Development and Evaluation of Accelerated Environmental Test Methods for Products with High Reliability Requirements

Reliability testing of electronics is performed to ensure that products function as planned in specific conditions for a specified amount of time. This is usually both time-consuming and expensive and therefore test time acceleration is often required. The acceleration may be realized by using more severe stress levels or higher use cycle frequencies, but at the same time the risk increases of inducing failure mechanisms not relevant to the use conditions. As a consequence, the accelerated reliability testing of products with markedly long lifetimes and high reliability is frequently challenging. In this thesis different methods for test time acceleration for products with high reliability requirements and long service lives were studied. Both standard tests and modifications of these were used. The effect of the accelerated tests used on the failure modes and mechanisms observed was examined and the limitations of the test methods discussed. The research in this work was conducted at both interconnection level and at device level. The interconnection level testing focused on anisotropically conductive adhesive (ACA) flex-on-board (FOB) attachments. In addition to the effect of the curing process on the mechanical strength of ACA FOB attachments, their applicability and long-term performance in industrial applications was studied. According to the real-time resistance measurement the assembly tested was observed to be extremely resilient in thermal cycling and hygrothermal aging. However, a significant decrease in the mechanical strength of the FOB attachment was also seen. Hydrolysis and embrittlement of the flex material was also observed to limit the applicability of harsher hygrothermal aging conditions. Clear ACA joint failures were only observed with moisture condensation testing, but this may not be a suitable test method for applications that are not susceptible to such a stressor. The device level testing comprised reliability analysis of two frequency converter models. The older generation device and its field failure data were used as the starting point in the development of a test method that could be used to minimize testing time and to induce comparable failure modes to those occurring in the use conditions of the devices. The tests showed that only with the simultaneous use of stresses could a significant reduction in the testing time be achieved. However, the application of the same test method to the newer generation device proved challenging because of differences in materials, components and layouts. Although similar failure modes were observed in both devices, the combined effect of the stresses used on the failure mechanisms requires further study. In addition, knowledge of the service conditions, the environmental stresses and their severity is critical. The main disadvantage of simultaneous stress testing was observed to be the interpretation of the test results, especially due to the complexity of the devices tested. Moreover, the results obtained may be highly application specific. However, regardless of the difficulties in the lifetime estimation, the use of combined stresses was observed to be a practical method to study the weaknesses in a product

Research priorities for advanced fibrous composites

Priorities for research in advanced laminated fibrous composite materials are presented. Supporting evidence is presented in two bodies, including a general literature survey and a survey of aerospace composite hardware and service experience. Both surveys were undertaken during 1977-1979. Specific results and conclusions indicate that a significant portion of contemporary published research diverges from recommended priorites

Development and reliability of a direct access sensor using flip chip on flex technology with anisotropic conductive adhesive

Technological developments in biomedical microsystems are opening up new opportunities to improve healthcare procedures. Swallowable diagnostic sensing capsules are an example of these. In none of the diagnostic sensing capsules, is the sensor’s first level packaging achieved via Flip Chip Over Hole (FCOH) method using Anisotropic Conductive Adhesive (ACA). In a capsule application with direct access sensor (DAS), ACA not only provides the electrical interconnection but simultaneously seals the interconnect area and the underlying electronics. The development showed that the ACA FCOH was a viable option for the DAS interconnection. Adequate adhesive formed a strong joint that withstood a shear stress of 120N/mm2 and a compressive stress of 6N required to secure the final sensor assembly in place before encapsulation. Electrical characterization of the ACA joint in a fluid environment showed that the ACA was saturated with moisture and that the ions in the solution actively contributed to the leakage current, characterized by the varying rate of change of conductance. Long term hygrothermal aging of the ACA joint showed that a thermal strain of 0.004 and a hygroscopic strain of 0.0052 were present and resulted in a fatigue like process. In-vitro tests showed that high temperature and acidity had a deleterious effect of the ACA and its joint. It also showed that the ACA contact joints positioned at around or over 1mm would survive the gastrointestinal (GI) fluids and would be able to provide a reliable contact during the entire 72hr of the GI transit time. A final capsule demonstrator was achieved by successfully integrating the DAS, the battery and the final foldable circuitry into a glycerine capsule. Final capsule soak tests suggested that the silicone encapsulated system could survive the 72hr gut transition

Mechanical degradation of composite structures subjected to environmental effects.

