367 research outputs found

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

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    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

    Characterization of Flexible Hybrid Electronics Using Stretchable Silver Ink and Ultra-Thin Silicon Die

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    Flexible Hybrid Electronics (FHEs) offer many advantages to the future of wearable technology. By combining the dynamic performance of conductive inks, and the functionality of ultra-thinned, traditional IC technology, new FHE devices allow for development of applications previously excluded by relying on a specific type of electronics technology. The characterization and reliability analysis of stretchable conductive inks paired with ultra-thin silicon die in theµm range was conducted. A silver based ink designed to be stretchable was screen printed on a TPU substrate and cured using box oven, conveyor convection oven, and photonic curing processes. Reliability tests were conducted including a tape test, crease test, wash test, and abrasion test. Optimization of each curing process resulted in all three methods’ ability to achieve the ink sheet resistance specification of \u3c75mΩ/square/25µm. Reliability tests on the printing concluded that, if fully cured, all samples achieve similar reliability performance. Additionally, a series of 10 mm x 10 mm ultra-thin die were characterized using stylus profilometry and optical measurement in order to test the die quality and readiness for assembly. The die had been thinned from an initial thickness down of 600 µm to a target of 50 µm. A direct inverse relationship was shown between die thickness and die warpage, likely due to high levels of internal stress caused by the dicing and thinning process. Finally, an innovate pairing of serpentine copper clad traces on TPU was tested for reliability performance using traditional solder for die attachment

    Experimental analysis of atmospheric plasma treatment and resin optimization for adhesive bonding of carbon fibre/epoxy composites

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    Two objectives have been pursued: the effects of atmospheric pressure plasma treatment on the surface of carbon fiber composites reinforced with phenolic resin matrix, with the goal of increase wettability and adhesion of epoxy resin, and the search of the best air bubble removal techniques for epoxy resin. Measurements have been conducted with SEM, FTIR, drop contact angle and XPS. An increase of wettability is observed, confirmed by single lap shear strength test

    Mechanical and electrical characterisation of anisotropic conductive adhesive particles

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    This thesis presents research into the mechanical and electrical characterisation of Anisotropic Conductive Adhesive (ACA) particles and their behaviour within typical joints. A new technique has been developed for study of individual ACA particle mechanical and electrical performance when undergoing deformation. A study of the effects of planarity variations on individual electrical joints in real ACA assemblies is presented firstly, followed by the research on the mechanical deformation and electrical tests of individual ACA particles undergoing deformation. In the co-planarity research, experiments introducing deliberate rotation between a chip and substrate were designed and carried out to simulate planarity variations in ACA assemblies. There are two outputs from this part of the research. One is the planarity variation effects on individual electrical joints in ACA assemblies, and the other is the effect of bond thickness on the resistance of a real joint. [Continues.

    Isotropically conductive adhesive filled with silver metalised polymer spheres

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    Isotropic conductive adhesives (ICAs) have a growing range of applications in electronics packaging and have recently emerged as an important material in photo-voltaic module interconnections, particularly for thin-film and other non-silicon technologies where soldering processes are often unsuitable due to the nature of the metallisation or the limited maximum temperature the assembly can be exposed to. ICAs typically comprise of a high volume fraction of solid metallic flakes, usually silver, in an adhesive matrix because of its highly conductive oxide however, this thesis will focus on adhesives containing a large volume fraction of silver coated/metalised mono-sized polymer spheres (Ag-MPS). Incorporating silver coated mono-sized polymer spheres is anticipated to deliver specific advantages such as a significant reduction in the required silver content, improvement of the overall mechanical properties and flexibility to tune the properties of the filler according to the application compared with conventional flake filled adhesives. In this research advancements in the understanding of Ag-MPS filled ICAs, both through theory and experiments, have been made. Analytical models to predict an individual Ag-MPS resistance and Ag-MPS filled ICA resistance have been developed. The experiments based on the flat punch nanoindentation technique have been conducted to determine individual Ag-MPS resistances. The theoretical and experimental studies establish Ag-MPS diameter, coating resistivity, coating thickness, contact radius, and contact geometry as the main contributors towards the resistance of an Ag-MPS filled ICAs. These studies showed that Ag-MPS resistance decreases with increasing coating thickness and contact radius but increases with increasing coating resistivity. The experiments have also been conducted to investigate the effect of Ag-MPS volume fraction, diameter, coating thickness, curing conditions and shrinkage (affecting contact radius) on ICA conductivity and comparisons are made with flake filled and commercial ICAs. The results showed that ICA conductivity increases with increasing volume fraction and coating thickness but decreases with diameter. More importantly the results showed that conductivities similar to those of flake filled ICAs, including those commercially available, can be obtained using 70% less silver. The results show that, Ag content can be reduced further to just 7% with use of larger 30ÎĽm Ag-MPS but with a lower resulting conductivity. Thus for applications where very high conductivity is not required larger Ag-MPS may offer even greater potential cost benefits, which is something flake filled ICAs cannot offer. This is a significant achievement which can allow tuning of ICA formulations according to the demands of the application, which is not possible with the use of silver flakes as there is only a limited range of silver flake volume fractions that will yield useful levels of conductivity

