Investigating Dielectric Properties of Sintered Polymers for Rapid Manufacturing

Selective Laser Sintering (SLS) of polymers is the leading technology in the growing field of Rapid Manufacturing. High Speed Sintering (HSS) is a process that offers the potential to reduce costs and processing times and thus open significant new markets for Rapid Manufactured parts. Much academic research has been performed with respect to mechanical properties of Rapid Manufactured parts, however the area of electrical properties has received little attention to date. Electrical properties are obviously important in applications that will involve embedding of circuits with Rapid Manufactured 3D objects. However electrical properties are also important for a wide variety of electrical products where Rapid Manufactured parts can be used as housings etc. This paper focuses on the dielectric properties of parts made by SLS and HSS and compares properties with those for conventionally processed polymers. Dielectric strength results show that SLS parts are comparable with injection moulded parts, while HSS parts are inferior to SLS parts. Dielectric constant and dissipation factor results show that HSS parts are comparable with injection moulded parts, whilst SLS parts have superior properties. The presence of porosity (SLS and HSS) and the presence of carbon (HSS) are suggested as reasons behind the variation in dielectric properties when compared with injection moulded parts.Mechanical Engineerin

Electrical method of monitoring percolation and abrasion of conducting spheres due to shear flow of a dense suspension in a narrow gap

This letter describes a method for studying the behavior of rigid particles in a dense suspension when they are forced into contact during flow within a narrow gap. The particles form transient percolating networks spanning the boundary walls, and will be crushed together. The method involves measuring the dc electrical resistance across the gap. The suspension e.g., solder paste consists of electrically conducting particles suspended in an insulating fluid. The electrical resistance drops when the particles are in contact with each other and the walls, and the insulating films on the surface of the conductors have been broken through. The results show a dramatic change in behavior as the ratio of gap to particle diameter is varied

Electrical conductive, characteristics of ACA bonding: a review of the literature, current challenges and future prospects.

Anisotropic Conductive Adhesives (ACAs) have been used in fine pitch electronics packaging for over a decade and provide a high density and low temperature bonding method in a range of niche applications. The principal objective of this paper is to provide significant insights into the basic conductive characteristics of ACAs based on a review of previously reported scientific research, and to identify the current challenges and future prospects for this technology. In order to provide a concise, structured overview of this topic, many detailed conductive models, mathematical solutions and research methodologies are presented based on the reviewed literature. These models can partially explain the conductive mechanisms of an ACA particle, but make a number of important sirnplifying assumptions. However, one model was developed and can be used to explain the conductive mechanism of an ACA particle more successfully. In conclusion, existing computational models, mathematical models and physical models have been used to estimate the resistance of an ACA particle and the particle contact area, and therefore constriction resistance, for a given degree of particle deformation, thereby almost achieving a model for the whole resistance of an ACA joint. The paper will close by identifying other research challenges remaining for this important electronics interconnection technology

Influence of etching solvent evaporation on the size of micro-via holes in PVP thin films

Via holes are a necessary component in traditional PCBs and IC interconnections. Such structures will also be required in organic electronics to achieve vertical communication between multiple layers. Inkjet printing has demonstrated its applicability in both hole creation and for other pattern generation requirements in various polymeric layers. However, the technique has not been systematically investigated. This paper is focused on a study of the effect of solvent evaporation rate on the size of inkjet-etched via holes for organic electronics, which is part of a more extensive investigation and evaluation of inkjet etching as a via hole fabrication technique. In this work, holes were etched in thin layers of poly(4-vinyl phenol) (PVP), which is a potential dielectric material for organic electronic structures. Ethanol, isobutanol and ethylene glycol were used as the etchants in order to study the effect of solvent boiling point and vapour pressure on the size evolution of via holes with the total number and the frequency of the solvent drops used to dissolve them. Isobutanol and ethylene glycol have higher boiling points than ethanol, leading to slower evaporation, which is believed to allow the dissolved polymer to flow backwards to the central area before complete solvent evaporation, resulting in hole refill. However it will be shown that applying temperatures higher than room temperature can accelerate solvent evaporation and eliminate the refill issue

The penetration limit of poly(4-vinyl phenol) thin films for etching via holes by inkjet printing

This article was published in the journal, Applied Physics Letters [© American Institute of Physics] and the definitive version is available at: http://dx.doi.org/10.1063/1.4772796This paper reports the penetration limit of via holes through dissolving dielectric polymer thin films by inkjet printing. It was found that both the outer diameter of via holes and the polymer thickness affect the penetration depth from the experimental results. Based on this finding, a more accurate relationship between the inner diameter of via holes and the diameter of in-flight droplets for different polymer thicknesses is obtained

The impact of substrate temperature on the size and aspect ratio of inkjet-dissolved via holes in thin poly(4-vinyl phenol) dielectric layers

This article was published in the journal, Applied Physics Letters [© American Institute of Physics] and may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The article may be found at: http://dx.doi.org/10.1063/1.4795447The authors demonstrate the effect of substrate temperature on the relationship between the inkjet-etched via hole size and the number of drops of etchant dispensed. A mechanism for the different via hole size evolution versus the number of drops is proposed. An explanation for the interrelationship between the solvent evaporation rate and polymer re-deposition is presented. The aspect ratio of via holes produced is found to increase with the substrate temperature. Therefore, higher temperatures can be used to reduce the size and increase the aspect ratio of via holes fabricated by inkjet etching

