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Title: Process characterization and reliability modeling of Rigid-Flex Printed Circuits (RFPCs)
Authors: Huang, Shiqing
Degree: Ph.D.
Issue Date: 2011
Abstract: The trend in electronic packaging systems is driven by the increasing demand for miniaturization, functional densification and integration, and high reliability. To fulfil these higher requirements for electronic products and achieve higher reliability, great attention has been paid to Rigid-Flex Printed Circuits (RFPCs). The combination of rigid and flexible materials in RFPC with divergent characteristics brings technological process challenges and raises reliability concerns. The objective of this thesis is to characterize the key fabrication processes (including plasma etching and lamination), and to develop reliability models (including finite element models and analytical models) so that a reliable robust RFPC can be fabricated and its useful life extended. The plasma etching process is one of the key processes that need to be characterized and optimized due to its limited productivity and poor etching uniformity in RFPC manufacturing. Design of experiment (DOE) was employed and the analysis of variance (ANOVA) results showed that etching rate and etching uniformity were strongly dependent on the gas proportions of the tetrafluoromethane (CF{210}) used in the plasma etching process. Empirical models were subsequently developed to optimize the process. The optimum set of etching process parameters was determined using the constraint optimization algorithm, which was designed to simultaneously increase the etching rate while maximizing its uniformity. Lamination is another critical process in RFPC manufacturing which has technological challenges. Resin flow plays an important role in RFPC lamination. Too great a resin flow will lead to sharp resin squeeze-out, while inadequate resin flow will result in lamination related failures. Differential Scanning Calorimetry (DSC) analysis was used to study the process of resin curing, from resin softening, melting and cross-linking to resin final curing. Rheological characterization and flow model analysis revealed that the viscosity of the resin changes as a function of temperature, and time. The DOE study reveals that the heating rate, pressure, and conformal material have significant effects on the resin flow in RFPC lamination. Besides the processing challenges, PTH reliability is recognized as the prime reliability problem of RFPC due to the induced stresses resulted from the large CTE difference between the PTH plated copper and the surrounding dielectric material. Once the induced stress exceeds the plated copper elastic limits, failures like lifted pads and barrel copper cracks occur. To examine the PTH reliability of RFPC, empirical studies were conducted and mathematical models were developed.
Firstly, the effect of materials and build-ups (constructions and composition of the multilayer board) on PTH reliability of RFPC was investigated through experimental approach. RFPC specimens using various materials and different types of build-ups were fabricated and thermal cycling tests were performed. The thermal cycling results and the Weibull plots indicated that the higher glass transition temperature (Tg) and lower CTE materials resulted in higher reliability. Build-up material using no flow prepreg and eliminating the coverlayer in the rigid section of RFPC also resulted in better reliability (with a larger number of cycles to failure). Secondly, a finite element model (FEM) was developed to simulate the stress and strain distribution along the PTH of RFPC. The FEM results showed that the peak strain in the PTH of RFPC was associated with different layers of the multilayer board that consist of different dielectric materials. The highest plastic strain occurred at the acrylic adhesive layer, and its value was one order of magnitude higher than the strain induced by the epoxy and polyimide. A parametric analysis using the Taguchi method further found that using high Tg low CTE bonding material, plating with higher thickness, a board with smaller thickness, and reducing the hole size, would result in smaller strains and better PTH reliability in RFPCs. Thirdly, an integrated analytical model was developed to formulate the thermo-mechanical stress and strain in PTH. The barrel deformation ( Δδi ) was calculated by taking the PTH structure as a coupled spring system, while the copper pad deflection ( Δwi ) was formulated by treating the copper pad as a thin circular plate. The integrated analytical model combines both the barrel deformation and the copper pad deflection together to formulate the maximum strain in the PTH barrel as well as to predict the fatigue life of RFPC. A good correlation in the maximum strain and fatigue life was observed when comparing the parametric Taguchi analysis between the analytical model, finite element model and thermal cycling test results. This comprehensive study on the RPFC fabrication process and PTH reliability is significant to both basic research and industry. The optimized process parameters can be used as a reference for RFPC manufacturing or related process studies. The mathematical models provide good insights into the thermo-mechanical behaviour of PTH and are useful for evaluating the reliability and for predicting the lifetime of RFPCs.
Subjects: Flexible printed circuits.
Hong Kong Polytechnic University -- Dissertations
Pages: xxii, 251 leaves : ill. (some col.) ; 30 cm.
Appears in Collections:Thesis

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