Comparison of Sintering Methods and Conductive Adhesives for Interconnections in Inkjet-Printed Flexible Electronics

Increasing demands for flexibility and stretchability for electronic devices are driving the research for novel fabrication technologies. Inkjet-printing is one of these novel electronics fabrication techniques studied and developed globally in recent years and it has some interesting benefits over traditional lithography-based techniques, mainly its additive and digital nature. Traditional manufacturing methods are mature techniques and the processes are well defined and optimized for large scale manufacturing and inkjet-printing is not going to replace the lithography as such for large scale manufacturing. Inkjet-printing does, however, enable whole new ways of electronics fabrication, such as high part-to-part customization and 3D processability, which have previously been either very challenging or even impossible.So far research has focused mainly on inkjet-printing itself and the jetting process is understood fairly well. However, at the moment printed semiconductor materials are far inferior to traditional semiconductor components and can not enable the same level of functionality or connectivity. Hybrid systems, combining the high performance of traditional semiconductor components and benefits of inkjet-printing, are studied as a solution for fabricating high performance devices with novel fabrication techniques. Hybrid systems require the ability to attach external components to the printed structures and this integration was chosen as one of the main topic for this thesis work as it had not been studied previously and the knowledge was required for developing inkjet-printing.This thesis analyzes inkjet-printed hybrid systems and focuses on system level integration. The work is done on interconnections including both the sintering of metallic nanoparticles as well as external component interconnections and circuit board to circuit board connections. Sintering research is focused on alternative sintering methods to traditional thermal sintering and evaluation of their usability in electronics fabrication. Electrically conductive adhesives are studied as the main method of forming external connection to components and to other circuit boards.In the research related to this thesis alternative sintering methods were found to be suitable replacements for traditional thermal sintering with the advantages and disadvantages varying between different technologies. Laser and intense pulsed lighting were generally found to be the most promising techniques for inkjet-printed structures. External connections to traditional surface mounted components as well as other printed circuit boards were also successfully demonstrated in the related publications using electrically conductive adhesive materials. Both the electrical performance and long term reliability of the conductive adhesives were found to be inferior to solder-based interconnections but observations show that the difference is caused by the adhesive material itself, not by the use of inkjet-printing. Thus adhesives can be considered as a viable method for forming external interconnections on inkjet-printed structures

Technological Integration in Printed Electronics

Conventional electronics requires the use of numerous deposition techniques (e.g. chemical vapor deposition, physical vapor deposition, and photolithography) with demanding conditions like ultra-high vacuum, elevated temperature and clean room facilities. In the last decades, printed electronics (PE) has proved the use of standard printing techniques to develop electronic devices with new features such as, large area fabrication, mechanical flexibility, environmental friendliness and—potentially—cost effectiveness. This kind of devices is especially interesting for the popular concept of the Internet of Things (IoT), in which the number of employed electronic devices increases massively. Because of this trend, the cost and environmental impact are gradually becoming a substantial issue. One of the main technological barriers to overcome for PE to be a real competitor in this context, however, is the integration of these non-conventional techniques between each other and the embedding of these devices in standard electronics. This chapter summarizes the advances made in this direction, focusing on the use of different techniques in one process flow and the integration of printed electronics with conventional systems

The role of printed electronics and related technologies in the development of smart connected products

The emergence of novel materials with flexible and stretchable characteristics, and the use of new processing technologies, have allowed for the development of new connected devices and applications. Using printed electronics, traditional electronic elements are being combined with flexible components and allowing for the development of new smart connected products. As a result, devices that are capable of sensing, actuating, and communicating remotely while being low-cost, lightweight, conformable, and easily customizable are already being developed. Combined with the expansion of the Internet of Things, artificial intelligence, and encryption algorithms, the overall attractiveness of these technologies has prompted new applications to appear in almost every sector. The exponential technological development is currently allowing for the ‘smartification’ of cities, manufacturing, healthcare, agriculture, logistics, among others. In this review article, the steps towards this transition are approached, starting from the conceptualization of smart connected products and their main markets. The manufacturing technologies are then presented, with focus on printing-based ones, compatible with organic materials. Finally, each one of the printable components is presented and some applications are discussed.This work has been supported by NORTE-06-3559- FSE-000018, integrated in the invitation NORTE59-2018-41, aiming the Hiring of Highly Qualified Human Resources, co-financed by the Regional Operational Programme of the North 2020, thematic area of Competitiveness and Employment, through the European Social Fund (ESF), and by the scope of projects with references UIDB/05256/2020 and UIDP/05256/2020, financed by FCT—Fundação para a Ciência e Tecnologia, Portugal

