Digitally driven microfabrication of 3D multilayer embedded electronic systems

The integration of multiple digitally driven processes is seen as the solution to many of the current limitations arising from standalone Additive Manufacturing (AM) techniques. A technique has been developed to digitally fabricate fully functioning electronics using a unique combination of AM technologies. This has been achieved by interleaving bottom-up Stereolithography (SL) with Direct Writing (DW) of conductor materials alongside mid-process development (optimising the substrate surface quality), dispensing of interconnects, component placement and thermal curing stages. The resulting process enables the low-temperature production of bespoke three-dimensional, fully packaged and assembled multi-layer embedded electronic circuitry. Two different Digital Light Processing (DLP) Stereolithography systems were developed applying different projection orientations to fabricate electronic substrates by selective photopolymerisation. The bottom up projection orientation produced higher quality more planar surfaces and demonstrated both a theoretical and practical feature resolution of 110 μm. A top down projection method was also developed however a uniform exposure of UV light and planar substrate surface of high quality could not be achieved. The most advantageous combination of three post processing techniques to optimise the substrate surface quality for subsequent conductor deposition was determined and defined as a mid-processing procedure. These techniques included ultrasonic agitation in solvent, thermal baking and additional ultraviolet exposure. SEM and surface analysis showed that a sequence including ultrasonic agitation in D-Limonene with additional UV exposure was optimal. DW of a silver conductive epoxy was used to print conductors on the photopolymer surface using a Musashi dispensing system that applies a pneumatic pressure to a loaded syringe mounted on a 3-axis print head and is controlled through CAD generated machine code. The dispensing behaviour of two isotropic conductive adhesives was characterised through three different nozzle sizes for the production of conductor traces as small as 170 μm wide and 40 μm high. Additionally, the high resolution dispensing of a viscous isotropic conductive adhesive (ICA) also led to a novel deposition approach for producing three dimensional, z-axis connections in the form of high freestanding pillars with an aspect ratio of 3.68 (height of 2mm and diameter of 550μm). Three conductive adhesive curing regimes were applied to printed samples to determine the effect of curing temperature and time on the resulting material resistivity. A temperature of 80 °C for 3 hours resulted in the lowest resistivity while displaying no substrate degradation. ii Compatibility with surface mount technology enabled components including resistors, capacitors and chip packages to be placed directly onto the silver adhesive contact pads before low-temperature thermal curing and embedding within additional layers of photopolymer. Packaging of components as small as 0603 surface mount devices (SMDs) was demonstrated via this process. After embedding of the circuitry in a thick layer of photopolymer using the bottom up Stereolithography apparatus, analysis of the adhesive strength at the boundary between the base substrate and embedding layer was conducted showing that loads up to 1500 N could be applied perpendicular to the embedding plane. A high degree of planarization was also found during evaluation of the embedding stage that resulted in an excellent surface finish on which to deposit subsequent layers. This complete procedure could be repeated numerous times to fabricate multilayer electronic devices. This hybrid process was also adapted to conduct flip-chip packaging of bare die with 195 μm wide bond pads. The SL/DW process combination was used to create conductive trenches in the substrate surface that were filled with isotropic conductive adhesive (ICA) to create conductive pathways. Additional experimentation with the dispensing parameters led to consistent 150 μm ICA bumps at a 457 μm pitch. A flip-chip bonding force of 0.08 N resulted in a contact resistance of 2.3 Ω at a standoff height of ~80 μm. Flip-chips with greater standoff heights of 160 μm were also successfully underfilled with liquid photopolymer using the SL embedding technique, while the same process on chips with 80 μm standoff height was unsuccessful. Finally the approaches were combined to fabricate single, double and triple layer circuit demonstrators; pyramid shaped electronic packages with internal multilayer electronics; fully packaged and underfilled flip-chip bare die and; a microfluidic device facilitating UV catalysis. This new paradigm in manufacturing supports rapid iterative product development and mass customisation of electronics for a specific application and, allows the generation of more dimensionally complex products with increased functionality

Modeling the SAC microstructure evolution under thermal, thermomechanical and electrical constraints