Polymeric materials have inherent advantages thanks to the mechanical properties that they lend to a structure enhancing its useful life in factors of safety, reliability and aesthetics. Nevertheless, the durability may be affected by other considerations including environmental attack resulting in unexpected failures and maintenance costs, making it therefore essential to accurately predict the overall performance of these structures. This study was designed to evaluate the joint strength of an adhesively bonded composite Single Lap Joint (SLJ), exposed to a hostile environment i.e. cycles of temperature and moisture, mechanical damage and fatigue. The aged joints under hygrothermal cycles were tested under static and dynamic loads. A combined experimental-numerical Cohesive Zone Model (CZM) was calibrated to predict the joint strength degradation, and damage propagation. The composite SLJ of T800/M21 bonded with FM94 was subjected to hygrothermal cycles in an environmental chamber (maximum 70 °C and minimum - 20 °C), at maximum 85 % Relative Humidity (RH). The results showed that the strength degraded consequent to the increasing number of cycles. The strength reduced by 42 % under static load after 714 cycles in comparison to unaged joints. The fatigue life was evaluated at 30%, 40% and 45% ultimate static load to a maximum of one million cycles, resulting in a continuous fatigue life reduction with the increase in the number of aging cycles. A characterisation of the moisture diffusion parameters was performed on adhesive (FM94) and composite laminate (T800/M21) subjected to hygrothermal cycles. A displacement-diffusion analysis was conducted to determine the effect of moisture on the elasticity of the adhesive. The displacement-diffusion model results and shear lap test results were employed to establish the degradation parameters of the CZM, thus predicting the degradation of the joint with an accuracy of 13 % at 714 hygrothermal cycles.PhD in Manufacturin

A new mixed model based on the enhanced-Refined Zigzag Theory for the analysis of thick multilayered composite plates

The Refined Zigzag Theory (RZT) has been widely used in the numerical analysis of multilayered and sandwich plates in the last decay. It has been demonstrated its high accuracy in predicting global quantities, such as maximum displacement, frequencies and buckling loads, and local quantities such as through-the-thickness distribution of displacements and in-plane stresses [1,2]. Moreover, the C0 continuity conditions make this theory appealing to finite element formulations [3]. The standard RZT, due to the derivation of the zigzag functions, cannot be used to investigate the structural behaviour of angle-ply laminated plates. This drawback has been recently solved by introducing a new set of generalized zigzag functions that allow the coupling effect between the local contribution of the zigzag displacements [4]. The newly developed theory has been named enhanced Refined Zigzag Theory (en- RZT) and has been demonstrated to be very accurate in the prediction of displacements, frequencies, buckling loads and stresses. The predictive capabilities of standard RZT for transverse shear stress distributions can be improved using the Reissner’s Mixed Variational Theorem (RMVT). In the mixed RZT, named RZT(m) [5], the assumed transverse shear stresses are derived from the integration of local three-dimensional equilibrium equations. Following the variational statement described by Auricchio and Sacco [6], the purpose of this work is to implement a mixed variational formulation for the en-RZT, in order to improve the accuracy of the predicted transverse stress distributions. The assumed kinematic field is cubic for the in-plane displacements and parabolic for the transverse one. Using an appropriate procedure enforcing the transverse shear stresses null on both the top and bottom surface, a new set of enhanced piecewise cubic zigzag functions are obtained. The transverse normal stress is assumed as a smeared cubic function along the laminate thickness. The assumed transverse shear stresses profile is derived from the integration of local three-dimensional equilibrium equations. The variational functional is the sum of three contributions: (1) one related to the membrane-bending deformation with a full displacement formulation, (2) the Hellinger-Reissner functional for the transverse normal and shear terms and (3) a penalty functional adopted to enforce the compatibility between the strains coming from the displacement field and new “strain” independent variables. The entire formulation is developed and the governing equations are derived for cases with existing analytical solutions. Finally, to assess the proposed model’s predictive capabilities, results are compared with an exact three-dimensional solution, when available, or high-fidelity finite elements 3D models. References: [1] Tessler A, Di Sciuva M, Gherlone M. Refined Zigzag Theory for Laminated Composite and Sandwich Plates. NASA/TP- 2009-215561 2009:1–53. [2] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. Assessment of the Refined Zigzag Theory for bending, vibration, and buckling of sandwich plates: a comparative study of different theories. Composite Structures 2013;106:777–92. https://doi.org/10.1016/j.compstruct.2013.07.019. [3] Di Sciuva M, Gherlone M, Iurlaro L, Tessler A. A class of higher-order C0 composite and sandwich beam elements based on the Refined Zigzag Theory. Composite Structures 2015;132:784–803. https://doi.org/10.1016/j.compstruct.2015.06.071. [4] Sorrenti M, Di Sciuva M. An enhancement of the warping shear functions of Refined Zigzag Theory. Journal of Applied Mechanics 2021;88:7. https://doi.org/10.1115/1.4050908. [5] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. A Multi-scale Refined Zigzag Theory for Multilayered Composite and Sandwich Plates with Improved Transverse Shear Stresses, Ibiza, Spain: 2013. [6] Auricchio F, Sacco E. Refined First-Order Shear Deformation Theory Models for Composite Laminates. J Appl Mech 2003;70:381–90. https://doi.org/10.1115/1.1572901