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

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

Silkkipainetuille johtimille toteutetun flip-chip-liitoksen taivutettavuus

The world is heading towards the IoE (Internet of Everything) where everything will be connected to each other. New flexible, light-weight and low-cost electronic devices are needed to add intelligence everywhere in our surroundings. Conventional silicon-based manufacturing is not the best solution because silicon is mechanically rigid and expensive. One way to manufacture these devices cost-effectively in a very large scale is by roll-to-roll screen printing on flexible substrates. However, due to the low calculation performance of current printed electronics, silicon ICs are still needed to act as brains of the devices. Many studies about chip-on-flex or chip-on-film (COF) attachments are available but information about the integration of a silicon chip directly on a screen printed substrate is needed. This thesis investigates bare chip attachment on printed flexible circuitry and evaluates its subsequent level of bendability. The chip was attached on the high-density rotary screen printed circuitry using a flip-chip technique with anisotropic conductive adhesives (ACP and ACF). A selection of chips was stud bumped with gold. Chips without bumps were more challenging to bond due to the surface roughness of the screen printed lines and a small marginal of the suitable bonding pressure. A chip should be exactly parallel with the substrate while bonding so that the pressure is correct on all pads. After finding the suitable bonding parameters, approximately 90 % of the ACP bonded and 96 % of the ACF bonded interconnections worked without bumps. Stud bumping increased the yield almost to 100 % and decreased the contact resistances approximately 75% making the contacts more reliable. Calendering was tested for printed lines to increase their uniformity and decrease the pad height deviation by heating and pressing them with high force. Calendering reduced the line heights by approximately 1 μm and decreased the surface roughness, but following this process there still existed at least a 2 μm variation in the line heights (nominal line height 5 μm). Bending reliability of the chip attachments on flexible plastic substrates was determined using a self-built bending test set-up which bends the sample between two rigid plates. All chip attachments studied withstood at least a 2.5 cm bending radius. The main results of this thesis were to demonstrate bare die integration on screen printed circuitry and to show its suitability for flexible hybrid electronic applications. Still further development of the bonding process and materials are needed to achieve more reliable long-term solutions