[no abstract

Micro-mechanical characteristics and dimensional change of Cu-Sn interconnects due to growth of interfacial intermetallic compounds

Sn-based solder alloys are extensively used in electronic devices to form interconnects between different components to provide mechanical support and electrical path. The formation of a reliable solder interconnects fundamentally relies on the metallurgic reaction between the molten solder and solid pad metallization in reflowing. The resultant IMC layer at the solder/pad metallization interface can grow continuously during service or aging at an elevated temperature, uplifting the proportion of IMCs in the entire solder joint. However, the essential mechanical properties of interfacial IMC (i.e. Cu6Sn5, Cu3Sn) layers, such as Young s modulus and hardness, are drastically different in comparison with Sn-based solder and substrate. Therefore, the increasing fraction of interfacial IMCs in the solder joint can lead to significant deformation incompatibility under exterior load, which becomes an important reliability concern in the uses of solder joints for electronic interconnects. In the past decades, extensive research works were implemented and reported regarding the growth of interfacial IMC layers and its effect on the mechanical integrity of solder joints. But, the following fundamental issues in terms of mechanical and microstructural evolution in the uses of solder joints still remain unclear, demanding further research to elaborate: (1) The protrusion of IMCs: Though the growth of interfacial IMC layers along the diffusion direction in solder joints were studied extensively, the growth of IMCs perpendicular to the diffusion direction were reported in only a few papers without any further detailed investigation. This phenomena can crucially govern the long-term reliability of solder interconnects, in particular, in the applications that require a robust microstructural integrity from a solder joint. (2) Fracture behaviour of interfacial IMC layers: The fracture behaviour of interfacial IMC layers is a vital factor in determining the failure mechanism of solder joints, but this was scarcely investigated due to numerous challenges to enable a potential in-situ micro-scale tests. It is therefore highly imperative to carry out such study in order to reveal the fracture behaviour of interfacial IMC layers which can eventually provide better understanding of the influence of interfacial IMC layers on the mechanical integrity of solder joints. (3) Volume shrinkage: The volume shrinkage (or solder joint collapse) induced by the growth of interfacial IMC layers was frequently ascribed as one of the main causes of the degradation of mechanical reliability during aging due to the potentially resulted voids and residual stress at the solder/substrate interface. However, very few experimental works on the characterisation of such type of volume shrinkage can be found in literatures, primarily due to the difficulties of observing the small dimensional changes that can be encountered in the course of IMCs growth. (4) Residual stress: The residual stress within solder joints is another key factor that contributes to the failure of solder joints under external loads. However, the stress evolution in solder joints as aging progresses and the potential correlation between the residual stress and the growth of interfacial IMC layers is yet to be fully understood, as stress/strain status can fundamentally alter the course of total failure of a solder joint. (5) Crack initiation and propagation in solder joints: Modelling on the mechanical behaviour of solder joints is often undertaken primarily on the stress distribution within solder joints, for instance, under a given external loading. But there is lack of utilising numerical analysis to simulate the crack initiation and propagation within solder joints, thus the effect of interfacial IMC layers on the fracture behaviour of the solder joints can be elaborated in further details. In this thesis, the growth of interfacial IMCs in parallel and perpendicular to the interdiffusion direction in the Sn99Cu1/Cu solder joints after aging was investigated and followed by observation with SEM, with an intention of correlating the growth of IMCs along these two directions with aging durations based on the measured thickness of IMC layer and height of perpendicular IMCs. The mechanism of the protrusion of IMCs and the mutual effect between the growth of IMCs along these two directions was also discussed. The tensile fracture behaviour of interfacial Cu6Sn5 and Cu3Sn layers at the Sn99Cu1/Cu interface was characterised by implementing cantilever bending tests on micro Cu6Sn5 and Cu3Sn pillars prepared by focused ion beam (FIB). The fracture stress and strain were evaluated by finite element modelling using Abaqus. The tensile fracture mechanism of both Cu6Sn5 and Cu3Sn can then be proposed and discussed based on the observed fracture surface of the micro IMC pillars. The volume shrinkage of solder joints induced by the growth of interfacial IMC layers in parallel to the interdiffusion direction in solder joint was also studied by specifically designed specimens, to enable the collapse of the solder joint to be estimated by surface profiling with Zygo Newview after increased durations of aging. Finite element modelling was also carried out to understand the residual stress potentially induced due to the volume shrinkage. The volume shrinkage in solder joints is likely to be subjected to the constraint from both the attached solder and substrate, which can lead to the build-up of residual stress at the solder/Cu interface. Depth-controlled nanoindentation tests were therefore carried out in the Sn99Cu1 solder, interfacial Cu6Sn5 layer, Cu3Sn layer and Cu with Vickers indenter after aging. The residual stress was then evaluated in the correlation with aging durations, different interlayers and the locations in the solder joint. Finally, finite element models incorporated with factors that may contribute to the failure of solder joints, including microstructure of solder joints, residual stress and the fracture of interfacial IMC, were built using Abaqus to reveal the effect of these factors on the fracture behaviour of solder joints under applied load. The effect of growth of IMC layer during aging on the fracture behaviour was then discussed to provide a better understanding of the degradation of mechanical integrity of solder joints due to aging. The results from this thesis can facilitate the understanding of the influence of interfacial IMC layers on the mechanical behaviour of solder joints due to long-term exposure to high temperatures

