Towards Large-Area Electronic Systems Using Non-Conventional Substrate and Conductor Materials

Xiaotian Li

Main supervisor: Assoc. Prof. Johan Sidén Co-supervisor: Assoc. Prof. Henrik Andersson

Faculty of Science, Technology and Media Thesis for Doctoral Degree in Electronics Mid Sweden University Sundsvall, 28 May 2020

Akademisk avhandling som med tillstånd av Mittuniversitetet i Sundsvall fram- läggs till offentlig granskning för avläggande av teknologie doktorsexamen torsdag, den 28 maj 2020, kl. 09.30, C312, Mittuniversitetet Sundsvall. Seminariet kommer att hållas på engelska.

Towards Large-Area Electronic Systems Using Non-Conventional Substrate and Conductor Materials

Faculty of Science, Technology and Media Mid Sweden University, Holmgatan 10, SE-85170 Sundsvall, Sweden Phone: +46 (0)10 142 80 00 Mid Sweden University Doctoral Thesis 321

Abstract

Flexible circuits, also known as flexible printed circuit boards, were originally developed in the 1950s for interconnection between multiple electronic devices when flexibility and movement were required. Nowadays, flexible circuits can be used for implementing electronic systems much more compli- cated than just interconnections. A commonly seen material combination of flexible circuits is copper foils laminated on polyimide substrates, although these solutions are both expensive and environmentally hazardous. With developments in printed electronics, many non-conventional materials can be used in fabricating flexible circuits that have advantages such as increased flexibility, low cost, a small environmental impact, etc. In addition, fast and efficient manufacturing methods can produce flexible electronics in large volumes. This opens a window of opportunity to create electronic systems over geometrically large areas. This thesis proposes methods and guidelines for how to implement large- area electronic devices using non-conventional flexible materials and technologies. The thesis specifically focuses on electronic systems that integrate both digital and analogue signals. Further, it demonstrates and provides examples of how signals in the microwave frequencies, commonly requiring expensive materials, can be handled with non-conventional materials and technologies. Several conductor-substrate material combinations are used, which are fabricated using industrial processes. The conductor materials include conductive inks, copper foils, and aluminium foils, while the substrate materials comprise papers, a nonwoven fabric, and a polyimide. In particular, methods are investigated in order to achieve a low DC resistance in printed conductive- ink-based tracks, which opens the possibilities for them to be used in high- current applications. Several surface mounting techniques are developed for incorporating surface mount devices within the fabricated flexible circuits, including the use of low-temperature solder paste, isotropic conductive adhesives, and anisotropic conductive adhesives. Some of the techniques have achieved sufficiently low contact resistance and adequate component bonding strengths, and thus can be used in implementing hybrid electronic systems. In addition, most of the techniques have the potential to be used in automated component assembly lines. As demonstrators, two antenna systems for commercial RFID readers operating at high frequency (13.56 MHz) and ultra-high frequency (867 MHz)

iii

Abstract bands are implemented, which comprise both digital and analogue signals. The two antenna systems are designed as part of SP4T switching networks using standard antenna elements as the loads of the network. It is shown in the results that both antenna systems have low RF attenuations, the potential to perform passive RFID tag positioning, and the possibility to be expanded to larger areas. Based on the characterisations to the two antenna systems, discussions are made about how large the antenna system areas can be as well as how many antenna elements can be achieved in a single antenna system. This thesis provides a material-to-system approach and demonstrates that non-conventional flexible materials and printed electronic technologies are suitable choices for large-area electronics.