A Digital Manufacturing Process For Three-Dimensional Electronics

Additive manufacturing (AM) offers the ability to produce devices with a degree of three-dimensional complexity and mass customisation previously unachievable with subtractive and formative approaches. These benefits have not transitioned into the production of commercial electronics that still rely on planar, template-driven manufacturing, which prevents them from being tailored to the end user or exploiting conformal circuitry for miniaturisation. Research into the AM fabrication of 3D electronics has been demonstrated; however, because of material restrictions, the durability and electrical conductivity of such devices was often limited. This thesis presents a novel manufacturing approach that hybridises the AM of polyetherimide (PEI) with chemical modification and selective light-based synthesis of silver nanoparticles to produce 3D electronic systems. The resulting nanoparticles act as a seed site for the electroless deposition of copper. The use of high-performance materials for both the conductive and dielectric elements created devices with the performance required for real-world applications. For printing PEI, a low-cost fused filament fabrication (FFF); also known as fused deposition modelling (FDM), printer with a unique inverted design was developed. The orientation of the printer traps hot air within a heated build environment that is open on its underside allowing the print head to deposit the polymer while keeping the sensitive components outside. The maximum achievable temperature was 120 °C and was found to reduce the degree of warping and the ultimate tensile strength of printed parts. The dimensional accuracy was, on average, within 0.05 mm of a benchmark printer and fine control over the layer thickness led to the discovery of flexible substrates that can be directly integrated into rigid parts. Chemical modification of the printed PEI was used to embed ionic silver into the polymer chain, sensitising it to patterning with a 405 nm laser. The rig used for patterning was a re-purposed vat-photopolymerisation printer that uses a galvanometer to guide the beam that is focused to a spot size of 155 µm at the focal plane. The positioning of the laser spot was controlled with an open-sourced version of the printers slicing software. The optimal laser patterning parameters were experimentally validated and a link between area-related energy density and the quality of the copper deposition was found. In tests where samples were exposed to more than 2.55 J/cm^2, degradation of the polymer was experienced which produced blistering and delamination of the copper. Less than 2.34 J/cm^2 also had negative effect and resulted in incomplete coverage of the patterned area. The minimum feature resolution produced by the patterning setup was 301 µm; however, tests with a photomask demonstrated features an order of magnitude smaller. The non-contact approach was also used to produce conformal patterns over sloped and curved surfaces. Characterisation of the copper deposits found an average thickness of 559 nm and a conductivity of 3.81 × 107 S/m. Tape peel and bend fatigue testing showed that the copper was ductile and adhered well to the PEI, with flexible electronic samples demonstrating over 50,000 cycles at a minimum bend radius of 6.59 mm without failure. Additionally, the PEI and copper combination was shown to survive a solder reflow with peak temperatures of 249°C. Using a robotic pick and place system a test board was automatically populated with surface mount components as small as 0201 resistors which were affixed using high-temperature, Type-V Tin-Silver-Copper solder paste. Finally, to prove the process a range of functional demonstrators were built and evaluated. These included a functional timer circuit, inductive wireless power coils compatible with two existing standards, a cylindrical RF antenna capable of operating at several frequencies below 10 GHz, flexible positional sensors, and multi-mode shape memory alloy actuators

Microfluidics for Soft Electronics

Microfluidics- based soft electronic systems have the potential to assist conventional rigid devices and circuits to achieve extreme levels of elasticity in wearable electronics and other applications. The goal of employing microfluidics-based approach among other existing methods is to enhance users comfort through fulfillment of wearable’s mechanical performance requirements such as flexibility, twistability, and stretchability. This chapter presents a brief survey of different solutions for developing elastic electronics and a thorough review of the progress in microfluidics-based approaches. This chapter mainly focuses on the description of the fabrication process, design, and measurement steps of different antennas and complex systems realized using microfluidic interconnects

Matkapuhelimessa käytettävän inkjet-tulostetun taipuisan piirilevyn analyysi

Flexible circuit boards are becoming more and more common in small and lightweight consumer electronics devices. Manufacturers are seeking novel technologies for flexible circuit manufacturing. Cost-effciency, environmental effects of the manufacturing technology and reducing process steps in manufacturing are the key drivers for development. Printed electronics, especially inkjet technology, has been an emerging viable candidate for flexible circuit board manufacturing over the last few years. Additive processing, minimal material consumption and digitally guided processing are the main attractors of inkjet printing technology. The objective of this thesis work is to demonstrate a flexible circuit board of a commercial mobile phone with inkjet printing technology. The design of the flexible printed circuit board is transferred to inkjet printing technology. The assembly process of the flex module is presented, including laser processing of the substrate, inkjet printing of the circuit layout, component attachment using isotropically conductive adhesive and epoxy and circuit coating. The end result is a flexible circuit, similar to the original design, produced with a new manufacturing technology. The inkjet printed demonstrators were then electrically tested by the original manufacturer of the device. Testing showed that functional prototypes with close to the same performance of the original devices were manufactured. The target of the thesis work was reached. Functional demonstrators with good performance were manufactured with inkjet printing technology. As other outcome knowledge of manufacturing electronics with inkjet printing was further increased, as methods for the design and assembly phase were developed. A specifc image masking algorithm for processing of the inkjet printer bitmap images was introduced in the device design phase. Method for making vias on substrate with inkjet printing technology was demonstrated. Also a method for component attachment on flexible inkjet printed circuits was demonstrated. /Kir1