Environmental Degradation Study of FRP Composites Through Evaluation of Mechanical Properties

The performance of fibre-reinforced composites is, to a large extent, controlled by the properties of fibre-matrix interfaces. The interface chemistry and character is vital to a composite material. Good interfacial properties are essential to ensure efficient load transfer from matrix to reinforcement, which helps to reduce stress concentrations and improves overall sustainability of mechanical properties. The strength of composite materials depends not only on the substrate but also on the interface strength. The interface here does not have unique fracture energy unlike homogeneous materials.Consequently, there is a great interest in developing new concepts for tailoring the strength of fibre-matrix interface. Some of researchers have been reported the mechanisms responsible for improved fibre-matrix interface adhesion is removing weak boundary layer, and thereby improving wettability. However, a high performance composite functions because a weaker interface or matrix stops a crack running continuously between the strong brittle reinforcements. Fibre reinforced composite materials do, however, suffer some serious environmental limitations. Environmental exposures include temperature, moisture, radiations, UV and other different alkali treatments, which cause deterioration in the mechanical and/or physical behaviour, adhesion between fibre/matrix interface regions of the composite material over a period of time. The aim of the current investigation is to present the variation of mechanical properties of glass fiber/epoxy composite under the synergistic effect of temperature and rate of loading. In case of temperature we performed 2 types of cases as above and below glass transition temperature (Tg) and in second case abobe and below-ambient temperature. Glass fibre reinforced polymer composites (GFRP), carbon fibre reinforced polymer composites (CFRP) and Kevlar fibre reinforced polymer composites were fabricated by hand-lay up method followed by compression molding press. The composite specimens were subjected to elevated and high temperatures as +60°C,+100°C,+150°C and +200°C temperatures. 3-Point short beam shear test and 4-point short beam shear test were conducted in order to characterize the mechanical behavior of laminated composite and to determine the influence of loading rate on interlaminar shear strength. To understand the interactions between various failure mechanisms in the fiber, matrix and fiber/matrix interface, microscopic analyses were conducted. In second case we performed in-depth analysis of interlaminar shear test and failure mechanisms of glass fibre/epoxy, carbon fibre/epoxy and Kevlar fibre/epoxy composites under +50°C,-50°C,+100°C and-100°C temperatures and different crosshead velocity. Different high and low temperature conditioning were performed using Instron with environmental chamber providing additional information regarding in-situ failure of laminated composites. Following the test, the fracture surfaces of the samples were scanned under SEM to understand the dominating failure modes. Microstructural assessments can also reveal the response of each constituent viz. fibre, matrix resin and the interface/interphase; under temperature and mechanical loading. This section comprehensively presents the mechanical behaviour and structural changes in fibrous polymeric composite systems during the mechanical loading under high and low temperature service environment. We specifically tailored this potential to describe the contradiction and confusion at polymer composite interface which may not be underestimated by material scientists. Fibre/matrix adhesion involves very complex physical and chemical mechanisms. One of the most important physical aspects is the geometry of reinforcing fibres, which influences adhesion between fibre and matrix, stress transfer and local