Sammanfattning

Flexibla kretsar, även kända som flexibla tryckta kretskort, utvecklades ur- sprungligen på 1950-talet för sammankoppling mellan flera elektroniska en- heter när flexibilitet och rörelse var nödvändig. Numera kan flexibla kretsar användas för att implementera elektroniska system som är mycket mer kom- plicerade än bara sammankopplingar. En vanligt sett materialkombination av flexibla kretsar är kopparfolier laminerade på polyimid-substrat, även om dessa lösningar är både dyra och miljöfarliga. Med utvecklingen inom tryckt elektronik kan många icke-konventionella material användas för att tillverka flexibla kretsar som har fördelar såsom ökad flexibilitet, låg kostnad, en liten miljöpåverkan, etc. I tillägg kan snabba och effektiva tillverkningsmetoder producera flexibel elektronik i stora voly- mer. Detta öppnar ett fönster av möjligheter att skapa elektroniska system över geometriskt stora områden. Flera kombinationer av material för ledare och substrat används i denna avhandling, som tillverkas med industriella processer. Ledarmaterialen inklu- derar ledande bläck, kopparfolier och aluminiumfolier, medan substraten innefattar papper, ett nonwoven-tyg och en polyimid. I synnerhet undersöks metoder för att uppnå låg DC-resistans i tryckta bläckbaserade ledare, vilket också möjliggör användning i högströmstillämpningar. Flera ytmonteringsmetoder utvecklas för att införliva ytmonterade kompo- nenter i de tillverkade flexibla kretsarna, inklusive användning av lödpasta med låg temperatur, isotropa ledande lim och anisotropa ledande lim. Vissa av teknikerna har uppnått tillräckligt lågt kontaktmotstånd och adekvata komponentbindningsstyrkor och kan således användas vid implementering av hybridelektroniska system. Dessutom har de flesta tekniker potentialen att användas i automatiserade komponentmonteringslinjer. Som demonstrator implementeras två antennsystem för kommersiella RFID-läsare som arbetar med högfrekvensband (13.56 MHz) och ultrahögfre- kvensband (867 MHz), som innefattar både digitala och analoga signaler. De två antennsystemen är konstruerade som en del av SP4T-nätverk med stan- dardantennelement som nätverksbelastningar. Det visas i resultaten att båda antennsystemen har låga RF-dämpningar, potentialen att utföra passiv RFID- taggpositionering och möjligheten att utökas till större områden. Baserat på resultaten diskuteras hur stora antennsystemområdena kan vara och hur många antennelement som kan uppnås i ett enda antennsystem.

Sammanfattning

Denna avhandling ger ett material-till-system-tillvägagångssätt och demon- strerar att icke-konventionella flexibla material och tryckt elektronisk teknik är lämpliga val för storskalig elektronik.

Table of Contents

Abstract ...... iii Sammanfattning ...... v Table of Contents ...... vii Abbreviations ...... xi List of Figures...... xiii List of Tables ...... xv List of Papers ...... xvii Acknowledgement ...... xix Part A ...... 1 1 Introduction ...... 3 1.1 Thesis scope and outline ...... 3 2 Materials and Technologies of Flexible Circuits ...... 5 2.1 Substrate materials ...... 5 2.1.1 Polymer-based substrates ...... 5 2.1.2 Cellulose-based substrates ...... 5 2.1.3 Textile substrates ...... 6 2.2 Conductor materials ...... 6 2.2.1 Metal foils ...... 6 2.2.2 Conductive inks based on metal particles ...... 7 2.2.3 Conductors for e-textiles ...... 8 2.3 Flexible circuit manufacturing ...... 8 2.3.1 Subtractive techniques ...... 8 2.3.2 Additive techniques ...... 9 2.4 Flexible circuits and systems ...... 9 2.4.1 Printed antennas ...... 9 2.4.2 Printed sensors and printed components ...... 11 2.5 Hybrid flexible circuits ...... 12

vii Table of Contents

2.5.1 SMTs using low temperature solder pastes ...... 12 2.5.2 SMTs using conductive adhesives ...... 12 2.5.3 Other SMTs ...... 13 2.6 Large-area electronics ...... 13 3 Problem Formulation and Objective ...... 15 3.1 Problem formulation ...... 15 3.1.1 Materials and manufacturing ...... 15 3.1.2 Surface mounting techniques ...... 15 3.1.3 Large-area electronic systems ...... 16 3.2 Objective ...... 17 4 Papers Included in This Thesis ...... 19 4.1 Paper I – Soldering Surface Mount Components on Screen- Printed Ag Patterns on Paper and Polyimide Substrates for Hybrid Printed Electronics ...... 19 4.2 Paper II – Soldering Surface Mount Components onto Inkjet Printed Conductors on Paper Substrate Using Industrial Processes ...... 19 4.3 Paper III – Enabling Paper-Based Flexible Circuits with Aluminium and Copper Conductors ...... 20 4.4 Paper IV - Flexible Circuits Based on Aluminium Conductor and Nonwoven Substrate ...... 21 4.5 Paper V – Low-Resistance Screen-Printed Nanoparticle Ink Tracks for High-Current Applications ...... 21 4.6 Paper VI – Flexible Circuits and Materials for Large-Area RFID Reader Antenna Systems ...... 22 4.7 Paper VII – UHF RFID Shelf Reader Antennas for Object Classification and Distance Estimation for Non-Tagged RFID Objects ...... 23 4.8 Paper VIII – A Paper-Based Screen Printed HF RFID Reader Antenna System ...... 23