Proceeding Of Mechanical Engineering Research Day 2016 (MERD’16)

This Open Access e-Proceeding contains a compilation of 105 selected papers from the Mechanical Engineering Research Day 2016 (MERD’16) event, which is held in Kampus Teknologi, Universiti Teknikal Malaysia Melaka (UTeM) - Melaka, Malaysia, on 31 March 2016. The theme chosen for this event is ‘IDEA. INSPIRE. INNOVATE’. It was gratifying to all of us when the response for MERD’16 is overwhelming as the technical committees received more than 200 submissions from various areas of mechanical engineering. After a peer-review process, the editors have accepted 105 papers for the e-proceeding that cover 7 main themes. This open access e-Proceeding can be viewed or downloaded at www3.utem.edu.my/care/proceedings. We hope that these proceeding will serve as a valuable reference for researchers. With the large number of submissions from the researchers in other faculties, the event has achieved its main objective which is to bring together educators, researchers and practitioners to share their findings and perhaps sustaining the research culture in the university. The topics of MERD’16 are based on a combination of fundamental researches, advanced research methodologies and application technologies. As the editor-in-chief, we would like to express our gratitude to the editorial board and fellow review members for their tireless effort in compiling and reviewing the selected papers for this proceeding. We would also like to extend our great appreciation to the members of the Publication Committee and Secretariat for their excellent cooperation in preparing the proceeding of MERD’16

Carbon Nanotube Based Interconnect Material for Electronic Applications

Carbon nanotubes (CNTs) are considered as a candidate material for future electronic interconnect applications. This thesis summarizes the research work on the fabrication and characterization of CNT-based interconnect systems, and explores the possibilities of integrating CNTs into various electronic interconnect scenarios. CNT material properties and fabrication methods are introduced as well as its potential for solving the future interconnect challenges. The technology development works are presented in detail in four categories: synthesis, densification, coating and transfer. The principles of the chemical vapor deposition (CVD) method for producing the CNTs are described and discussed. Densification methods are developed in order to increase the volume density of the pristine porous CVD-grown CNTs. Two techniques, vapor-based densification and paper-mediated wet densification, have been proposed and characterized. CNT transfer techniques are developed in order to decouple the harsh CVD growth environment from the target application devices. Two kinds of transfer medium materials, indium and polymer, have been proposed and optimized. To improve the electrical performance of the pristine CNTs, metallic coating techniques for both vertically aligned and randomly dispersed CNTs are developed and characterized. Finally, three different CNT-based interconnect scenarios: bumps, through silicon vias, and flexible conductors, are demonstrated and characterized, using the as-developed processes. The integration technologies developed in this thesis not only improve the CNT process compatibility with the conventional electronics manufacture flows, but also offers state-of-the-art electrical and mechanical performance for the non-conventional flexible and stretchable interconnect applications

