Rheological characterisation of solder pastes and isotropic conductive adhesives used for flip-chip assembly

Solder pastes and isotropic conductive adhesives (ICAs) are widely used as a principal bonding medium in the electronic industry. This study investigates the rheological behaviour of the pastes (solder paste and isotropic conductive adhesives) used for flip-chip assembly. Oscillatory stress sweep test are performed to evaluate solid characteristic and cohesiveness of the lead-free solder pastes and isotropic conductive adhesive paste materials. The results show that the G' (storage modulus) is higher than G '' (loss modulus) for the pastes material indicating a solid like behaviour. It result shows that the linear visco-elastic region for the pastes lies in a very small stress range, below 10 Pa. in addition, the stress at which the value of storage modulus is equal to that of loss modulus can be used as an indicator of the paste cohesiveness. The measured cross-over stress at G'=G '' shows that the solder paste has higher stress at G'=G '' compared to conductive adhesives. Creep-recovery test method is used to study the slump behaviour in the paste materials. The conductive adhesive paste shows a good recovery when compared to the solder pastes. (C) 2008 Elsevier B.V. All rights reserved.

Preparation of o/w emulsions stabilized by solid particles and their characterization by oscillatory rheology

The purpose of this investigation was to examine the preparation and characterisation of hexane-in-water emulsions stabilised by clay particles. These emulsions, called Pickering emulsions, are characterised by the adsorption of solid particles at the oil/water (o/w) interface. The development of an elastic film at the o/w interface following the adsorption of colloidal particles helps to promote emulsion stability. Three different solid materials were used: silica sand, kaolin, and bentonite. Particles were added to the liquid mixtures in the range of 0.5–10 g dm−3. Emulsions were prepared using o/w ratios of 0.1, 0.2, 0.3, and 0.4. The effect of sodium chloride, on the stability of the prepared emulsions, was assessed in the range of 0–0.5 mol dm−3. In addition the use of a cationic surfactant hexadecyl-trimethylammonium bromide (CTAB) as an aid to improving emulsion stability was assessed in the concentration range of 0–0.05% (w/v). Characterisation of emulsion stability was realised through measurements of rheological properties including non-Newtonian viscosity, the elastic modulus, G', the loss modulus, G", and complex modulus, G*. The stability of the emulsions was evaluated immediately after preparation and 4 weeks later. Using the stability criteria, that for highly stable emulsions: G' > G" and both G' and G" are independent of frequency (varpi) it was concluded that highly stable emulsions could be prepared using a bentonite concentration of 2% (or more); an o/w ratio greater than 0.2; a CTAB concentration of 0.01%; and a salt concentration of 0.05 M or less—though salt was required.

The impact of ocean acidification and warming on the skeletal mechanical properties of the sea urchin Paracentrotus lividus from laboratory and field observations

Increased atmospheric CO2 concentration is leading to changes in the carbonate chemistry and the temperature of the ocean. The impact of these processes on marine organisms will depend on their ability to cope with those changes, particularly the maintenance of calcium carbonate structures. Both a laboratory experiment (long-term exposure to decreased pH and increased temperature) and collections of individuals from natural environments characterized by low pH levels (individuals from intertidal pools and around a CO2 seep) were here coupled to comprehensively study the impact of near-future conditions of pH and temperature on the mechanical properties of the skeleton of the euechinoid sea urchin Paracentrotus lividus. To assess skeletal mechanical properties, we characterized the fracture force, Young's modulus, second moment of area, material nanohardness, and specific Young's modulus of sea urchin test plates. None of these parameters were significantly affected by low pH and/or increased temperature in the laboratory experiment and by low pH only in the individuals chronically exposed to lowered pH from the CO2 seeps. In tidal pools, the fracture force was higher and the Young's modulus lower in ambital plates of individuals from the rock pool characterized by the largest pH variations but also a dominance of calcifying algae, which might explain some of the variation. Thus, decreases of pH to levels expected for 2100 did not directly alter the mechanical properties of the test of P. lividus. Since the maintenance of test integrity is a question of survival for sea urchins and since weakened tests would increase the sea urchins' risk of predation, our findings indicate that the decreasing seawater pH and increasing seawater temperature expected for the end of the century should not represent an immediate threat to sea urchins vulnerability.

