Function Of Filler Content Research Articles

Towards model-based engineering of optoelectronic packaging materials: dielectric constant modeling

Increases in data transmission speeds of optoelectronic devices have consequently increased high-frequency requirements for optoelectronic packaging materials including substrate, EMC/EMI shielding, adhesive and encapsulant (molding and underfill) materials. Most of those materials are polymer/filler composites, and critical materials properties for the device design and packaging include the effective dielectric constant, dielectric loss and their frequency and filler concentration dependence. This work presents a systematic theoretical investigation of the effective dielectric constant of polymer/filler composite materials, and its dependence on the filler concentration, the filler/polymer interaction, and the size of fillers. Our results demonstrate that, in contrary to the prevailing views, the filler concentration dependence of the effective dielectric constant is non-monotonic. Depending on the dielectric constant ratio between filler and polymer matrix, and the degree of interaction between filler and matrix, the effective dielectric constant exhibits an extreme as a function of filler concentration. In addition, our model is demonstrated to contain the Maxwell–Wagner formulation as an asymptotic limit. The present results have significant implications to the targeted formulation of optoelectronic packaging materials.

Effective elastic modulus of underfill material for flip-chip applications

In this paper, a micromechanics model based on the Mori-Tanaka method was developed to estimate the elastic modulus of underfill materials. An explicit expression of the underfill modulus was derived as a function of filler content and the properties of the matrix and the fillers. Predictions of the modulus from this theory were compared with experimentally measured values. Excellent agreement was observed.

Yielding and Fracture Toughness of Glass Microballoon-Filled Epoxy Composites

The yielding and fracture behaviour of epoxy/glass microballoon composites has been studied as a function of filler content, temperature and strain rate. No increase in elastic modulus, yield stress and fracture toughness was observed. The compressive yield stress of the composites showed strain rate dependence with more than one rate-activated yielding process. The fracture toughness parameters Gc and Kc were found to be temperature insensitive. The variations in the measured mechanical quantities are discussed in terms of the observed morphology and thermal behaviour of the epoxy composites.

Towards model-based engineering of underfill materials: CTE modeling

Polymeric composite based underfill materials, with well-controlled coefficient of thermal expansion (CTE) are critical to flip-chip and other advanced high-density integrated circuit packaging technologies. The use of underfills beneath the flip-chip integrated circuits leads to an increase in reliability by reducing the strain on the solder bumps during thermal cycling imposed by the CTE mismatch between the chip and substrate. A fundamental understanding of the composite CTE of underfill materials is critical to the manufacture of high performance underfill materials and is critical to market expansion of flip-chip technology for high density packaging applications. This work presents a novel model for predicting the effective CTE of underfills and other polymeric composite materials by considering the effect of an interphase zone surrounding the filler particles in a polymer matrix. A microscopic model is also introduced for the volume fraction of the interphase as a function of filler concentration as well as filler–filler overlapping. The CTE model resolves several conflicts regarding the effect of filler concentration, filler size and filler–polymer interaction on the effective CTE of underfill and other polymeric composite materials. The results are demonstrated to be critical for accurate flip-chip reliability predictions based on finite-element and other modeling techniques.

The influence of filler concentration on the compaction and filtration properties of Zirfon®-composite ultrafiltration membranes

Abstract During ultrafiltration, a transmembrane pressure difference is applied across a polymeric membrane. This pressure difference results in solute transport through the membrane and mechanical compression of the membrane. The ultrasonic time-domain reflectometry (UTDR) technique is used to measure mechanical compression of the membrane structure in real time while water permeability is simultaneously measured during ultrafiltration in a specially designed crossflow filtration cell. The membranes used in this study are Zirfon® organo-mineral membranes fabricated using 0.9-μm zirconia particles at three different loadings. Thickness changes and the permeability of composite ultrafiltration membranes are obtained simultaneously and in real-time as a function of filler concentration and transmembrane pressure. Results indicate that the presence of ZrO 2 particles in the polymeric matrix influences the membrane permeability as well as compression of the membrane structure. Increasing the filler concentration decreases the elastic strain component but increases the time-dependent strain component. Both the initial and the time-dependent strain increase with increasing pressure. The flux decline is more severe at higher pressures and appears to be due to localized deformation in the membrane skin layer. This study provides the first quantitative information regarding compaction of organo-mineral Zirfon® membranes.