Metamorphic stretchable electronics

Die jüngsten Fortschritte auf dem Gebiet der Elektronik wenden sich der Realisierung mechanischer dehnbarer Elektroniken zu. Diese sind in der Lage sich umzuwandeln um neue Formfaktoren anzunehmen. Um eine nahtlose Integration der Elektronik in unsere Alltagsgegenstände und viele weitere Anwendungsfelder zu ermöglichen, bei denen herkömmliche starre elektronische Systeme nicht ausreichen, ist mechanische Dehnbarkeit notwendig. Diese Arbeit zielt darauf ab, eine dehnbare Leiterplattentechnologie (sPCB) zu demonstrieren, die mit industriellen Herstellungsprozessen kompatibel ist. Idealerweise soll das starre Trägersubstrat der konventionellen Elektronik durch ein dehnbares Gummisubstrat mit dehnbaren Leiterbahnen ersetzt werden. Zunächst wurde eine Methode entwickelt, um eine industrietaugliche, einlagige, dehnbare Leiterplatte zu realisieren. Der dargestellte Ansatz unterscheidet sich von anderen Methoden in diesem Bereich, welche die Metallisierung auf dem Gummisubstrat aufbringen und die Komponenten anschließend darauf montieren. Dadurch leiden diese unter einer geringeren Ausrichtung und Fixierung. Stattdessen wird im dargestellten Ansatz ein harter Träger verwendet, der den Einsatz des dehnbaren Gummimaterials bis ans Ende der Prozesskette verschiebt. Diese Single-Layer-Methode wurde weiterentwickelt, um mehrschichtige, integrierte sPCB zu realisieren, bei der verschiedene Metallisierungsebenen durch vertikalen Durchkontaktierungen (VIA) miteinander verbunden werden. Auch dieses Verfahren verwendet konventionelle starre Träger für den Herstellungsprozess. Wie in der konventionellen Leiterplattentechnologie ist auch die Herstellung auf starren Trägern wichtig, da sie Folgendes ermöglicht: Ausrichtung und Registrierung, Hochtemperaturprozesse, konventionelle Chip-Bestückung durch Roboter und "On-Hard-Carrier"-Bauteiltests. Darüber hinaus ermöglicht die dargestellte Methode den direkten Einsatz handelsüblicher SMDs, was für die einfache Realisierung komplexer elektronischer Schaltungen wichtig ist. Als Endsubstrat kommt ein hochelastisches Silikonmaterial (EcoFlex) zum Einsatz, welches die Bauelementebenen einkapselt. Um die Bauelementebenen vom harten Träger auf das weiche Substrat zu übertragen, wird ein einstufiges, waferbasiertes und lösungsmittelfreies Ablöseverfahren eingesetzt, bei dem die differentielle Grenzflächenadhäsion einer Multi-Opferschichten genutzt wird. Für die hochelastischen Leiterbahnen wurde ein neues Mäander-Metallbahndesign entwickelt, welches als "spannungsadaptiv" bezeichnet wird. Die neue Mäander-Metallbahn variiert in ihrer Breite, um das einwirkende Drehmoment in den Metallbahnen, aufgrund der ungleichmäßigen Spannungsverteilung über die Mäander-Schleifen, aufzunehmen. Das spannungsadaptive Design zeigt eine signifikante Verbesserung der Spannungsverteilungen auf den Metallbahnen und führt experimentell zu einem höheren Niveau der maximalen Dehnung und der Anzahl der Dehnungszyklen. Es wurde eine breite Palette von dehnbaren Systemen demonstriert, darunter Elektronik, Optoelektronik, Akustoelektronik und Sensor-Arrays. Die Demonstratoren, auf Basis einer einzigen Metallisierungsschicht in einer Gummimatrix, enthalten Arrays mit gehäusten SMDs, LED-Nacktchips, laborgefertigte Si [my]-Transistoren und MEMS-Mikrofone. Weiterhin wird eine integrierte Multilayer-sPCB mit Chip-großen LEDs und Transistoren demonstriert, um eine adressierbare aktive Matrix zu realisieren. Dieser Prototyp demonstriert die Machbarkeit von integrierten Multilayer-sPCB und wird im Prinzip dazu führen, dass jedes heute bekannte elektronische System in ein äquivalentes dehnbares System überführt werden kann. Schließlich stellt diese Arbeit das bahnbrechende Konzept der metamorphen Elektronik vor, welche sich umwandeln kann um neue Topologien und Formfaktoren anzunehmen. Es werden verschiedene Arten von Deformationsmechanismen demonstriert, darunter das Aufblasen von gleichförmigen oder strukturierten Gummimembranen, 3D-geführte Deformationen und Vakuumformung