A dynamic mechanical method for determining the silicone elastomer solubility of drugs and pharmaceutical excipients in silicone intravaginal drug delivery rings

The silicone elastomer solubilities of a range of drugs and pharmaceutical excipients employed in the development of silicone intravaginal drug delivery rings (polyethylene glycols, norethisterone acetate, estradiol, triclosan, oleyl alcohol, oxybutynin) have been determined using dynamic mechanical analysis. The method involves measuring the concentration-dependent decrease in the storage modulus associated with the melting of the incorporated drug/excipient, and extrapolation to zero change in storage modulus. The study also demonstrates the effect of drug/excipient concentrations on the mechanical stiffness of the silicone devices at 37°C.

Processing, Annealing and Sterilisation of Poly-L-Lactide.

Poly--lactide (PLLA) is one of the most significant members of a group of polymers regarded as bioabsorbable. Degradation of PLLA proceeds through hydrolysis of the ester bonds in the polymer chains and is influenced significantly by the polymer's molecular weight and crystallinity. To evaluate the effects of processing and sterilisation on these properties, PLLA pellets were either compression moulded or extruded, subjected to annealing at 120°C for 4 h and sterilised by ethylene oxide (EtO) gas. Procedures were used to evaluate the mechanical properties, molecular weight and crystallinity. Upon processing, the crystallinity of the material fell from 61% for the PLLA pellets to 12% and 20% for the compressed and extruded components, respectively. After annealing, crystallinity increased to 43% for the compression-moulded material and 40% for the extruded material. Crystallinity further increased upon EtO sterilisation. A slight decrease in molecular weight was observed for the extruded material through processing, annealing and sterilisation. Young's modulus generally increased with increasing crystallinity, and extension at break and tensile strength decreased. The results from this investigation suggest that PLLA is sensitive to processing and sterilisation, altering properties critical to its degradation rate.

Plug Force Monitoring for the Control and Optimisation of the Thermoforming Process

Thermoforming processes generally employ sheet temperature monitoring as the primary means of process control. In this paper the development of an alternative system that monitors plug force is described. Tests using a prototype device have shown that the force record over a forming cycle creates a unique map of the process operation. Key process features such as the sheet modulus, sheet sag and the timing of the process stages may be readily observed, and the effects of changes in all of the major processing parameters are easily distinguished. Continuous, cycle-to-cycle tests show that the output is consistent and repeatable over a longer time frame, providing the opportunity for development of an on-line process control system. Further testing of the system is proposed.

Towards An Acoustic Model-Based Poroelastic Imaging Method: I. Theoretical Foundation

The ultrasonic measurement and imaging of tissue elasticity is currently under wide investigation and development as a clinical tool for the assessment of a broad range of diseases, but little account in this field has yet been taken of the fact that soft tissue is porous and contains mobile fluid. The ability to squeeze fluid out of tissue may have implications for conventional elasticity imaging, and may present opportunities for new investigative tools. When a homogeneous, isotropic, fluid-saturated poroelastic material with a linearly elastic solid phase and incompressible solid and fluid constituents is subjected to stress, the behaviour of the induced internal strain field is influenced by three material constants: the Young's modulus (E(s)) and Poisson's ratio (nu(s)) of the solid matrix and the permeability (k) of the solid matrix to the pore fluid. New analytical expressions were derived and used to model the time-dependent behaviour of the strain field inside simulated homogeneous cylindrical samples of such a poroelastic material undergoing sustained unconfined compression. A model-based reconstruction technique was developed to produce images of parameters related to the poroelastic material constants (E(s), nu(s), k) from a comparison of the measured and predicted time-dependent spatially varying radial strain. Tests of the method using simulated noisy strain data showed that it is capable of producing three unique parametric images: an image of the Poisson's ratio of the solid matrix, an image of the axial strain (which was not time-dependent subsequent to the application of the compression) and an image representing the product of the aggregate modulus E(s)(1-nu(s))/(1+nu(s))(1-2nu(s)) of the solid matrix and the permeability of the solid matrix to the pore fluid. The analytical expressions were further used to numerically validate a finite element model and to clarify previous work on poroelastography.