viii Table of Contents

5 Methodology ...... 25 5.1 Materials and fabrication methods ...... 25 5.1.1 DC current carrying capacity test ...... 27 5.2 Surface mounting techniques ...... 27 5.3 Large-area RFID reader antenna systems ...... 29 6 Discussion ...... 33 6.1 Materials and manufacturing ...... 33 6.1.1 Materials ...... 33 6.1.2 Manufacturing ...... 33 6.2 Qualities of the developed SMTs ...... 34 6.2.1 Contact resistance ...... 34 6.2.2 Component bonding strength ...... 34 6.2.3 Automatic component assembling ...... 36 6.3 RFID reader antenna systems ...... 36 6.3.1 Antenna performance when using conductive inks ...... 36 6.3.2 Antenna system limitations ...... 36 6.3.3 Potential applications ...... 37 6.4 Social and ethical considerations ...... 39 6.4.1 Environmental impact ...... 39 6.4.2 Ethics when using RFID techniques ...... 39 6.4.3 Social benefit ...... 39 7 Conclusion and Future Work ...... 41 7.1 Conclusion ...... 41 7.2 Future work ...... 42 7.3 Authors’ contributions ...... 43 Bibliography ...... 45 Part B ...... 51 PAPER I - Soldering Surface Mount Components on Screen-Printed Ag Patterns on Paper and Polyimide Substrates for Hybrid Printed Electronics ...... 53

ix Table of Contents

PAPER II - Soldering Surface Mount Components on Inkjet Printed Conductors on Paper Substrate Using Industrial Processes ...... 67 PAPER III - Enabling Paper-Based Flexible Circuits with Aluminium and Copper Conductors ...... 77 PAPER IV - Flexible Circuits Based on Aluminium Conductor and Nonwoven Substrate ...... 95 PAPER V - Low-Resistance Screen-Printed Nanoparticle Ink Tracks for High-Current Applications ...... 101 PAPER VI - Flexible Circuits and Materials for Large-Area UHF RFID Antenna Systems ...... 107 PAPER VII - UHF RFID Shelf Reader Antennas for Object Classification and Distance Estimation of Non-Tagged RFID Objects ...... 123 PAPER VIII - A Paper-Based Screen Printed HF RFID Reader Antenna System ...... 129

Abbreviations

Notation Description ACA Anisotropic conductive adhesive DC Direct current HF High frequency IC Integrated circuit I2C Inter-integrated circuit ICA Isotropic conductive adhesive LTSP Low-temperature solder paste MUX Multiplexer NFC Near-field communication NP Nanoparticle OFET Organic field-effect transistor OLED Organic light-emitting diode PCB Printed circuit board PEN Polyethylene naphthalate PET Polyethylene terephthalate PI Polyimide PLA Polylactic acid RFID Radio-frequency Identification RH Relative humidity SEM Scanning electron microscopy SMD Surface mounted devices SMT Surface-mount technology SOIC Small outline integrated circuit SP4T Single-pole-four-throw UHF Ultra high frequency VIA Vertical interconnect accesses WLAN Wireless local area network WSN Wireless sensor network

List of Figures

Figure 1.1 A typical flex-rigid PCB where a flexible circuit is used as the interconnector...... 4 Figure 1.2 An Arduino platform implemented on PET, acquired from [1]. ... 4 Figure 2.1 Configuration of a single-layer flexible circuit...... 6 Figure 3.1 Interrelations of the research questions...... 16 Figure 5.1 Flowchart of methodology...... 26 Figure 5.2 A three-tier single-pole-4-throw switching network...... 30

xiii

List of Tables

Table 2.1 A comparison of printing techniques...... 10 Table 2.2 Advantages and disadvantages of printing techniques...... 10 Table 2.3 A summary of SMTs based on LTSPs and conductive adhesives. 13 Table 5.1 List of flexible circuit types...... 26 Table 5.2 List of the developed SMTs...... 28 Table 5.3 A comparison between the UHF and HF RFID reader antenna systems...... 31 Table 6.1 Achieved contact resistance and component bonding strength of the SMTs developed in this thesis...... 35 Table 7.1 List of authors ...... 44 Table 7.2 Authors’ contributions ...... 44

