Solder balling of lead-free solder pastes

Solder balling in Sn/Ag/Cu solder pastes was studied in this work. Three different solder pastes, several different reflow profiles and conditions, and two stencil thicknesses were used in the investigation. During the first phase, called the verification phase, the solder pastes were checked to ensure they met the minimum requirements. In the process-screening phase, the reflow profile was varied. Results show that besides flux chemistry, reflow atmosphere plays the major role in solder balling. The average number of solder balls with the best paste was one fifth of that with the worst paste. Furthermore, with all the pastes, the number of solder balls dropped close to zero when nitrogen atmosphere was used. Another finding during the reflow process screening was the influence of the stencil thickness on the solder-balling result. With a thinner stencil, two of the pastes exhibited significant solder balling. This is assumed to be caused by the different ability of fluxes to withstand oxidation during the preheating in the reflow process. In the last phase, the effect of the solder-paste particle size on solder balling was studied more closely. The flux chemistry was kept unchanged, and the solder particle size was varied between type 3 and type 4. The results show that, with type 4 paste, significantly more solder balls are formed compared to type 3 paste. It was also confirmed that, regarding the reflow profile, the ramp-up rate from 150°C to 217°C and the reflow atmosphere were the most significant factors that determine the solder-ball formation for both types of paste.     

Emulated single-mode fiber-optic link by use of a linearly chirpedfiber Bragg grating

A compact and low-cost device that emulates a standard and dispersion-shifted 100-km single-mode fiber-optic link at 1550 nm is proposed. The device, composed of a chirped fiber grating and a monitor coupler, is analyzed by simulation and experimentally proved. A chirped fiber grating synthesis procedure and an explanation for both cases are also introduced     

Progressive multifocal leukoencephalopathy in a patient with hypogammaglobulinemia

Baker's yeast reduction of methyl and ethyl (2-oxocyclohexyl) acetates proceeded with enantio- and diastereo-selectivity, affording the corresponding (2S)-trans-alcohols (major), (2S)-cis-alcohols (minor), and the unaltered (1S)-ketones with high optical purity.     

Effects of the chemical structure on the miscibility level and properties of phenoxy/polymethacrylate blends

The important effect that even a small change in the nature of the side chain of a component of a blend has in its miscibility level was observed in a series of blends of phenoxy (Ph) with poly(methacrylates). Thus, while the Ph/poly(methyl methacrylate) blends are miscible and the Ph/poly(ethyl methacrylate) blends partially miscible, Ph/poly(butyl methacrylate) blends were almost fully immiscible. The observed miscibility of Ph/poly(butylmethyl methacrylate) indicates that the change in a component of a miscible blend of some pendant units that give rise to miscibility, by those from a different second component, which give rise to immiscibility is less important. The observed decrease in the strength of the β secondary transition of Ph was clearly related to the miscibility level of the blends. The negative effects on properties of a very low molecular weight material can be overcame by blending with a miscible second component, rendering the overall molecular weight of the blend above the critical value. The change in the nature of the side chain, apart from the negative effect on fracture properties such as ductility, also had considerable effect on the short‐term mechanical properties such as modulus of elasticity and yield stress. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2978–2986, 2000     

Effects of injection temperature on the mixing level and properties of injection-mixed PMMA–phenoxy blends

Polymethyl methacrylate-phenoxy blends have been obtained in an injection machine at different injection temperatures in order to determine the possibility of direct blending in these processing machines and also the effect that different blending levels have in the mechanical properties of these miscible blends. High injection temperatures, but in the range used in production of the components, provided fairly well homogenized blends comparable both in transparency and mechanical properties to those obtained by kneading and subsequent compression molding. Very good mechanical properties, although less transparent, were obtained when processing at low injection temperatures.     