    Electromagnetic Energy Coupled to Nanomaterial Composites for Polymer Manufacturing

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    Polymer nano-composites may be engineered with specific electrical properties to achieve good coupling with electromagnetic energy sources. This enables a wide range of novel processing techniques where controlling the precise thermal profile is critical. Composite materials were characterized with a variety of electrical and thermographic analysis methods to capture their response to electromagnetic energy. COMSOL finite element analysis software was used to model the electric fields and resultant thermal profiles in selected samples. Applications of this technology are demonstrated, including the use of microwave and radio frequency energy to thermally weld the interfaces of 3D printed parts together for increased interlayer (Z) strength. We also demonstrate the ability to bond various substrates with carbon nanotube/epoxy composite adhesives using radio frequency electromagnetic heating to rapidly cure the adhesive interface. The results of this work include 3D printed parts with mechanical properties equal to injection molded samples, and RF bonded joints cured 40% faster than traditional oven curing

    Novel fine pitch interconnection methods using metallised polymer spheres

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    There is an ongoing demand for electronics devices with more functionality while reducing size and cost, for example smart phones and tablet personal computers. This requirement has led to significantly higher integrated circuit input/output densities and therefore the need for off-chip interconnection pitch reduction. Flip-chip processes utilising anisotropic conductive adhesives anisotropic conductive films (ACAs/ACFs) have been successfully applied in liquid crystal display (LCD) interconnection for more than two decades. However the conflict between the need for a high particle density, to ensure sufficient the conductivity, without increasing the probability of short circuits has remained an issue since the initial utilization of ACAs/ACFs for interconnection. But this issue has become even more severe with the challenge of ultra-fine pitch interconnection. This thesis advances a potential solution to this challenge where the conductive particles typically used in ACAs are selectively deposited onto the connections ensuring conductivity without bridging. The research presented in this thesis work has been undertaken to advance the fundamental understanding of the mechanical characteristics of micro-sized metal coated polymer particles (MCPs) and their application in fine or ultra-fine pitch interconnections. This included use of a new technique based on an in-situ nanomechanical system within SEM which was utilised to study MCP fracture and failure when undergoing deformation. Different loading conditions were applied to both uncoated polymer particles and MCPs, and the in-situ system enables their observation throughout compression. The results showed that both the polymer particles and MCP display viscoelastic characteristics with clear strain-rate hardening behaviour, and that the rate of compression therefore influences the initiation of cracks and their propagation direction. Selective particle deposition using electrophoretic deposition (EPD) and magnetic deposition (MD) of Ni/Au-MCPs have been evaluated and a fine or ultra-fine pitch deposition has been demonstrated, followed by a subsequent assembly process. The MCPs were successfully positively charged using metal cations and this charging mechanism was analysed. A new theory has been proposed to explain the assembly mechanism of EPD of Ni/Au coated particles using this metal cation based charging method. The magnetic deposition experiments showed that sufficient magnetostatic interaction force between the magnetized particles and pads enables a highly selective dense deposition of particles. Successful bonding to form conductive interconnections with pre-deposited particles have been demonstrated using a thermocompression flip-chip bonder, which illustrates the applicable capability of EPD of MCPs for fine or ultra-fine pitch interconnection

    Investigation of cohesive and interfacial properties of structural adhesive materials by advanced acoustic methods

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    This research study has examined the feasibility of using acoustic methods for evaluation of the adhesive mechanical properties. The first method is based on the measurement of bulk longitudinal sound velocity during the process of the adhesive cure reaction. Glass transition temperature Tg depends on the extent of reaction of adhesive polymerization; acoustic parameters reflect viscoelastic properties of the material. Acoustic characteristics such us sound velocity or attenuation reflect changes in the adhesive mechanical properties and predict cohesive strength of the adhesive joint. Experimental results show the validity of this assumption. Methodology for monitoring the viscoelastic properties of the adhesive was developed. It was shown that sound velocity in epoxy adhesive correlates with the cohesive strength of the adhesive. The second method is scanning acoustic microscopy which quantitatively allows visualization of the intact adhesive/steel interface. Changes in the microstructure on the intact metal-adhesive interface were investigated. Two dimensional Fourier transforms allow us to determine the main sizes of the granular structure which is 200ÎĽm. It was shown that changes in brightness of the images correspond to changes in the reflection coefficient on the adhesive/metal during polymerization reaction. Adhesive adjacent to the interface has Young\u27s modulus slightly higher than the adhesive in the middle of the layer. Conditions optimal for visualization of the major defects of the adhesive structure were determined. The capability of scanning acoustic microscopy to detect and dynamically monitor small changes in both structure of the metal/epoxy interface and bulk adhesive material was demonstrated

