder for the growth of interfacial IMC layer and flattening of the solder:IMC layer during isothermal aging of the solder joint. © 1998 Elsevier Science S.A. All rights reserved.

Growth kinetic studies of Cu–Sn intermetallic compound and its effect on shear strength of LCCC SMT solder joints

The effect of Cu-Sn intermetallic compounds (IMC) on the fatigue failure of solder joints during thermal cycling has been studied. The samples consist of components [leadless ceramic chip carrier (LCCC)] soldered onto FR-4 printed circuit board (PCB), and are prepared by conventional reflow soldering using a 63Sn-37Pb solder paste. The specimens are subjected to thermal cycling between -35/spl deg/C and 125/spl deg/C with a frequency of two cycles per hour until failure. The results indicate that the fatigue lifetime of the solder joints depends on the thickness of IMC's layer between Cu-pad and bulk solder, and the relation of the lifetime to the thickness can be described as a monotonically decreasing curve. The lifetime is very sensitive to the thickness of the IMC when the thickness is less than 1.4 /spl mu/m. During thermal cycling the thickness of the IMC layer increases and then the interface between IMC and solder becomes gradually flatter. The results of X-ray diffraction and scanning electron microscope (SEM) analysis show that cracks propagate along the interface between the IMC layer and the solder joint. The Cu/sub 3/Sn (/spl epsiv/-phase) is also found to form between the Cu-pad and /spl eta/-phase during thermal cycling. On the basis of the above results, the thick and flattened IMC layer is shown to responsible for the fatigue failure of solder joint during thermal cycling. The results of this paper can be used to optimize the reflow soldering process for the fabrication of robust solder joints.

Y.C. Chan

Papers

Failure mechanisms of solder interconnects under current stressing in advanced electronic packages

Research advances in nano-composite solders

Growth kinetic studies of Cu–Sn intermetallic compound and its effect on shear strength of LCCC SMT solder joints

Effect of intermetallic compounds on the thermal fatigue of surface mount solder joints

Growth kinetics of intermetallic compounds in chip scale package solder joint