List of Papers

This thesis is based on the following papers, as attached in Part B: Paper I Soldering Surface Mount Components on Screen-Printed Ag Patterns on Paper and Polyimide Substrates for Hybrid Printed Electronics Xiaotian Li, Henrik Andersson, Johan Sidén and Thomas Schön IOP Flexible and Printed Electronics, Volume 3, 2018 53 Paper II Soldering Surface Mount Components onto Inkjet Printed Conductors on Paper Substrate Using Industrial Processes Henrik Andersson, Johan Sidén, Vincent Skerved, Xiaotian Li and Linnea Gyllner IEEE Transactions on Components, Packaging and Manufacturing Technology, Volume 6, 2016 67 Paper III Enabling Paper-Based Flexible Circuits with Aluminium and Copper Con- ductors Xiaotian Li, Johan Sidén, Henrik Andersson, Anurak Sawatdee, Robert Öh- man, Jeanette Eriksson and Tove Genchel IOP Flexible and Printed Electronics, Volume 4, 2019 77 Paper IV Flexible Circuits Based on Aluminium Conductor and Nonwoven Substrate Xiaotian Li, Johan Sidén, Henrik Andersson, Jawad Ahmad, Anurak Sawatdee, Robert Öhman, Jeanette Eriksson and Tove Genchel IEEE International Flexible Electronics Technology Conference, Aug. 2019, Vancouver, Canada_ 95 Paper V Low-Resistance Screen-Printed Nanoparticle Ink Tracks for High-Current Applications Xiaotian Li, Henrik Andersson, and Johan Sidén In manuscript and planned to be submitted to IEEE International Flexible Electronics Technology Conference 2020, Aug. 2020, Shanghai, China 101

xvii List of Papers

Paper VI Flexible Circuits and Materials for Large-Area RFID Reader Antenna Sys- tems Xiaotian Li, Johan Sidén and Henrik Andersson IOP Flexible and Printed Electronics, Volume 3, 2018 107 Paper VII UHF RFID Shelf Reader Antennas for Object Classification and Distance Estimation for Non-Tagged RFID Objects Xiaotian Li, Johan Sidén and Henrik Andersson IEEE International Conference on Communications and Electronic Systems (IEEE COMCAS) Nov. 2017, Tel Aviv, Israel 123 Paper VIII A Paper-Based Screen Printed HF RFID Reader Antenna System Xiaotian Li, Johan Sidén, Henrik Andersson and Thomas Schön IEEE Journal of Radio Frequency Identification, Volume 2, 2018 129

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Acknowledgement

I would like to thank my supervisors, Dr. Johan Sidén and Dr. Henrik Anders- son, for offering me the opportunity to be a Ph.D. student. I want to thank their guidance, supports, kindness and encouragements during these years. Especially, I am grateful to their trusts when I had tough moments in my doctoral education. This thesis would have been impossible without them. Swedish KK Foundation (KK-stiftelsen) has my sincere gratitude for fund- ing this work. I would like to address my appreciations to the supports from partner companies: Skultuna Flexible AB, Atlas Industrial Print AB, and Inis- sion Stockholm AB. I thank my colleagues at the STC research centre for their dedications at work. A tack så mycket goes to Dr. Börje Norlin for his internal review. I wish my officemate and my friend, Jawad Ahmad, all the best in his Ph.D. studies. I would also like to acknowledge the helps and assistances from all the colleagues whose names are not mentioned here. The supports of my friends here in Sundsvall are vital for me during the past years. I thank them for bearing me through tough times. I thank them for being there for me. We may come from different corners on this planet, but I am glad that we found each other. With you guys, I will never walk alone. I wish all the best to my fellow countrymen whom I met in Sweden. I wish them success in their careers and life-long happiness. I am also grateful to my friends from BISTU for the time when we were studying hard and pursuing our dreams together. I am extremely grateful to the unconditional love from my parents. I thank them for supporting and trusting me in whatever I determine to do. Every effort and decision they made in my life have made me the person today. This thesis was written during the COVID-19 pandemic. I would like to address my highest respect to all medical staff around the world for their hard work and sacrifice so that we are safe and sound. Thank you for being the heroes of our time! My gratitude also goes to the volunteers and organizations that have been protecting the world from collapsing during this troubling time. May hard times come again no more.

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Part A

1 Introduction

Flexible circuits, also known as flexible printed circuit boards (PCBs), were originally developed in the 1950s for interconnecting electronic devices. In the beginning, flexible circuits were mostly used for interconnections between two PCBs as shown in Figure 1.1. The combinations of rigid PCBs and flexible circuits are commonly known as flex-rigid PCBs. For these type of circuits, the main functionalities are implemented on the rigid PCBs, which are based on copper (Cu) and rigid substrates such as FR-4. Flex-rigid circuits can be found in many of today’s electronic products such as laptops, smart phones, printers, etc. Commercially available flexible circuits nowadays are mostly fabricated from polyimide (PI) substrates with Cu conductors (hereafter denoted as Cu- PI circuits). They are a popular choice because PI is resistant to high temperature, which enables the use of many mature surface mounting techniques (SMTs) that were originally developed for rigid PCBs. Nowadays, materials that are non-conventional compared to what are used for Cu-PI circuits and rigid PCBs can be utilised in implementing electronics devices. An example is shown in Figure 1.2 where a small microcontroller system, an Arduino Mini, is implemented on a polyethylene terephthalate (PET) substrate. Non-conventional materials such as PET, cellulose, polyethylene naphthalate (PEN), aluminium (Al), and conductive ink can also be utilised, which further enriches the material options of electronic devices. In addition, the fabrication methods of flexible circuits are becoming more diverse than just chemical etching that is used for Cu-PI circuits. Fabrication methods such as direct printing have shown possibilities to process using high-throughput techniques such as roll-to-roll and sheet-to-sheet. Thus, it is believed that flexible circuit materials, together with technologies related to printed electronics, can be used to create electronic devices that have significantly larger areas, which is challenging and inefficient when using conventional materials.

1.1 Thesis scope and outline This thesis provides general methods and design guidelines on how to implement large-area electronic devices using non-conventional flexible materials and flexible circuit technologies. The thesis specifically focuses on electronic systems that integrate both digital and analogue signals. Further, the thesis

3 Chapter 1. Introduction

Figure 1.1 A typical flex-rigid PCB where a flexible circuit is used as the interconnector. © https://www.greencircuits.com/

Figure 1.2 An Arduino platform implemented on PET, acquired from [1]. demonstrates and provides examples of how signals in the microwave frequencies, commonly requiring expensive materials, can be handled with non- conventional materials. The remainder of the thesis is divided into six chapters, with the main contents as follows:  Chapter 2 provides a general overview of the materials and technologies of flexible circuits, including materials, circuit manufacturing, hybrid flexible circuits, printed electronic devices, and large-area electronics.  Chapter 3 states the problem formulations and the objectives.  Chapter 4 summarises the included papers with their scientific im- pacts.  Chapter 5 summarises the methodologies and characterization methods used.  Chapter 6 provides a discussion based on the results produced.  Chapter 7 concludes this thesis in accordance with the problem formulations.

2 Materials and Technologies of Flexible Circuits

A flexible circuit commonly comprises thin flexible substrate films on which patterned conductor materials are adhered, as shown in Figure 2.1. This chapter provides introduction and general overview to: - Commonly used flexible circuit materials - Manufacturing methods - Flexible devices and electronic systems - SMTs to implement hybrid flexible circuits - large-area electronic systems

2.1 Substrate materials Substrate materials are used to support conductor materials and to isolate signals between different layers. The following characteristics should be fulfilled for a substrate material to be useful: flexibility, good electrical isolation, sufficient mechanical strength, good thermal resistance, and good bonding com- patibility with conductor materials. 2.1.1 Polymer-based substrates Polymer materials are popular choices for flexible circuits, and typical choices include PI, PEN, and PET. These substrates are organic and lightweight. Pol- ymer substrates can also be biodegradable, e.g. polylactic acid (PLA). 2.1.2 Cellulose-based substrates Cellulose has been gaining more attention as a substrate material, and one form of cellulose is paper. Paper possesses the benefits of being environmental friendly, recyclable, renewable, and ultra-low-cost, which are advantages compared to polymer materials [2]. Electronic systems implemented on paper substrates are referred to as paper electronics. Papers directly made from pulp are not suitable for implementing electronic devices due to their rough surface features, which creates difficulties in depositing conductive inks. Thus, for printed electronics, coating materials are commonly applied on top of papers in order to smoothen the surface, e.g. photo papers, which further assists the deposition and absorption of conductive inks. Despite many advantages of paper substrates, a reported problem is that paper substrates can absorb moisture from the surrounding environment. The

5 Chapter 2. Materials and Technologies of Flexible Circuits

Figure 2.1 Configuration of a single-layer flexible circuit. absorbed moisture can decrease the electrical isolation of the paper, as well as change the dielectric constant. This is particularly problematic for radio frequency (RF) and microwave systems where the characteristic impedance of the systems is determined by the material properties. 2.1.3 Textile substrates Textile products can also be used as substrates, and electronic systems implemented on textile products are referred to as e-textiles. One of the main purposes of e-textiles is to enable wearable electronic devices. When using textiles as substrates, electronic devices, e.g. sensor platforms that monitor human bodies, can be integrated into daily clothes and articles, which has a great value for medical uses.

2.2 Conductor materials A high DC resistance in conductor tracks results in energy waste, which is shown in the form of Joule heat (P = I2R). When conducting high currents, this might cause excessive temperatures that damage the substrate materials and waste energy. Also, the high DC resistance could cause a large voltage drop in the conductors, which makes it difficult to use low-voltage sources such as flexible batteries. 2.2.1 Metal foils Metal foils are usually the first choices as a conductor material mainly due to their excellent electrical characteristics [3], e.g. low resistivity (< 1 × 10-7 Ω-m). This means that the power/energy losses in conductors are low, which obviously benefits systems related to RF, microwave, and power electronics. In addition to this, metal foils can be laminated to flexible substrates using high- speed manufacturing methods such as roll-to-roll. The most commonly used metal conductor material is Cu, due to its high solderability and low resistivity (1.68 × 10-8 Ω-m). Al is second to Cu since it has a lower material cost and a comparable resistivity (2.65 × 10-8 Ω-m). Due to Al has a dense surface oxide layer that results in low solderability, Al is not popularly used as the main

6 2.2. Conductor materials

conductor material of electronic systems. Other metals, e.g. silver (Ag) and gold (Au), can be used. However, they are only used on small scales, e.g. in electronic connectors, because of the high material costs. 2.2.2 Conductive inks based on metal particles Another option for conductor materials is inks that have electrical conductivities, which is achieved by mixing conductive particles together with binder materials. The conductive inks can be applied to substrates by printing methods. Conductive inks can be divided into three categories: metal particle based, carbon based, and conductive polymer based. According to [4], metal particle based conductive inks possess the lowest resistivity among conductive inks. Therefore, a brief review of metal particle based conductive inks is given in this section. 2.2.2.1 Ag-based conductive inks Ag-based conductive inks are the most reported inks for printed electronics [5]. Ag-based inks with particle sizes in the micrometre range, hereafter referred to as Ag flake inks, can offer a resistivity of 2.5-3.8 × 10-7 Ω-m, which is 15.6 – 23.5 times compared to bulk Ag. Ag flake inks are designed for screen printing and stencil printing, which requires a high viscosity. Such inks can be cured by standard methods such as hot plates or convection ovens. The dry film thickness of Ag flake inks is a few µm for single-layer coatings. Ag nanoparticle (NP) inks, with Ag particles in the nanometre range, possess significantly lower resistivity than Ag flake inks. The reason for this is that Ag NPs can coalesce and merge during the sintering when most of the capping layer and solvent are removed, while Ag flake inks rely on mechanical contact between flakes. Most of the Ag NP inks are designed for inkjet printers, which requires them to have a low viscosity. Some of the lowest reported resistivities are 3-16 × 10-8 Ω-m [6-8], which is 1.9-10 times to bulk Ag. However, one problem for inkjet-printed Ag NP inks is that they have a very thin dry film thickness, ~ 1 µm for single-layer coatings. The total achievable DC resistance of printed tracks is still similar to Ag flake inks, despite the lower resistivity. To further reduce the resistivity, novel sintering techniques can be applied to Ag NP inks [9-11], e.g. low-pressure argon plasma, rapid electrical sintering, photonic sintering, microwave radiation, laser radiation, infrared radiation, and chemical sintering. It is shown in [11] that a combination of plasma and microwave flash sintering can reduce the resistivity of an Ag NP ink from 3.7 times that of bulk Ag to 1.7 times.

7 Chapter 2. Materials and Technologies of Flexible Circuits

2.2.2.2 Cu- and Au-based conductive inks Cu NPs can also be used to form conductive inks as an alternative to Ag NPs [12]. However, Cu NPs are easily oxidized, which deteriorates the performance of Cu inks. An alternative for this is to use CuO ink with a reducing agent. Cu NP ink with resistivity as low as 1.1 × 10-7 Ω-m (6.5 times that of bulk Cu) [13], 4.1 × 10-8 Ω-m (2.4 times that of bulk Cu) [14], and 3.6 × 10-8 Ω- m (2.1 times that of bulk Cu) [15], are reported. A thick-film processes is presented in [16] that screen-printed Cu paste tracks onto Al2O3 ceramic substrate with a thickness up to 100 µm and a resistivity of 2.8 × 10-8 Ω-m (1.7 times to bulk Cu). These tracks are capable of carrying a DC current up to 26 A with the conductor temperature raised to 75 °C. However, the thick-film technology requires a firing process to cure the Cu pastes, which temperature used can damage PET, PEN, paper, and nonwoven fabric substrates. A gold (Au) NP ink is presented in [17], and a resistivity of 3×10-8 Ω-m (~1.2 times that of bulk Au) is reported. Au-based inks are less frequently used than Ag and Cu based inks due to high material costs. 2.2.3 Conductors for e-textiles In addition to conductive inks, customized conductors are developed for e- textiles in order to fully integrate conductors with substrates. Fine conductive threads with diameter of a few micrometres are sewed into, stitched into or wrapped around materials such as cotton and polyester. The materials for the threads can be conductive polymers and metal wires. Depending on the materials used, these conductors can offer resistivity ranged 10-8 – 101 Ω-m [18].

2.3 Flexible circuit manufacturing This subsection briefly introduces some flexible circuit manufacturing techniques which can be generally divided into additive techniques and subtractive techniques. 2.3.1 Subtractive techniques Subtractive techniques are traditional and highly popular in industry, and these are common in fabricating Cu-PI circuits or rigid PCBs. When manufacturing these circuits, subtractive techniques are achieved by protecting the desired circuit layouts and then removing unprotected/unwanted conductors, and chemical etching methods are the most commonly used. Subtractive techniques are effective when patterning metal foils such as Cu and Al. Other subtractive techniques such as dry phase patterning [19] and e-beam evaporation

8 2.4. Flexible circuits and systems

[20] have also been reported. For dry phase patterning, unwanted conductors are mechanically removed. For e-beam evaporation, the circuit areas that do not require conductors are covered by masks before applying metal films. The masks and the conductors applied over the masks are subsequently removed, which results in the metal films being applied in the desired areas and remain- ing on the substrates. However, due to the removal of unwanted conductor materials, a well-known disadvantage of subtractive techniques is that they lead to a large amount of material wastes. 2.3.2 Additive techniques Additive techniques are achieved by directly depositing conductive/dielectric materials at desired positions, which are commonly incorporated with conductive inks. By doing so, material wastes can be significantly reduced compared to subtractive techniques. Another advantage of additive techniques is that they can be used with very fast manufacturing methods such as sheet-to- sheet and roll-to-roll. Printing techniques are widely used for additive manufacturing, including screen printing, stencil printing, inkjet printing, gravure printing, offset printing, and flexographic printing. A comprehensive expla- nation of these printing techniques is given in [21,22], and a comparison of the techniques in given in [2], as presented in Table 2.1. The advantages and disadvantages of each printing techniques are summarised and presented in Ta- ble 2.2. In addition to printing techniques, coating techniques [21,23,24] can also be used when homogeneously applying a material over a large area, including spin coating, slot-die coating, and spray coating. However, it is challenging for these techniques to achieve the same resolutions of printing techniques.

2.4 Flexible circuits and systems 2.4.1 Printed antennas Radio Frequency Identification (RFID) antennas operating on high frequency (HF)1 (13.56 MHz) and ultrahigh frequency (UHF) (865-928 MHz) bands have been screen-, inkjet-, and gravure-printed on different, especially low-cost, substrates such as PET, paper, and textiles. For the HF band, [25-28] have proposed several loop-shaped antennas for use in RFID tags and RFID readers. For the UHF band, multiple types of antennas have been proposed, such as

1 Near Field Communication (NFC) is included due to it has the same resonant frequency (13.56 MHz) as HF RFID.

9 Chapter 2. Materials and Technologies of Flexible Circuits

Table 2.1 A comparison of printing techniques. Printing Wet film Ink vis- Printing Minimum fea- speed thickness cosity method ture size (µm) (m/min) (µm) (m Pa·s) Screen 50-100 10-100 3-100 500-50,000 Inkjet 20-50 1-100 0.3-20 1-40 Gravure 20-75 20-1000 0.1-5 50-200 20,000- Offset 20-50 15-1000 0.5-2 100,000 Flexographic 30-75 50-500 0.5-8 50-500

Table 2.2 Advantages and disadvantages of printing techniques. Printing Advantages Disadvantages method Low printing resolution Large range of film thick- Screen High viscosity ink needed ness (depending on screen) Only flat printing Low printing speed Low viscosity ink needed High printing resolution (Low solid content in ink) Inkjet Digitally controlled Low film thickness Printing optimisation needed for fine features Costly for small production High printing speed Gravure High pressure needed for (Roll-to-roll) printing fine features High printing speed Costly for small production Offset (Roll-to-roll) High requirements for inks Fine printing resolution Flexo- Fine printing speed Costly for small production graphic (Roll-to-roll) Low viscosity ink needed microstrip patch antennas [29], dipole antennas [30], Quasi-Yagi antennas [31], and monopole antennas [32]. Flexible printed antennas have also been used for other frequencies and applications such as: wireless sensor networks (WSN) (2.4 GHz) [31,32] and wireless local area network (WLAN) (2.4 GHz) [33,34]. It is reported in the above literature that these RFID antennas made from conductive inks commonly suffer from decreased performance compared to antennas made from metal foils, e.g. the radiation efficiency of UHF antennas

10 2.4. Flexible circuits and systems

and the quality (Q) factor of HF antennas. The reason for this is the non-negligible resistivity of conductive inks compared to metal foils, which creates an unwanted RF power loss on conductors. Different methods have been proposed to reduce ink resistivity by applying multiple conductive ink layers [30,35] and by investigating ink curing/sintering conditions [36]. Flexible antennas are also seen in wearable applications [37,38], where the conductor materials of the antennas are printed, laminated, or sewn onto textile materials. These antennas are integrated with daily clothes and connected to other electronic circuitries as a part of e-textiles. They are designed to have wide bandwidths in order to avoid being sensitive to bending. Such devices are connected to the Internet by wireless network protocols such as Bluetooth and WLAN, which enables daily clothes to be a part of the Internet-of-Things. 2.4.2 Printed sensors and printed components One important usage of non-conventional materials is in the construction of sensors that are based on the principle that environmental changes are trans- ferred into electrical property changes, such as impedance (piezoresistive effects). Many types of sensors have been developed based on this principle, e.g. force/pressure sensors [4,39], temperature sensors [40], relative humidity (RH) sensors [40,41], and pH sensors [42]. They are incorporated with external read-out electronic devices such as microcontrollers. Sensors that are integrated into RFID devices are also frequently reported in the literature. Several designs are proposed in [43] for integrating an inkjet- printed resistive RH sensor unit directly into UHF RFID tag antennas. Thus, the impedance of the tag antennas changes with the RH, which is subsequently determined by the reflected power of the tags at RFID readers. For HF, [44] proposed integrating capacitive RH sensing arrays (screen- and inkjet-printed) into RFID tag antennas. The change in RH results in a shift in the resonant frequency of the RFID tag in a linear relationship. Printing techniques and non-conventional materials have also been used for fabricating thin and multi-layered electronic components, e.g. solar cells [45,46], organic light emitting diodes (OLEDs) [47], organic field effect tran- sistors (OFETs) [48]. However, compared to the devices made from traditional Si-based materials, these devices have deteriorated functionalities such as lower efficiency (solar cells), shorter life span (OLEDs), and limited performance (OFETs).

11 Chapter 2. Materials and Technologies of Flexible Circuits

2.5 Hybrid flexible circuits One of the approaches to overcoming the deteriorated functionalities is to combine standard Si-based surface mount device (SMD) components with printed conductive tracks on flexible substrates to form hybrid flexible circuits. Therefore, the high performance of SMD components and the benefits of flexible circuits are combined. Many works focusing on hybrid flexible circuits have been presented. In [50], an active (battery-powered) HF RFID multi-sensor tag is proposed, which comprises PEN substrates, a screen-printed tag antenna and inkjet-printed RH, temperature and NH3 sensors. The multi-sensor tag comprises two SMD components, a microcontroller and an RFID tag integrated circuit (IC), the purposes of which are to collect sensor data and communicate with the RFID interrogator. Similarly, a photo-paper-based active WSN sensing node, operating at 904 MHz, is presented in [32]. The node includes an inkjet-printed dipole antenna and SMD components such as an RF transceiver and temperature sensor. In order to form hybrid flexible circuits, the techniques to assemble SMD components onto flexible circuits need to be addressed. For Cu-PI based circuits, many mature techniques that are developed for rigid PCBs are available, e.g. reflow soldering, hot-air soldering, laser soldering, and conductive adhesives. For tracks made from conductive inks, some of the SMTs are not applicable due to excessive temperature that could deform substrates. 2.5.1 SMTs using low temperature solder pastes One method to avoid high temperature during reflow soldering process is to use low-temperature solder pastes (LTSPs). Commercially available LTSPs can offer a melting temperature as low as 138 ˚C with a Sn42/Bi57.6/Ag0.4 content. Related works addressing soldering SMD components onto flexible circuits using LTSPs are summarised in Table 2.3. 2.5.2 SMTs using conductive adhesives SMD components can be assembled using conductive adhesives that are in forms of glues and tapes. Due to conductive adhesives have milder curing conditions, e.g. lower temperature, than LTSPs, they can be applied to a wide range of materials and especially those are sensitive to heat, e.g. PET. In addition, conductive adhesives can be good choice when the surface of a conductor has a poor wetting to solder. Isotropic conductive adhesives (ICA) and anisotropic conductive adhesives (ACA) can be chosen, where the ICAs are typically filled with Ag particles and the ACAs can contain metal particles such as Au, Ag, or Ni [55]. Due to the conductivities in z-direction, ACAs are

12 2.6. Large-area electronics

Table 2.3 A summary of SMTs based on LTSPs and conductive adhesives. SMT Reference Conductor material Substrate material [51][52] Ag NP ink Paper [52] Ag NP ink PI LTSP [53] Conductive threads Fabric [54] Ag flake ink Fabric [32][51][52] