On the treatment of boundary conditions by the method of artificial parameters

A procedure to deal with various boundary conditions in the Rayleigh-Ritz method is discussed herein, namely, the method of artificial parameters. The method is then applied to solve a simple string problem; results obtained compare very nicely with exact solutions.     

Muscle Wasting in Chronic Kidney Disease: Mechanism and Clinical Implications—A Narrative Review

Muscle wasting, known to develop in patients with chronic kidney disease (CKD), is a deleterious consequence of numerous complications associated with deteriorated renal function. Muscle wasting in CKD mainly involves dysregulated muscle protein metabolism and impaired muscle cell regeneration. In this narrative review, we discuss the cardinal role of the insulin-like growth factor 1 and myostatin signaling pathways, which have been extensively investigated using animal and human studies, as well as the emerging concepts in microRNA- and gut microbiota-mediated regulation of muscle mass and myogenesis. To ameliorate muscle loss, therapeutic strategies, including nutritional support, exercise programs, pharmacological interventions, and physical modalities, are being increasingly developed based on advances in understanding its underlying pathophysiology.     

Investigation of growth behavior of Al-Cu intermetallic compounds in Cu wire bonding

In this work, we investigate the intermetallic compound formation in Cu wire bonded device. Voids near the Cu side at the bond interface are clearly seen. Nevertheless, these voids do not seem to interfere with the function of the unit. High temperature storage test (HTST) results show that there are Al₂Cu and AlCu in the damaged unit while Al₂Cu and Al₄Cu₉ appear in the good unit. The results clearly show that the Al layer is exhausted in the damaged unit while those with Al₄Cu₉ on the Cu side pass HTST with unconsumed Al. Theoretical calculations indicate that AlCu and Al₄Cu₉ are energetically more favorable than Al₂Cu, which is consistent with the reported IMC forming sequence. Formation energy of AlCu is compatible but slightly lower to that of Al₄Cu₉, suggesting AlCu tends to be the most stable phase among all. The reason why the Al layer is completely consumed in one case and some Al layer remains in the other is due to the fact that the formation of AlCu requires more than twice the amount of Al than Al₄Cu₉ for the same amount of Cu consumed. The complete consumption of Al is proposed as the reason responsible for the failure of the damaged unit.     

A 90–96 GHz CMOS down-conversion mixer with high conversion gain and excellent LO–RF isolation

A 90–96 GHz down-conversion mixer for 94 GHz image radar sensors using standard 90 nm CMOS technology is reported. RF negative resistance compensation technique, i.e. NMOS LC-oscillator-based RF transconductance (GM) stage load, is used to increase the output impedance and suppress the feedback capacitance C_gd of RF GM stage. Hence, conversion gain (CG), noise figure (NF) and LO–RF isolation of the mixer can be enhanced. The mixer consumes 15 mW and achieves excellent RF-port input reflection coefficient of ?10 to ?36.4 dB for frequencies of 85–105 GHz. The corresponding -10 dB input matching bandwidth is 20 GHz. In addition, for frequencies of 90–96 GHz, the mixer achieves CG of 6.3–9 dB (the corresponding 3-dB CG bandwidth is greater than 6 GHz) and LO–RF isolation of 40–45.1 dB, one of the best CG and LO–RF isolation results ever reported for a down-conversion mixer with operation frequency around 94 GHz. Furthermore, the mixer achieves an excellent input third-order intercept point of 1 dBm at 94 GHz. These results demonstrate the proposed down-conversion mixer architecture is very promising for 94 GHz image radar sensors.     

Thermal stability investigations of AlGaN/GaN HEMTs with various high work function gate metal designs

The operation of high power RF transistor generates a huge amount of heat and thermal effect is a major consideration for improving the efficiency of power transistors. AlGaN/GaN high electron mobility transistors (HEMTs) on silicon substrates have been studied extensively because of their high thermal conductivity. This study comprehensively investigates the DC, low frequency noise, microwave and RF power performance of Al_0.27Ga_0.73N/GaN HEMTs on silicon substrates at temperatures from room temperature to 100 °C using high work function metals such as palladium (Pd) and iridium (Ir) gate metals. Although the conventional Ni gate exhibited a good metal work function with AlGaN, which is beneficial for increasing the Schottky barrier height of HEMTs, the diffusion of Ni metal toward the AlGaN and GaN layers influences the DC and RF stability of the device at high temperatures or over long-term operation. Pd and Ir exhibited less diffusion at high temperature than Ni, resulting in less degradation of device characteristics after high temperature operation.     

Spatial-resolution enhancement in computed tomography

We propose an approach that combines an asymmetric fan-beam configuration and a new reconstruction algorithm to enhancing spatial resolution in computed tomography (CT). The asymmetric configuration can be achieved by changing the center of rotation (COR) from the conventional symmetric configuration. It does not, however, require new detectors and X-ray source nor alter the relative geometry between the detector and the X-ray source. By effectively reducing the distance of the COR to the X-ray source, the asymmetric configuration can increase the effective sampling density in projection data without reducing the size of the field of view (FOV). The proposed algorithm, on the other hand, can reconstruct images directly from data acquired with this asymmetric configuration. We performed numerical studies to demonstrate and validate the proposed acquisition/reconstruction approach. Results in these studies confirm that the proposed approach can lead to enhanced spatial resolution in reconstructed images. The proposed acquisition/reconstruction approach may find applications in micro-CT and industrial CT in which the CORs may be changed.     