Rinnakkainen yhdistelmätestausmenetelmä elektronisten kokoonpanojen kokonaisvaltaisempaan ja tehokkaampaan luotettavuustestaukseen

Perinteisesti elektroniikan kokoonpanojen luotettavuutta on testattu kiihdytetyillä elinaikatesteillä yhtä rasitusparametriä hyväksikäyttäen. Tässä diplomityössä pyrittiin kehittämään uusi rinnakkainen yhdistelmätestausmenetelmä, joka simuloisi tuotteiden todellisia käyttöolosuhteita todenmukaisemmin. Rasituksien yhdistämisen uskotaan myös nopeuttavan vauriomekanismejä ja täten lyhentävän testaukseen käytettyä aikaa. Työ keskittyi termomekaanisten ja mekaanisten rasitusten yhdistämiseen. Työn kirjallisuusosa käsitteli kiihdytettyjä elinaikatestejä. Oleelliset yhden rasituksen testimenetelmät käytiin läpi ennen syventymistä useita rasitusparametrejä hyödyntäviin testeihin kirjallisuusselvityksen avulla. Työn kokeellisessa osassa yhdistettiin tehosyklaus ja tärinätestaus yhdeksi testimenetelmäksi. Ennen rasitusten yhdistämistä suoritettiin yhden rasitusparametrin tehosykli- ja tärinätestit varsinaisten testiparametrien määrittämiseksi ja rinnakkaistestauksen vikaantumismekanismien selvittämiseksi. Tulokset olivat yhtäpitäviä aikaisempien asiasta suoritettujen tutkimusten kanssa, elinaikojen yhdistelmätestauksessa ollessa huomattavasti lyhyempiä kuin oli oletettu erillisten yhden rasitusparametrin testien tulosten perusteella. Yhdistelmätestauksessa havaitut vikaantumismoodit olivat hyvin samankaltaisia tärinätesteissä havaittujen moodien kanssa ja vaurioitumisnopeuden kasvun oletettiin johtuvan juoteliitoksien lämpötilasta aiheutuvien mekaanisten ominaisuuksien muutoksista. Tulokset osoittivat, että testiparametrien huolellisella valinnalla voidaan uudella menetelmällä saavuttaa todellisia käyttöympäristöjä todenmukaisemmin edustava rasitusympäristö sekä lyhentää testiaikoja merkittävästi. On myös huomioitava, että yksittäin lähes merkityksettömillä rasituksilla saattaa olla merkittäviä yhteisvaikutuksia luotettavuuteen, joita ei voida huomioida perinteisillä yhden rasituksen testeillä.Traditionally the reliability and lifetime predictions of electronic assemblies have been conducted by employing single load accelerated life tests. This thesis aimed to develop a new concurrent reliability testing method that would offer a more realistic representation of actual use environments. Additionally, the combination of several loadings is expected to accelerate the damage accumulation, thus decreasing the associated testing times and costs. The focus was on the combination of thermomechanical and mechanical loads. Accelerated life tests were analyzed in the literature part. The relevant conventional accelerated life tests utilizing a single loading parameter were discussed before addressing multiple loading tests by giving a literature review on the current status of the subject. The experimental part combined power cycling and vibration loading into a single concurrent test method. To determine the actual test parameters and to clarify the failure modes and mechanisms in concurrent loading, single loading tests were conducted before the loads were combined. The experimental results were consistent with the previous results reported in literature with observed lifetimes considerably shorter than expected based on the single load test results. Observed failure modes in concurrent testing closely resembled those observed in pure mechanical loading, but the accelerated damage accumulation rate was attributed to the temperature related change of material properties. It was concluded that when the loading parameters are carefully set, significant improvements in both the lifelikeness of the loading conditions and in effectiveness can be achieved with the new test method. The results indicated that even under relatively small magnitude single loads the interactions of the various loadings can have significant effects on reliability that cannot be accounted for with single load tests

Qualification of Metallized Optical Fiber Connections for Chip-Level MEMS Packaging