mechanisms of failure. In addition to chemical bonding, the fibre/matrix bond strength in shear is largely dependent on the roughness of the fibre surface and the fibre/matrix contact area. At cryogenic temperatures, due to difference in coefficient of thermal expansion between the fibre and the matrix phase, microcracks initiate and propagate through the laminated composites. Therefore, knowledge of the resistance to different failure modes of woven fabric composites laminates at cryogenic temperatures is essential to the materials scientist and design analyst. The aim of this investigation was to study deformation and mechanical behaviour of glass fibre/epoxy composites subjected to 3-point short beam shear test at low and ultra-low temperature with different loading speeds. The laminates were tested at ambient (+27°C) temperature and at (-20°C,-40°C,-60°C) temperatures using liquid nitrogen in an environmental chamber installed on an Instron testing machine. Testing was carried out in different loading covering low to high medium speeds. Following the test the fracture surfaces were scanned under SEM microscope. A need probably exists for an assessment of mechanical performance of such potentially promising materials under the influence of changing environment and loading speed. Using fractography study to characterize the onset and growth of failure modes has become generally accepted method. During thermal cycling differential coefficient of thermal expansions and residual stresses is a prime cause in fibre reinforced polymer composites (FRP) material. The behavior of the interfacial contact between fibre and matrix is strongly influenced by the presence and nature of residual stresses. GFRP and CFRP composite laminates are used to analyze the thermal cycle effect on the mechanical behavior with different loading rates. 3-point short beam shear test was performed for the analyze the mechanical behavior. To study the failure modes which have great impact on mechanical behavior, Scanning electorn microscope (SEM) was used. The ensuing research revealed a number of key challenges regarding interface issues in producing polymer nanocomposites that exhibit a desired behavior. The greatest stumbling block to the large-scale production and commercialization of nanocomposites is the dearth of cost effective methods for controlling the dispersion of the nanoparticles in polymeric matrix. Current interest in alumina/epoxy nanocomposites, Cu nano particle and Multi walled carbon nanotube (MWCNT) has been generated and maintained because nanoparticles filled polymers exhibit unique combinations of properties not achievable with conventional composites. In the present study, glass fiber reinforced composites filled with nanoparticle have been prepared. 3-point short beam shear test was conducted to analyze the Interlaminar shear strength (ILSS) variation with different loading rate. Alumina nanoparticle was well dispersed in epoxy polymer matrix to achieved high mechanical performance. The results show that it is possible to improve the interlaminar shear strength with the loading rate variations. Clearly, no follow-up work in this area will be commendation for better understanding of effect of nanoparticle in FRP composites in assessment of loading rate sensitivity. Under these conditions, fibre reinforced polymer nanocomposites have been shown to exhibit two glass-transition temperatures, Tg: one associated with polymer chains far from the nanoparticles, and a second, larger Tg, associated with chains in the vicinity of the particles. To analyze different failure modes SEM analyses was conducted. Good interfacial properties are essential to ensure efficient load transfer from matrix to fillers, which helps to reduce stress concentrations and improves overall mechanical properties. Consequently, there is great interest in developing new concepts for improving the strength of fibre−matrix interface