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Title: Multifunctional vertical interconnections of multilayered flexible substrates for miniaturised POCT devices
Authors: Zhang, Xiaolong
Issue Date: 2016
Publisher: © Xiaolong Zhang
Abstract: Point-of-care testing (POCT) is an emerging technology which can lead to an eruptive change of lifestyle and medication of population against the traditional medical laboratory. Since living organisms are intrinsically flexible and malleable, the flexible substrate is a necessity for successful integration of electronics in biological systems that do not cause discomfort during prolonged use. Isotropic conductive adhesives (ICAs) are attractive to wearable POCT devices because ICAs are environmentally friendly and allow a lower processing temperature than soldering which protects heat-sensitive components. Vertical interconnections and optical interconnections are considered as the technologies to realise the miniaturised high-performance devices for the future applications. This thesis focused on the multifunctional integration to enable both electrical and optical vertical interconnections through one via hole that can be fabricated in flexible substrates. The functional properties of the via and their response to the external loadings which are likely encountered in the POCT devices are the primary concerns of this PhD project. In this thesis, the research of curing effect on via performance was first conducted by studying the relationship between curing conditions and material properties. Based on differential scanning calorimetry (DSC) analysis results, two-parameter autocatalytic model (Sestak-Berggren model) was established as the most suitable curing process description of our typical ICA composed of epoxy-based binders and Ag filler particles. A link between curing conditions and the mechanical properties of ICAs was established based on the DMA experiments. A series of test vehicles containing vias filled with ICAs were cured under varying conditions. The electrical resistance of the ICA filled vias were measured before testing and in real time during thermal cycling tests, damp heat tests and bending tests. A simplified model was derived to represent rivet-shaped vias in the flexible printed circuit boards (FPCBs) based on the assumption of homogenous ICAs. An equation was thus proposed to evaluate the resistance of the model. Vias with different cap sizes were also tested, and the equation was validated. Those samples were divided into three groups for thermal cycling test, damp heat ageing test and bending test. Finite element analysis (FEA) was used to aid better understanding of the electrical conduction mechanisms. Based on theoretical equation and simulation model, the fistula-shape ICA via was fabricated in flexible PCB. Its hollow nature provides the space for integrations of optical or fluidic circuits. Resistance measurements and reliability tests proved that carefully designed and manufactured small bores in vias did not comprise the performance. Test vehicles with optoelectrical vias were made through two different approaches to prove the feasibility of multifunctional vertical interconnections in flexible substrates. A case study was carried out on reflection Photoplethysmography (rPPG) sensors manufacturing, using a specially designed optoelectronic system. ICA-based low-temperature manufacture processes were developed to enable the integration of these flexible but delicate substrates and components. In the manufacturing routes, a modified stencil printing setup, which merges two printing-curing steps (vias forming and components bonding) into one step, was developed to save both time and energy. The assembled probes showed the outstanding performance in functional and physiological tests. The results from this thesis are anticipated to facilitate the understanding of ICA via conduction mechanism and provide an applicable tool to optimise the design and manufacturing of optoelectrical vias.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.
URI: https://dspace.lboro.ac.uk/2134/23269
Appears in Collections:PhD Theses (Mechanical, Electrical and Manufacturing Engineering)

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