in Kombination mit 3D-Schablonen. Die Palette der Topologien reicht dabei von halbkugelförmig, kugelförmig, konkav/konvex, pyramidenförmig, turmartig, bis hin zu komplexeren 3D-Formen, darunter Bienenaugen-Strukturen.Recent advancement in the field of electronics has taken a shift to enable the realization of mechanically stretchable electronics which morph to take on new form factors. Mechanical stretchability is necessary to have seamless integration of electronics in our daily life objects and many other purposes where conventional rigid electronic system is insufficient. This thesis aims to enable a stretchable printed circuit board (sPCB) technology that is compatible with industrial manufacturing. Ideally, the rigid carrier substrate of conventional electronics is intended to be replaced by stretchable rubber substrate with stretchable interconnects. Initially, a method has been developed to realize an industry compatible single layer stretchable PCB. The approach is different from other reported methods in this field, which apply the metallization to the rubber support and mount the components on top and, which suffer from a lower level of alignment and fixation. Instead, in the depicted method a hard carrier is used, which delays the use of the stretchable rubber support to the end of the processing sequence. The single layer method has been further developed to realize a multilayer integrated sPCB, where different metallization layers are connected through vertical interconnect access (VIA). The method uses hard carrier. Like conventional PCB technology, the hard carrier fabrication is important since it enables: alignment and registration, high temperature processing, conventional robotic chip placement, and “on-hard-carrier” device tests. Moreover, the depicted method enables direct use of commercially available SMDs which is important to realize complex electronic devices. As final substrate, highly stretchable silicone material (EcoFlex) is used which encapsulates the device layers. To transfer the device layers from hard carrier to soft substrate a single-step, wafer-level, and solvent-free detachment process has been developed which uses the differential interfacial adhesion in between the sacrificial layers. For highly stretchable interconnects a new meander metal track design is developed which is named as “stress adaptive” metal track. The new meander metal track varies in widths to accommodate produced torque in the metal tracks due to the non-uniform stress distribution over the meander loops. The stress adaptive design shows a significant improvement in the stress distributions over the metal tracks in computer simulated stress profile. And, experimental results show a higher level of maximum stretching (320%) and higher number of stretch-release cycles (11000) comparing with a reference design. A wide range of stretchable systems have been demonstrated including electronics, optoelectronics, acoustoelectronics and sensor arrays. The demonstrators contain arrays with packaged SMDs, bare dies integrated LEDs, lab-fabricated Si µ-transistors and MEMS microphones using a single metallization layer within a rubber matrix. Furthermore, an integrated multilayer sPCB is demonstrated using chip scale LEDs and transistors to realize an addressable active matrix. These prototypes of integrated multilayer electronics demonstrate method to enable multilayer sPCB technology which could lead to realize any electronic system known today to be stretchable. Finally, this thesis introduces a new type of electronics which morph to adapt to new topology and form factor. This shape-adaptive electronics is named as metamorphic electronics. Various types of deformation mechanisms have been demonstrated including inflation and/or deflation of uniform or patterned rubber membranes, 3D guided deformations, and vacuum forming in combination with 3D chaperons. The range of topologies includes hemispherical, spherical, concave/convex, pyramid, tower, bumble bee-eye, and more complex 3D shapes