Poly(epsilon-caprolactone) and poly(epsilon-caprolactone)-polyvinylpyrrolidone-iodine blends as ureteral biomaterials: characterisation of mechanical and surface properties, degradation and resistance to encrustation in vitro.

This study describes the physicochemical properties and in vitro resistance to encrustation of solvent cast films composed of either poly(epsilon-caprolactone) (PCL), prepared using different ratios of high (50,000) to low (4000) (molecular weight) m.wt., or blends of PCL and the polymeric antimicrobial complex, poly(vinylpyrrolidone)-iodine (PVP-I). The incorporation of PVP-I offered antimicrobial activity to the biomaterials. Films were characterised in terms of mechanical (tensile analysis, dynamic mechanical thermal analysis) and surface properties (dynamic contact angle analysis, scanning electron microscopy), whereas degradation (at 37degreesC in PBS at pH 7.4) was determined gravimetrically. The resistance of the films to encrustation was evaluated using an in vitro encrustation model. Reductions in the ratio of high:low-m.wt. PCL significantly reduced the ultimate tensile strength, % elongation at break and the advancing contact angle of the films. These effects were attributed to alterations in the amorphous content and the more hydrophilic nature of the films. Conversely, there were no alterations in Young's modulus, the viscoelastic properties and glass-transition temperature. Incorporation of PVP-I did not affect the mechanical or rheological properties of the films, indicative of a limited interaction between the two polymers in the solid state. Manipulation of the high:low m.wt. ratio of PCL significantly altered the degradation of the films, most notably following longer immersion periods, and resistance to encrustation. Accordingly, maximum degradation and resistance to encrustation was observed with the biomaterial composed of 40:60 high:low m.wt. ratios of PCL; however, the mechanical properties of this system were considered inappropriate for clinical application. Films composed of either 50:50 or 60:40 ratio of high:low m.wt. PCL offered an appropriate compromise between physicochemical properties and resistance to encrustation. This study has highlighted the important usefulness of degradable polymer systems as ureteral biomaterials

Co-melt fluidised bed granulation of pharmaceutical powders: Improvements in drug bioavailability

This study investigates the use of co-melt fluidised bed granulation for the agglomeration of model pharmaceutical powders, namely, lactose mono-hydrate, PEG 10000, poly-vinyl pyrolidone and ibuprofen as a model drug. Granulation within the co-melt system was found to follow a nucleationâ??steady growthâ??coating regime profile. Using high molecular weight PEG binder, the granulation mechanism and thus the extent of granulation was found to be significantly influenced by binder viscosity. The compression properties of the granulate within the hot fluidised bed were correlated using a novel high temperature experimental procedure. It was found that the fracture stress and fractural modulus of the materials under hot processing conditions were orders of magnitude lower than those measured under ambient conditions. A range of particle velocities within the granulator were considered based on theoretical models. After an initial period of nucleation, the Stokes deformation number analysis indicated that only velocities within the high shear region of the fluidised bed were sufficient to promote significant granule deformation and therefore, coalescence. The data also indicated that larger granules de-fluidised preventing agglomeration by coalescence. Furthermore, experimental data indicated that dissipation of the viscous molten binder to the surface was the most important factor in the latter stages of the granulation process. From a pharmaceutical perspective the inclusion of the model drug, ibuprofen, combined with PVP in the co-melt process proved to be highly significant. It was found that using DSC analysis on the formulations that the decrease in the heat of fusion associated with the melting of ibuprofen within the FHMG systems may be attributed to interaction between PVP and ibuprofen through inter-molecular hydrogen bonding. This interaction decreases the crystallinity of ibuprofen and facilitates solubilisation and bioavailability within the solid matrix.