Solid‐state structure and properties of poly(ethyl methacrylate)/phenoxy blends

Poly(ethyl methacrylate)/poly(hydroxy ether of bisphenol A) (PEMA/phenoxy or PEMA/Ph) blends were obtained by melt mixing to investigate their solid‐state characteristics and mechanical properties. The slight structural change from poly(methyl methacrylate) (PMMA) to PEMA spoiled the miscibility of PMMA/Ph blends leading to biphasic PEMA/Ph blends. It is proposed that an antiplasticizer in the case of PEMA, and a low molecular weight component in the case of Ph, as well as minor amounts of each component, migrated to the other phase during melt mixing. The mechanical properties of the blends were good, given that they were biphasic. The modulus of elasticity and yield stress values were found to be additive. Despite the below‐additivity ductility values, ductile behavior was observed. The minor amount of the other component in each phase, and the migration of the antiplasticizer of PEMA, are proposed as the main causes of the observed mechanical properties. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1539–1546, 1999     

Multiple axonal tracing: simultaneous detection of three tracers in the same section

It is appropriate that this Special Section in Journal of Adolescence should focus on the needs of adolescents within the care system. The year 1998 marks the fiftieth anniversary of the passing of the Children Act (1948), which established the modern structure of services in England and Wales for children and adolescents deprived of adequate parenting in their own homes. Anniversaries are an opportunity for reflection. Are our current services an improvement on the past? In learning new lessons have we possibly lost sight of things we knew better, and did better, previously?     

Real-time spectrum analysis in microstrip technology

We report on a time-domain analog in microwave lines to the spatial Fraunhofer (far-field) diffraction in paraxial conditions. Microstrip lines are used to design filtering configurations acting as spectrum analyzers. They are based on linearly chirped distributed Bragg coupling between the fundamental microstrip mode and the same but counterpropagating mode. Linearly chirped continuous impedance modulation in a microstrip line with varying upper plane strip-width is shown to yield a mode-coupling location and group delay linearly distributed in frequency. Under the condition of a temporal equivalent to the spatial Fraunhofer inequality, the energy spectral density of the input signal is directly recoverable from the average output (reflected) power. It is only necessary to take into account a linear axis-change, given by the dispersion coefficient (group-delay slope) of the structure, from time to Fourier frequency. Both pulsed and nonpulsed RF signals are studied. Sequential time-gated segments of the input have to be processed in the nonpulsed case. The maximum frequency resolution achievable in this situation is discussed. The devices developed here could have important potential applications in the field of temporal signal processing, such as filtering using time-division techniques.     

Theoretical chemical characterization of phosphate-metal-humic complexes and relationships with their effects on both phosphorus soil fixation and phosphorus availability for plants

BACKGROUND: Previous studies showed that phosphate can be complexed by humic acids (HA) through stable metal (M) bridges (PMHA). We studied the thermodynamic properties of PMHA and their relationships with the ability of PMHA to both decrease soil P fixation and increase P availability for plants. With this aim, we studied the theoretical stability of PFeHA, PAlHA and PCaHA by molecular modelling methods in relation to the degree and intensity of P absorption in soils and the ability of plants to take up complexed P. RESULTS: A density functional theory (DFT) quantum chemical study enabled us to obtain stable structures for the three PMHA complexes in water solution. The theoretical stabilities (ΔG⁰) were consistent with that for apparent stability obtained by Scatchard method, PFeHA ≥ PAlHA > PCaHA, though the differences were clearer by the DFT method. Also the reduction of soil P fixation and the release of P from PMHA in the presence of an anionic resin confirmed the stability order of the different PMHA. Plant studies confirmed the ability of diverse plant species to take up both P and metal complexed in PMHA. CONCLUSION: The results indicated the potential efficiency of PMHA‐based fertilizers to optimize P fertilization for crops cultivated in soils with high P fixation ability. Copyright © 2012 Society of Chemical Industry     

Analysis and design of periodic structures for microstrip lines by using the coupled mode theory

In this work, the coupled mode theory is formulated to analyze distributed periodic structures for microstrip lines. After it, several reasonable approximations are introduced giving rise to analytical solutions for the problem. The obtained results, both numerical and analytical, are checked against measurements showing very good agreement. The proposed method is very attractive for the study of these devices since it avoids the time-consuming full-wave electromagnetic simulations customarily employed and provides analytical solutions that are very useful for analysis and synthesis purposes.