Microstructure characterization of Sn-Ag solder joints between stud bumps and metal pads

The microstructure of the flip-chip solder joints fabricated using stud bumps and Pb-free solder was characterized. The Au or Cu stud bumps formed on Al pads on Si die were aligned to corresponding metal pads in the substrate, which was printed with Sn-3.5Ag paste. Joints were fabricated by reflowing the solder paste. In the solder joints fabricated using Au stud bumps, Au-Sn intermetallics spread over the whole joints, and the solder remained randomly island-shaped. The δ-AuSn, ε-AuSn₂, and η-AuSn₄ intermetallic compounds formed sequentially from the Au stud bump. The microstructure of the solder joints did not change significantly even after multiple reflows. The AuSn₄ was the main phase after reflow because of the fast dissolution of Au. In the solder joints fabricated using Cu stud bumps, the scallop-type Cu₆Sn₅ intermetallic was formed only at the Cu interface, and the solder was the main phase. The difference in the microstructure of the solder joints with Au and Cu stud bumps resulted from the dissolution-rate difference of Au and Cu into the solder.     

A 1.8-V 6/9-GHz reconfigurable dual-band quadrature LC VCO in SiGe BiCMOS technology

This paper presents a dual-band voltage-controlled oscillator (VCO) that can be reconfigured between 6- and 9-GHz frequency bands. It comprises a 6-GHz LC-tuned VCO, two 1/2 dividers, two mixers, and two 3-GHz notch filters. The 9-GHz output is generated based on the analog frequency multiplication method by mixing the 6-GHz VCO output with its divide-by-two signal. The VCO, implemented in a 0.18-/spl mu/m SiGe BiCMOS technology, achieves a fast reconfiguration time of 3.6 ns. The measured VCO phase noises are -106 and -104 dBc/Hz at 1-MHz offset for 6- and 9-GHz modes, respectively, while draining 10.8 mA from a 1.8-V supply.     

Generalized construction of binary bent sequences with optimal correlation property

We generalize the construction method of the family of binary bent sequences introduced by Olsen, Scholtz, and Welch (1982) to obtain a family of generalized binary bent sequences with optimal correlation and balance property by using the modified trace transform. Then, the conventional binary bent sequence becomes a special case of our construction method. Several families of the generalized binary bent sequences are constructed by using the bent functions on the intermediate field. Using some of the generalized binary bent sequences, new families of binary sequences with optimal correlation and balance property can be constructed by the lifting idea similar to No (1988) sequences, which are referred to as binary bent-lifted sequences.     

Antenna-assisted round robin scheduling for MIMO cellular systems

Packet scheduling is investigated for the downlink of a multiple-input multiple-output (MIMO) cellular system. We propose an antenna-assisted round robin scheduling (AA-RRS) scheme to improve the conventional RRS scheme. The AA-RRS scheme provides fair channel access chance to users as the RRS scheme, whereas it increases the system capacity through the effective use of multiple antennas in achieving a diversity effect from multiple users. Simulation results show that the AA-RRS scheme achieves a significant capacity gain over the RRS scheme. We also investigate the effects of long-term power control on the system capacities for the proposed scheduling scheme.     

Meshing wireless personal area networks: Introducing IEEE 802.15.5

This article presents an overview of the IEEE project 802.15.5 that targets providing mesh capabilities to both high-rate and low-rate wireless personal area networks. Low-rate mesh is built on IEEE 802.15.4 MAC, while high-rate mesh utilizes IEEE 802.15.3 MAC. We seek to share our insights and motivations of the approach adopted in the major components of the standard instead of presenting a la carte items drawn in the specification. We hope this article helps readers of the 802.15.5 standard to better understand the rationale and intent of the protocol design.     

Non-functionalized soluble diketopyrrolopyrrole: Simplest p-channel core for organic field-effect transistors

We report the synthesis, characterization and behavior in field-effect transistors of non-functionalized soluble diketopyrrolopyrrole (DPP) core with only a solubilizing alkyl chain (i.e. –C₁₆H₃₃ or –C₁₈H₃₇) as the simplest p-channel semiconductor. The characteristics were evaluated by UV–vis and fluorescence spectroscopy, X-ray diffraction, cyclic voltammetry (CV), thermal analysis, atomic force microscopy (AFM) and density functional theory (DFT) calculation. For top-contact field-effect transistors, two types of active layers were prepared either by a solution process (as a 1D-microwire) or thermal vacuum deposition (as a thin-film) on a cross-linked poly(4-vinylphenol) gate dielectric. All the devices showed typical p-channel behavior with dominant hole transports. The device made with 1D-microwiress of DPP-R18 showed field-effect mobility in the saturation region of 1.42 × 10^?2 cm²/V s with I_ON/I_OFF of 1.82 × 10³. These findings suggest that the non-functionalized soluble DPP core itself without any further functionalization could also be used as a p-channel semiconductor for low-cost organic electronic devices.     

Strain‐Visualization with Ultrasensitive Nanoscale Crack‐Based Sensor Assembled with Hierarchical Thermochromic Membrane

As eidetic signal recognition has become important, displaying mechanical signals visually has imposed huge demands for simple readability and without complex signal processing. Such visualization of mechanical signals is used in delicate urgent medical or safety‐related industries. Accordingly, chromic materials are considered to facilitate visualization with multiple colors and simple process. However, the response and recovery time is very long, such that rapid regular signals are unable to be detected, i.e., physiological signals, such as respiration. Here, the simple visualization of low strain ≈2%, with ultrasensitive crack‐based strain sensors with a hierarchical thermochromic layer is suggested. The sensor shows a gradient color change from red to white color in each strain, which is attributed to the hierarchical property, and the thermal response (recovery) time is dramatically minimized within 0.6 s from 45 to 37 °C, as the hierarchical membrane is inspired by termite mounds for efficient thermal management. The fast recovery property can be taken advantage of in medical fields, such as monitoring regular respiration, and the color changes can be delicately monitored with high accuracy by software on a mobile phone.     

A Novel Uplink MIMO Transmission Scheme in a Multicell Environment

Uplink multiple-input multiple-output (MIMO) transmission scheme is developed for time division duplex (TDD) systems in a multicell environment. We propose a precoding scheme that maximizes the total achievable rate and works in the decentralized manner with only locally available channel state information (CSI) at each transmitter. We first establish and solve a decentralized optimization problem for the case of multiple-input single-output (MISO) channels, introducing a new precoding design metric called signal to generated interference plus noise ratio (SGINR). By extending the result to general MIMO channels, we propose an SGINR-based precoding scheme where the number of transmit streams is selected adaptively to the surrounding environments. Simulation results confirm that the proposed precoding scheme offers significant throughput enhancement in multicell environments.     

Energy Loss Mechanism in Organic and Inorganic Light-Emitting Diodes

We report that there exists a similar energy loss mechanism in fluorescent/phosphorescent organic light-emitting diodes (F/P OLEDs) and inorganic semiconductor optoelectronic devices [1310-nm InGaAsP-InP superluminescent diodes (SLDs)]. The loss of energy in inorganic SLDs based on thickness-altered asymmetric multiple quantum-well (QW) structures occurs depending sensitively on the sequence of QWs, an analogous behavior also observed in F/P OLEDs depending on the sequence of phosphorescent dopants for different colors. It is shown that such an energy (power) loss is evitable by placing long-wavelength QWs near the p-side in inorganic SLDs and similarly long-wavelength phosphors near the hole-transporting layer in F/P OLEDs.