A MEMS-based Safety and Arming (S&A) device is being developed for the next generation of Navy torpedoes. The MEMS-based S&A consists of a high aspect ratio MEMS chip fabricated by deep reactive ion etching (DRIE) of silicon on insulator substrates (SOI). The micro-machined structures, which include environmental sensors, actuators, and optical components, are susceptible to stiction related failures. A robust package is essential to transform the fragile MEMS S&A device into a rugged package capable of reliably functioning throughout the military stockpile to target sequence. To adequately protect the MEMS device from deleterious effects of the external environment, the package must be housed in a hermetic, organic-free package. This dissertation presents the design of, analyzes, and qualifies a die-level fluxless packaging concept. The die-level package consists of a metallized seal ring patterned around the perimeter of the chip, including the fiber groove, sidewalls, and base. The fiber grooves provide a fiber optic interconnect between the microstructure area and the macro-environment. A cap chip, with a matching seal ring, completes the clamshell package. Solder is deposited onto the seal ring and in the grooves at the wafer-level on the device and cap chips. A fluxless, and hence organic-free, soldering process joins and seals the fiber-chip assembly on the chip-level. The conditions that govern fluxless soldering are addressed and tailored for success in the developed design. Surface energy models are used to understand the fluxless soldering conditions and to study the geometric stability of fluid solder joints at the fiber to chip interface. Several techniques for fabrication of the chips and assembly of the packages are investigated. The effects of leak rate of the package seal on the internal package environment are discussed in detail to establish an acceptable leak rate of small volume MEMS packages. The calculations are then furthered to determine the acceptable leak path dimensions to ensure moisture does reach unacceptable levels during the package life. The presented work represents the first reported organic-free (fluxless) die-level package seal with optical fibers that cross the seal boundary

Numerical modelling of additive manufacturing process for stainless steel tension testing samples

Nowadays additive manufacturing (AM) technologies including 3D printing grow rapidly and they are expected to replace conventional subtractive manufacturing technologies to some extents. During a selective laser melting (SLM) process as one of popular AM technologies for metals, large amount of heats is required to melt metal powders, and this leads to distortions and/or shrinkages of additively manufactured parts. It is useful to predict the 3D printed parts to control unwanted distortions and shrinkages before their 3D printing. This study develops a two-phase numerical modelling and simulation process of AM process for 17-4PH stainless steel and it considers the importance of post-processing and the need for calibration to achieve a high-quality printing at the end. By using this proposed AM modelling and simulation process, optimal process parameters, material properties, and topology can be obtained to ensure a part 3D printed successfully

Étude de nouveaux matériaux polymères pour membranes de séparation et actionneurs contrôlés par stimuli