A better channel coding for a Link‐16 waveform in pulsed‐noise interference

Link‐16 is a tactical data link currently used by North Atlantic Treaty Organization (NATO) countries, the United States and its allies. The Link‐16 waveform features Reed–Solomon codes for channel coding, cyclic code‐shift keying for 32‐ary baseband symbol modulation, minimum‐shift keying for waveform modulation, and frequency hopping for transmission security. In addition to the original errors‐only decoding of Reed–Solomon codes, both an errors‐and‐erasures decoding (EED) and a special concatenated coding are proposed in this paper to determine a better channel coding scheme for a Link‐16 waveform with noncoherent detection in the presence of pulsed‐noise interference (PNI). The investigation is first carried out both analytically and by simulation for the original Link‐16 waveform transmitted over AWGN. It is then accomplished analytically for the proposed waveforms in both AWGN and PNI. The results show that EED achieves the best error rate performance for a Link‐16 waveform in both AWGN and PNI when the signal‐to‐noise ratio is relatively small. When both the signal‐to‐noise ratio is sufficiently large and the fraction active time of PNI is small, the proposed concatenated coding outperforms both EED and errors‐only decoding. Copyright © 2012 John Wiley & Sons, Ltd.     

Effects of underfill materials on the reliability of low-K flip-chip packaging

The reliability of low-K flip-chip packaging has become a critical issue owing to the low strength and poor adhesion qualities of the low-K dielectric material when compared with that of SiO₂ or fluorinated silicate glass (FSG). The underfill must protect the solder bumps and the low-K chip from cracking and delamination. However, the material properties of underfill are contrary to those required for preventing solder bumps and low-K chip from cracking and delamination. This study describes the systematic methodologies for how to specify the adequate underfill materials for low-K flip-chip packaging. The structure of the test vehicle is seven copper layers with a low-K dielectric constant value of 2.7-2.9, produced by the chemical vapor deposition (CVD) process. Initially, the adhesion and the flow test of the underfill were evaluated, and then the low-K chip and the bumps stress were determined using the finite element method. The preliminary screened underfill candidates were acquired by means of the underfill adhesion and flow test, and balancing the low-K chip and the bumps stress simulation results. Next, the low-K chips were assembled with these preliminary screened underfills. All the flip-chip packaging specimens underwent the reliability test in order to evaluate the material properties of the underfill affecting the flip-chip packaging stress. In addition, the failed samples are subjected to failure analysis to verify the failure mechanism. The results of this study indicate that, of the underfill materials investigated, those with a glass transition temperature (T_g) and a Young’s modulus of approximately 70–80 °C and 8–10 GPa, respectively, are optimum for low-K flip-chip packaging with eutectic solder bumps.     

Interfacial reactions in the Sn-(Ag)/(Ni,V) couples and phase equilibria of the Sn-Ni-V system at the Sn-Rich corner

Ni-7wt.%V is commonly used as the barrier layer material in the under-bump metallurgy in the microelectronic industry. Although interfacial reactions between various solders with the nickel substrate have been investigated intensively, the effects of vanadium addition upon the solder/(Ni,V) interfacial reactions have not been studied. Sn/(Ni,V) and Sn-Ag/(Ni,V) interfacial reactions at 250°C were investigated in this study using the reaction couple technique. The vanadium contents of the (Ni,V) substrates examined in this study are 3 wt.%, 5 wt.%, 7 wt.%, and 12 wt.% and the reaction time is 12 h. The results indicate that when the vanadium contents in the (Ni,V) substrate are 5 wt.% and higher, the Sn/(Ni,V) and Sn-Ag/(Ni,V) interfacial reactions are different from those in the solder/Ni couples. Besides the Ni₃Sn₄ phase as commonly formed in the reaction with Ni substrate, a new ternary T phase has been found, and the reaction path is L/Ni₃Sn₄/T/(Ni,V). A 250°C Sn-Ni-V isothermal section is proposed based on the three constituent binary systems and limited experimental results obtained in this study. The reaction path is illustrated with the proposed Sn-Ni-V isothermal section. No stable ternary Sn-Ni-V phase is found from the phase equilibria study, and the new T phase is likely a metastable phase.     

Electrothermal coupling analysis of current crowding and Joule heating in flip-chip packages

Solder bumps serve as electrical paths as well as structural support in a flip-chip package assembly. Owing to the differences of feature sizes and electric resistivities between a solder bump and its adjacent traces, current densities around the regions where traces connect the solder bump increase in a significant amount. This current crowding effect along with the induced Joule heating would accelerate fatigue failure due to electromigration. In this paper we apply the three-dimensional electrothermal coupling analysis to investigate current crowding and Joule heating in a flip-chip package assembly carrying different constant electric currents under different ambient temperatures. Experiments are conducted to calibrate temperature-dependent electric resistivities of solder alloy, Al trace, and Cu trace, and to verify the numerical model by comparing calculated and measured maximum temperatures on the die surface. Through the electrothermal coupling analysis, effects of current crowding and Joule heating induced by different solder bump structures are examined and compared.