Polymer matrix composites filled with nanoporous metal powders: Preparation and electrical properties

The production of a new kind of silver powder for the preparation of electrically conducting polymers is described. This comprises the method of inert-gascondensation with forced gas flow in a closed-loop system and a quasi continuous collection of the nanoporous metal deposits via filter separation. After sieving, powders of various morphologies are obtained. The electrical resistance of polymers filled with two kinds of such powders as a function of filler content is presented, showing percolation at filler contents below 5 vol%. In a two component epoxy resin system, the metal-like resistance observed for samples cured at room temperature changes into a semiconductor-like resistance upon thermal annealing. However, a thermal pretreatment of the powder retains the metal-like conductivity.

Effective thermal conductivity of isotropic polymer composites

The effective thermal conductivity of tin powder filled high density polyethylene composites is investigated experimentally as a function of filler concentration and the measured values are compared with the existing theoretical and empirical models. Samples are prepared by compression molding process, up to 16% volumetric concentration of tin particles. The thermal conductivity is measured by a modified hot wire technique in a temperature range from about 0°C to 70°C. Experimental results show a region of low particle content, up to about 10% volume concentration, where the increase in thermal conductivity is rather slow. The filler particles are dispersed in the matrix material in this region, the thermal conductivity is best predicted by Maxwell's model and Nielsen's model with A=1.5, φ m=0.637. Whereas, at high filler concentrations, the filler particles tend to form agglomerates and conductive chains in the direction of heat flow resulting in a rapid increase in thermal conductivity. A model developed by Agari and Uno estimates the thermal conductivity in this region, using two experimentally determined constants.

A novel method to prepare high‐resistivity polymer composites

AbstractA novel method was developed for the preparation of high‐resistivity conductive polymer composites reproducibly. The conventional method which involves the simple mixing of a conductive filler and a polymer usually produces a conductive polymer composite having a loading curve with a region in which the resistivity changes rapidly as the filler concentration changes. Hence, it is very difficult to obtain reproducible materials in that region. This newly developed method involves the preparation of a conventional conductive polymer composite, which is prepared by the simple mixing of carbon black and a polymer as the first step. The resulting compound, which is crosslinked by either electron‐beam radiation or a chemical‐crosslinking agent, is ground into a fine powder (composite filler) with the particle size less than 250 microns. The composite filler is mixed with another polymer to form a conductive particulate composite which has a loading curve showing a more gradual change of resistivity as a function of filler concentration. The modification of the loading curve is controlled by the resistivity, the shape, and the size of the composite filler.

Thermal characteristics of silver powder‐filled polypropylene composites

AbstractThe thermal properties of silver powder‐filled polypropylene composites were studied as a function of filler concentration. Thermal conductivities of the composites increased with filler content. Comparison of the data with theoretical predictive models for two‐phase systems showed that the Nielson equation agreed reasonably well with the measured values. A TGA study showed that presence of silver powder increased the thermal stability of polypropylene through an increase in activation energy. The thermal expansion coefficient of the composites exhibited a decreased in the presence of Ag powder. Surface treatment of Ag powder with a titanate coupling agent marginally increased thermal conductivity and decreased thermal expansion coefficient values, implying a degree of enhance adhesion of the filler with the polymer. © 1994 John Wiley & Sons, Inc.

Effect of filler concentration on the phase separation in poly(vinyl acetate)‐poly(methyl methacrylate) mixtures

AbstractThe temperatures and kinetics of phase separation have been studied as a function of filler concentration in a polymer mixture of poly(vinyl acetate)‐poly(methyl methacrylate). The filler was shown to have a marked effect on the cloud point position and the phase separation kinetics at various temperatures. The effects observed are dependent upon the component ratio.

Effect of fibre surface treatment on yielding and fracture behaviour of glass fibre-polypropylene composite

Two parallel studies of the mechanical properties of composites based on modified polypropylene is presented. The fillers used were either uncoated-glassy fibres or glassy fibres coated with aluminium. The tensile properties and fracture behaviour are investigated as a function of filler content over a range of temperatures and strain rates. The measured elastic modulus, tensile strength, and fracture parameters, Gc and Kc, showed dependencies on filler content. The observed enhancement is found to be remarkable in the case of composites containing uncoated surface treated glass fibres. The tensile yield stress of all the composites prepared showed strain rate and temperature dependence. Stress activation volume and activation energy of single-activated yielding processes are determined. The observed fracture morphology could explain the reinforcement behaviour of the measured mechanical properties.