Stimuli-responsive materials have exhibited a wide range of applications, because the necessity of designing novel “smart” systems usually requires materials that can change their chemical and/or physical properties upon exposure to changed environmental conditions. Two representative applications are stimuli-responsive porous membranes and polymer actuators which are subjects of this thesis. On one hand, stimuli-responsive porous membranes are a class of membranes that can adjust their mass transfer and interfacial properties to manipulate permeability and selectivity in response to changing environmental conditions, such as pH, light, temperature, redox, and so on. There has been a rapid development in this field over the past decades or so. However, there are some limitations for these stimuli-responsive porous membranes. For example, for pH-responsive porous membranes, addition of acids and bases into the solution results in salt accumulation that can contaminate the system and diminish the switchability, while temperature or light responsive porous membranes may be damaged by the stimuli to a certain extent. Therefore, it is still urgent to develop environmentally friendly and cost-effective stimulation modes for stimuli-responsive porous membranes. On the other hand, polymer actuators have aroused scientists’ extensive research interest because of their potential applications in biomedicine, artificial muscles and soft robots. Instead of utilizing mechanical force to achieve motion or deformation, polymer actuators can move or deform under thermal, optical or electrical stimulation. Among them, photo-thermal-responsive polymer actuators based on liquid crystal polymer networks (LCNs) have emerged as a particularly promising materials system. For this kind of actuator, the photothermal agents are needed to convert optical energy into thermal energy to induce the LC-to-isotropic phase transition that drives the actuator to deform macroscopically. However, the photothermal reagents often have poor compatibility with organic polymeric matrices, resulting in a dilemma that reducing the doping percentages of photothermal reagents would weaken the photo-actuation speeds of the LCN actuators, whereas increasing the doping percentages would lead to serious phase segregations, and then sacrifice the mechanical properties of the LCN actuators. Moreover,fabricating photothermal-responsive LCN actuators that can perform light-driven caterpillar-type motion on untreated surfaces is also challenging. The main topic of this thesis is to learn from nature to design and fabricate the stimuli-responsive porous membranes for the size separation and nanofiltration and the stimuli-responsive polymer actuators based on LCN for remotely controlled motion. We introduce the CO2-responsive polymers into membrane separation, in a bid to develop a new external stimulus that can reversibly “open” and “close” the membrane pore sizes and further impact the size selectivity of the membrane. Compared with other stimuli, using CO2 as trigger has several merits, such as environmental friendliness, easy operation, excellent repeatabil-ity without any damage and contamination and good depth inside the membranes. Moreover, we designed a photothermal-responsive LCN-based trilayer actuator that can perform near-infrared (NIR) light -guided bending, moving waves and motion on un-treated, either horizontal or inclined surface. The research works in this thesis mainly contain above-mentioned two topics, presented in three chapters. In the first work, we proposed a novel concept that gas-tunable pore sizes as well as gas-controlled separation of species can be successfully realized by using CO2/Ar as trigger. In this work, a CO2-responsvie polymer, poly(N,N-diethylaminoethyl methacrylate) (PDEAEMA), was grafted to the polydopamine (PDA) modified polyvinylidene fluoride (PVDF) membrane to undergo reversible contraction/extension in response to CO2/Ar, resulting in the corresponding opening and closure of membrane pore. The reversible rejection of gold nanoparticles (AuNPs) with the diameter of 50 nm can be realized by alternating CO2/Ar bubbling time. This novel modality that integrates CO2-responsive polymers, tunable membrane pore size and membrane separation shows the great potential of developing smart membranes for applications requiring or involving tunable, size-selective separation of molecular species or nanoparticles. On the basis of the first work, in order to improve the separation capability of CO2-responsive polymer membranes, we developed a CO2-responsive nanofiltration mem-brane based on pyrene modified PDEAEMA (Py-PDEAEMA) and graphene oxide (GO) for water purification. This composite nanofiltration membrane had a number of attrib-utes potentially appealing for water treatment, such as the reversible, gas-tunable water permeability, both high water permeability and rejection of organic dye molecules and gas-tunable changes of the charge sign. To our knowledge, this is the first report about CO2-responsive nanofiltration membrane. This work combines the advantages of a CO2-responsive polymers and GO-based nanofiltration membrane, demonstrating new perspectives in developing smart stimuli-responsive nanofiltration membranes for water purification. In the third work, we designed a LCN-based actuator that performed NIR light-guided locomotion. The actuator had a trilayer structure, including a thin reduced graphene oxide (RGO) top layer, an inactive polymer middle layer and an active LCN bottom layer. When exposing the RGO side to a moving NIR laser, a moving wave along the strip actuator is generated, which makes the strip an effective caterpillar walker that could move on untreated, either horizontal or inclined surface under the guide of NIR laser. Moreover, while known actuators using photothermal effect are usually fabricated by mixing the nanofiller as NIR light heater with the polymer, which may weak the reversible deformation degree and raise the compatibility concern, the easy trilayer fabrication method laminates directly a “sheet” of RGO on thick polymer layers, which circumvents the potential problems

Some operational aspects of a rotating advanced-technology space station for the year 2025

The study of an Advanced Technology Space Station which would utilize the capabilities of subsystems projected for the time frame of the years 2000 to 2025 is discussed. The study includes tradeoffs of nuclear versus solar dynamic power systems that produce power outputs of 2.5 megawatts and analyses of the dynamics of the spacecraft of which portions are rotated for artificial gravity. The design considerations for the support of a manned Mars mission from low Earth orbit are addressed. The studies extend to on-board manufacturing, internal gas composition effects, and locomotion and material transfer under artificial gravity forces. The report concludes with an assessment of technology requirements for the Advanced Technology Space Station