    Evaluation of Hybrid Electrically Conductive Adhesives

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    An electrically conductive adhesive (ECA) is a composite material acting as a conductive paste, which consists of a thermoset loaded with conductive fillers (typically silver (Ag)). Many works that focus on this line of research were successful at making strides to improve its main weakness of low electrical conductivity. Most research focused on developing better silver fillers and co-fillers, or utilizing conductive polymers to improve its electrical conductivity, however, most of these works are carried out on small scale. In this work, we aim to produce larger quantities of hybrid ECA to successfully test its properties. Industry is interested in materials with superior physical properties. As such, rheological behavior and mechanical strength were explored as it has been theoretically hinted that incorporation of exfoliated graphene within the composite could impact those factors listed in a positive manner. In the first step of this project, pre-treated sodium dodecyl sulfate (SDS)-decorated graphene’s rheological properties were examined. An epoxy resin diglycidylether of bisphenol-A (DGEBA) was the main polymer used for this study: a well-known material that can behave either as a shear-thinning or shear-thickening material depending on the supplier. We showed how composites that contain graphene (Gr) had higher viscosities than ones that contained SDS decorated graphene Gr(s). Not only did we confirm that surfactant was a key factor in the decrease of viscosity, but we also report how Gr and Gr(s) had a special effect that suppresses the intrinsic shear thickening behavior of epoxy resin at weight concentrations (wt%) higher than 0.5 wt%. The results showed that Gr(s) is not only beneficial in terms of improving the conductivity of conventional ECAs, but it also acts as a solid lubricant that decreases the viscosity of the composite paste at higher weight concentrations. In the second step of the project, pre-treated SDS decorated graphene’s mechanical properties were examined. In specific, its lap-shear strength (LSS) as well as the effect of residual solvent when present in our hybrid ECA system were studied in order to follow up on the thermal results obtained from a previous study. We showed that our initial suspicion was correct as the LSS did decrease for all of the solvent-assisted formulations that contained Gr(s) ranging from 66 to 84%, however, we were not able to tell whether or not that decrease was caused by lower crosslinking density. Instead, we uncovered another reason for this decrease: bubble formation during the curing step. This suspicion was confirmed qualitatively through light microscopy and quantitatively through optical profilometry, where we present an increase in surface roughness for the solvent-assisted samples. Furthermore, by using SEM, we also confirmed that this bubble formation extends throughout the entire bulk material rather than just at the interface. Lastly, we investigated whether the use of solvent to assist in the mixing process significantly improves the electrical conductivity at a lower weight loading of Ag, and compared the electrical conductivity with that of the products prepared under the same higher weight loading of Ag using a solvent-free mixing method from previous work. Thirdly, we investigated another mechanical property of our hybrid ECAs through indentation tests, where we use Hertizan equations to characterize elastic modulus. Since we learned that the addition of Ag flakes is detrimental to the mechanical strength, we focused on the difference between the elastic moduli for Gr and Gr(s) in a solvent-free environment. In the last step of this project, we explored the use of a liquid-suspended co-filler (instead of carbon filler-based materials) in Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS): a conductive polymer that is frequently in conductive thin-films. We report that by using PEDOT:PSS as a conductive co-filler into the conventional ECA with 60 wt% of Ag, we observed higher conductivity equivalent to adding an extra 20 wt% of Ag into the system. Furthermore, we report that an increase of PEDOT:PSS in the composite appears to decrease the LSS of the material by 20%.

    Nanowires for 3d silicon interconnection – low temperature compliant nanowire-polymer film for z-axis interconnect

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    Semiconductor chip packaging has evolved from single chip packaging to 3D heterogeneous system integration using multichip stacking in a single module. One of the key challenges in 3D integration is the high density interconnects that need to be formed between the chips with through-silicon-vias (TSVs) and inter-chip interconnects. Anisotropic Conductive Film (ACF) technology is one of the low-temperature, fine-pitch interconnect method, which has been considered as a potential replacement for solder interconnects in line with continuous scaling of the interconnects in the IC industry. However, the conventional ACF materials are facing challenges to accommodate the reduced pad and pitch size due to the micro-size particles and the particle agglomeration issue. A new interconnect material - Nanowire Anisotropic Conductive Film (NW-ACF), composed of high density copper nanowires of ~ 200 nm diameter and 10-30 µm length that are vertically distributed in a polymeric template, is developed in this work to tackle the constrains of the conventional ACFs and serves as an inter-chip interconnect solution for potential three-dimensional (3D) applications
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