Correlations between fMRI activation and individual psychotic symptoms in un-medicated subjects at high genetic risk of schizophrenia

<p>Abstract</p> <p>Background:</p> <p>It has been proposed that different types of psychopathology in schizophrenia may reflect distinguishable pathological processes. In the current study we aimed to address such associations in the absence of confounders such as medication and disease chronicity by examining specific relationships between fMRI activation and individual symptom severity scores in un-medicated subjects at high genetic risk of schizophrenia.</p> <p>Methods:</p> <p>Associations were examined across two functional imaging paradigms: the Hayling sentence completion task, and an encoding/retrieval task, comprising encoding (at word classification) and retrieval (old word/new word judgement). Symptom severity was assessed using the positive and negative syndrome scale (PANSS). Items examined were hallucinations, delusions, and suspiciousness/persecution.</p> <p>Results:</p> <p>Associations were seen in the anterior middle temporal gyrus in relation to hallucination scores during the sentence completion task, and in the medial temporal lobe in association with suspiciousness/persecution scores in the encoding/retrieval task. Cerebellar activation was associated with delusions and suspiciousness/persecution scores across both tasks with differing patterns of laterality.</p> <p>Conclusion:</p> <p>These results support a role for the lateral temporal cortex in hallucinations and medial temporal lobe in positive psychotic symptoms. They also highlight the potential role of the cerebellum in the formation of delusions. That the current results are seen in un-medicated high risk subjects indicates these associations are not specific to the established illness and are not related to medication effects.</p

Thermo-mechanical damage accumulation during power cycling of lead-free surface mount solder joints

This is a conference paper [© IEEE]. It is also available from: http://ieeexplore.ieee.org/servlet/opac?punumber=4658784. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.It is well known that in surface mount technology (SMT), thermal strains in electronic assemblies are induced in the solder joints by the mismatch between the coefficients of thermal expansion (CTE) of the components, substrate and solder, both during their processing and in service. Therefore, thermo-mechanical damage is likely to occur in the solder and the principle reliability hazard in SMT assemblies is the resulting fatigue cracking of the solder fillet, caused by cyclic thermal stresses. These stresses may be caused by both cyclic variations in power dissipation within equipment and by external environmental temperature changes. Most work reported to date has focused on the effects of environmental temperature changes, although for many types of equipment power cycling may result in significant stresses. The present paper describes the experimental determination of the actual temperature distribution in a chip resistor assembly when it is powered. The paper also discusses the significance of such experimentally determined non-uniform temperature distributions in electronic assemblies to fatigue damage accumulation due to both power cycling and to cyclic variations in the ambient temperature whilst the chip resistor is powered. This fatigue damage accumulation study is carried out using finite element analysis

An epidemiological study of season of birth, mental health, and neuroimaging in the UK Biobank

Environmental exposures during the perinatal period are known to have a long-term effect on adult physical and mental health. One such influential environmental exposure is the time of year of birth which affects the amount of daylight, nutrients, and viral load that an individual is exposed to within this key developmental period. Here, we investigate associations between season of birth (seasonality), four mental health traits (n = 137,588) and multi-modal neuroimaging measures (n = 33,212) within the UK Biobank. Summer births were associated with probable recurrent Major Depressive Disorder (β = 0.026, pcorr = 0.028) and greater mean cortical thickness in temporal and occipital lobes (β = 0.013 to 0.014, pcorr&lt;0.05). Winter births were associated with greater white matter integrity globally, in the association fibers, thalamic radiations, and six individual tracts (β = -0.013 to -0.022, pcorr&lt;0.05). Results of sensitivity analyses adjusting for birth weight were similar, with an additional association between winter birth and white matter microstructure in the forceps minor and between summer births, greater cingulate thickness and amygdala volume. Further analyses revealed associations between probable depressive phenotypes and a range of neuroimaging measures but a paucity of interactions with seasonality. Our results suggest that seasonality of birth may affect later-life brain structure and play a role in lifetime recurrent Major Depressive Disorder. Due to the small effect sizes observed, and the lack of associations with other mental health traits, further research is required to validate birth season effects in the context of different latitudes, and by co-examining genetic and epigenetic measures to reveal informative biological pathways

Grey matter changes can improve the prediction of schizophrenia in subjects at high risk

BACKGROUND: We hypothesised that subjects at familial high risk of developing schizophrenia would have a reduction over time in grey matter, particularly in the temporal lobes, and that this reduction may predict schizophrenia better than clinical measurements. METHODS: We analysed magnetic resonance images of 65 high-risk subjects from the Edinburgh High Risk Study sample who had two scans a mean of 1.52 years apart. Eight of these 65 subjects went on to develop schizophrenia an average of 2.3 years after their first scan. RESULTS: Changes over time in the inferior temporal gyrus gave a 60% positive predictive value (likelihood ratio >10) of developing schizophrenia compared to the overall 13% risk in the cohort as a whole. CONCLUSION: Changes in grey matter could be used as part of a predictive test for schizophrenia in people at enhanced risk for familial reasons, particularly for positive predictive power, in combination with other clinical and cognitive predictive measures, several of which are strong negative predictors. However, because of the limited number of subjects, this test requires independent replication to confirm its validity