Design of granular pavements

The use of high-quality quarried crushed rock aggregates is generally required to comply with current specifications for unbound granular materials (UGMs) in pavements. The source of these high-quality materials can be a long distance from the site, resulting in high transportation costs. The use of more local sources of marginal materials or the use of secondary aggregates is not allowed if they do not fully comply with existing specifications. These materials can, however, be assessed for their suitability for use in a pavement by considering performance criteria such as resistance to permanent deformation and degradation instead of relying on compliance with inflexible specifications. The final thickness of the asphalt cover and the pavement depth are governed by conventional pavement design methods, which consider the number of vehicle passes, subgrade strength, and some material property, commonly the California bearing ratio or resilient modulus. A pavement design method that includes as a design criterion an assessment of the resistance to deformation of a UGM in a pavement structure at a particular stress state is proposed. The particular stress state at which the aggregate is to perform in an acceptable way is related to the in situ stress, that is, the stress that the aggregate is anticipated to experience at a particular depth in the pavement. Because the stresses are more severe closer to the pavement surface, the aggregates should be better able to resist these stresses the closer they are laid to the surface in the pavement. This method was applied to two Northern Ireland aggregates of different quality (NI Good and NI Poor). The results showed that the NI Poor aggregate performed at an acceptable level with respect to permanent deformation, provided that a minimum of 70 mm of asphalt cover was provided. It was predicted that the NI Good material would require 60 mm of asphalt cover.

The atmospheric parameters and chemical composition of early B-type giants in h and chi Persei

Atmospheric parameters and surface chemical compositions are presented for eight stars, classified as B1 or B2 but with a range of luminosity classes, in the northern double cluster h and chi Persei. Echelle spectroscopy (covering the wavelength region 3900 to 4700 Ä) and grating spectroscopy (of the Balmer, H? and Hß lines) were analysed using non-LTE synthetic spectra based on LTE line-blanketed atmosphere structures. High microturbulences are found in our sample, and this quantity must be included in the computation of the non-LTE level populations; its effect is generally to decrease the derived metal abundances by typically 0.1 dex but by up to 0.4 dex. Our absolute abundances are in reasonable agreement with those previously found for main sequence B-type stars, while we find some evidence for small abundance variations (particularly for nitrogen) within our sample. One star (BD+56 678) appears to be a spectrum variable and at two epochs shows a highly enriched nitrogen spectrum. Our atmospheric parameters imply that two stars have previously been mis-identified as main sequence objects and a distance modulus, at the higher end of the values previously deduced. The observational HR diagram is consistent with stellar evolutionary models that explicitly include the effects of rotation.

Physicochemical characterization of bioactive polyacrylic acid organogels as potential antimicrobial implants for the buccal cavity

This study describes the formulation and physicochemical characterization of poly(acrylic acid) (PAA) organogels, designed as bioactive implants for improved treatment of infectious diseases of the oral cavity. Organogels were formulated containing a range of concentrations of PAA (3-10% w/w) and metronidazole (2 or 5% w/w, representing a model antimicrobial agent) in different nonaqueous solvents, namely, glycerol (Gly), polyethylene glycol (PEG 400), or propylene glycol (PG). Characterization of the organogels was performed using flow rheometry, compressional analysis, oscillatory rheometry, in vitro mucoadhesion, moisture uptake, and drug release, methods that provide information pertaining to the nonclinical and clinical use of these systems. Increasing the concentration of PAA significantly increased the consistency, compressibility, storage modulus, loss modulus, dynamic viscosity, mucoadhesion, and the rate of drug release. These observations may be accredited to enhanced molecular polymer entanglement. In addition, the choice of solvent directly affected the physicochemical parameters of the organogels, with noticeable differences observed between the three solvents examined. These differences were accredited to the nature of the interaction of PAA with each solvent and, importantly, the density of the resultant physical cross-links. Good correlation was observed between the viscoelastic properties and drug release, with the exception of glycerol-based formulations containing 5 and 10% w/w PAA. This disparity was due to excessive swelling during the dissolution analysis. Ideally, formulations should exhibit controlled drug release, high viscoelasticity, and mucoadhesion, but should flow under minimal stresses. Based on these criteria, PEG 400-based organogels composed of 5% or 10% w/w PAA exhibited suitable physicochemical properties and are suggested to be a potentially interesting strategy for use as bioactive implants designed for use in the oral cavity. © 2008 American Chemical Society.

Liquid monomer-powder particle interaction in acrylic bone cement

It is known that the method used to mix the liquid monomer and powder of PMMA bone cement influences the quality of the cement that is used in total joint replacements. Mixing theory indicates that the interaction between the liquid monomer and the powder is affected by a number of parameters, such as cement viscosity and degree of agitation, with this knowledge utilized in the design of cement mixing devices. Therefore, the objectives of this study were to: (i) obtain information on the interaction of the liquid monomer and the powder in the case of an PMMA bone cement, (ii) show how this knowledge can be applied to the design of an automated cement mixing device, and (iii) compare the porosity, bending modulus, and bending strength of one commercially-available cement prepared using the automated mixer and prepared using a conventional mixer that is in current clinical use. Experimental data indicated that increasing the velocity and decreasing the viscosity of the systems produced cement that improved mechanical properties, which may contribute to better mechanical integrity and, hence, reduced tendency for aseptic loosening, of cemented hip implants.

On the ultracompact nature of 4U 1822-000

We report the discovery of a periodic modulation in the optical lightcurve of the candidate ultracompact X-ray binary 4U 1822-000. Using time-resolved optical photometry taken with the William Herschel Telescope we find evidence for a sinusoidal modulation with a semi-amplitude of 8 per cent and a period of 191 min, which is most likely close to the true orbital period of the binary. Using the van Paradijs & McClintock relation for the absolute magnitude and the distance modulus allowing for interstellar reddening, we estimate the distance to 4U 1822-000 to be 6.3 kpc. The long orbital period casts severe doubts on the ultracompact nature of 4U 1822-000.

Biaxial deformation behavior and mechanical properties of a polypropylene/clay nanocomposite

Polymer nanocomposites offer the potential of enhanced properties such as increased modulus and barrier properties to the end user. Much work has been carried out on the effects of extrusion conditions on melt processed nanocomposites but very little research has been conducted on the use of polymer nanocomposites in semi-solid forming processes such as thermoforming and injection blow molding. These processes are used to make much of today’s packaging, and any improvements in performance such as possible lightweighting due to increased modulus would bring signi?cant bene?ts both economically and environmentally. The work described here looks at the biaxial deformation of polypropylene–clay nanocomposites under industrial forming conditions in order to determine if the presence of clay affects processability, structure and mechanical properties of the stretched material. Melt compounded polypropylene/clay composites in sheet form were biaxially stretched at a variety of processing conditions to examine the effect of high temperature, high strain and high strain rate processing on sheet structure
and properties.

A biaxial test rig was used to carry out the testing which imposed conditions on the sheet that are representative of those applied in injection blow molding and thermoforming. Results show that the presence of clay increases the yield stress relative to the un?lled material at typical processing temperatures and that the sensitivity of the yield stress to temperature is greater for the ?lled material. The stretching process is found to have a signi?cant effect on the delamination and alignment of clay particles (as observed by TEM) and on yield stress and elongation at break of the stretched sheet.