Fatigue crack propagation in posterior dental composites and prediction of clinical wear

AbstractFatigue crack propagation in a notched specimen under cyclic loading was investigated for a series of experimental composite resins and six commercial composites. The fatigue resistance was measured as a function of filler content, water absorption, and post‐curing temperature. Constants A and m of the Paris law have been obtained by linear regression analysis. The plot of fatigue resistance against volumetric filler content, V shows a sigmoidal relationship wiht a rapid increase in the range of V = 40–50% then tapering off at higher V. A substantial increase in the constant m of the Paris law was found for water‐saturated commercial composite resins. Although increasing the post‐curing temperature from 37°C to 70°C results in a 10% increase in fracture resistance, a high post‐curing temperature is not favorable for highly filled composites (V > 60%) where more strained areas can be induced unless polymerization shrinkage is minimal. The ranking of fracture resistance for the commercial materials is: Occlusin > P30 ≃ Adaptic II ≃ Ful‐Fil > Isomolar > LC33. Based on the assumption that microcrack growth in the subsurface damage layer is the precursor of wear, fatigue resistance combined with fracture toughness satisfactorily predicted the clinical wear of commercial composites. Surface microfracture which amplifies the subsurface flaw size and creates undesirable stress concentrators can be subdued by high fracture toughness. In addition, high fatigue resistance retards the formation of wear particles, thereby reducing loss of material from the composite surface.

AC and DC dielectric properties of some polypropylene/calcium carbonate composites

AbstractAC dielectric properties and thermally stimulated polarization (TSP) and depolarization (TSD) currents were studied in a series of CaCO3‐filled polypropylene composites. The filler content (0 to 50 weight percent) and the average particle size (3.0 to 16.1 μm) at constant filler content (30 weight percent) were varied in separate groups of samples. In a third group of samples the filler (20 to 40 weight percent) was surface treated with stearates. The AC dielectric behavior of composites containing untreated fillers is largely determined by a small amount of adsorbed water. Upon heating, the dielectric properties show maxima (increasing with decreasing frequency) which disappear on cooling. In the case of stearate‐treated fillers the dielectric loss level is higher, the dispersion and loss curves on heating reflect a combination of dipolar and protonic processes with water desorption. In the dry state the onset of an audio frequency relaxation process is observed in the pre‐melting zone. The thermally stimulated currents of the composites containing treated and untreated fillers are also different. In the case of the untreated fillers the TSP curves show maxima indicating water desorption which are increasingly intense and roughly exponential with filler content. The high temperature conductivity and the intensity of the pre‐melting depolarization peak pass through a minimum as a function of filler content. Above 20 weight percent filler content the activation energy of high temperature conductivity decreases. In the case of the surface treated samples, the thermally stimulated response is different for “wet” and dried samples. The dry samples exhibit a relaxation between the amorphous and crystalline transitions of the matrix polymer which is probably due to interfacial relaxation caused by the enhanced surface conductivity of the stearate‐treated fillers.

Electron transport in polyvinylchloride-magnetite composites

Magnetite/polyvinylchloride composites with a variety of concentrations of magnetite were fabricated to investigate their electrical properties. The electrical conductivities of the specimens were measured as a function of filler concentration and temperature. In the temperature range of 130–293 K, the temperature dependence of the low-field conductivity showed that electrons were thermally activated. The current-voltage data measured at high fields showed a non-linear curve, indicating that an electron emission process is dominant in the electrical conduction. The charging current variation with time showed a current peak due to trap centers. The electrical transport in the composite is explained by a band model. Surface states on the folded chains and the localized states in the amorphous region were found to play an important role in the electrical transport in the composites.

The electrical properties of magnetite-loaded polyethylene composites

Magnetite–polyethylene composites containing various concentrations of magnetite have been prepared and their electrical properties have been investigated. The electrical resistivities of the specimens were studied as function of filler concentration and temperature. The current density-electric field characteristic and the current variation with time were measured. The thermionic and field emission models provide good explanations for the electrical conduction in the specimens, and space-charge-limited conduction is discussed based on the results.

Mechanical properties of drawn poly(methyl methacrylate) filled with ultrafine particles

AbstractThe elastic and yield properties of drawn poly(methyl methacrylate) (PMMA) filled with ultrafine SiO2 are described as functions of filler content and size. The drawn PMMA composites were made by uniaxially drawing to x4.0 at 100°C and at a rate of 20 mm/min. Four compliance values, i.e., S33, S11, S13, and S44 were determined. These values decreased with filler content and decreasing filler size. The relative compliance values Sde/Sdo(Sde is the compliance of drawn PMMA composites and Sdo is that of drawn unfilled PMMA) are almost equivalently changed with changes in filler content. The elastic properties of drawn PMMA composites are thus reinforced isotopically. This is characteristic of PMMA which has a large side group. The yield behavior of drawn PMMA composites have similar filler size and content dependence to those of elastic properties except that the transverse yield stresses become more brittle with filler content. The anisotropy in yield stress is relatively larger than that of elastic properties. This is probably because the anti‐reiforcing effect, such as fibrillation becomes prominent with increasing filler content in the perpendicular direction.

Resistivity behavior of filled electrically conductive crosslinked polyethylene

AbstractThe electrical resistivity of crosslinked high‐density polyethylene loaded with conductive blacks and carbon fibers was studied as a function of filler concentration and temperature. A thermoelectric switching phenomenon (a sudden resistivity increase in the vicinity of the polyethylene melting point) in such semicrystalline conductive systems was investigated. Significant switching behavior was exhibited by compounds containing very low concentrations of carbon fibers. Some crosslinked compounds filled with mixtures of carbon black and carbon fibers were also studied.

Mechanical properties of oriented polyvinylchloride composites filled with ultrafine particles

The tensile stress-strain behaviour of undrawn and drawn polyvinylchloride (PVC) composites filled with ultrafine SiO2 and micron sized glass particles were discussed as functions of filler content and size. For the undrawn PVC composites filled with ultrafine SiO2 particles, Young's moduli, yield and breaking stresses increased with filler content and decreasing filler size. Whereas for the composites filled with micron sized glass particles, their Young's moduli slightly increased with filler content but both the yield and breaking stresses decreased. Oriented PVC composites were made by uniaxially drawing to × 2.5 at 100 °C. Anisotropic mechanical properties of oriented specimens were discussed in terms of compliance tensor and yield stress measured in the direction of 0°, 45° and 90° to the original stretching direction at room temperature. In the case of 70A SiO2, all the compliance decreased with filler content, whereas those of 65μ glass, this relation was reversed. The yield stress of the oriented PVC composites showed filler size dependence similar to Young's moduli. The anisotropic yield stress of oriented PVC composites were reasonably analysed by Hill's yield criterion.

Bulk modulus and thermal expansivity of melt polymer composites: Statistical versus macro‐mechanics

AbstractPhysical properties of composites as a function of filler content have been described in the past through models requiring the solution of macroscopic equations of motion for an elastic continuum or, on a more empirical level, by the assumption of various compositional averaging schemes. We discuss a model drawn from our molecular mixture theory. This allows us to avoid the formal problems attending concentrated suspensions and introduces molecular filler‐medium interaction parameters. Comparisons with experimental material and empirical equations involving bulk modulus and thermal expansivity will be given. The theory opens the possibility of determining interfacial strengths from appropriate mechanical and thermal measurements and of predicting one set from another set of data.

Mechanical behavior and dilatation of particulate‐filled thermosets in the rubbery state

AbstractThe stress–strain–dilatational response of glass‐bead‐filled, amorphous, network polyurethanes was investigated above their glass transition temperature. The mechanical–dilatational behavior was studied as functions of filler content, particle size, crosslink density of the polymer, and the surface treatment of the filler. It was found that the stress–strain properties are strongly affected by separation of the filler from the matrix. Measurements of the vacuole formation and growth processes enabled modeling the stress–strain response as well as understanding the ultimate behavior of the composites. Also, it was found that the dilatational response of the composites can be shifted on a single curve given by the second integral of the Gaussian function.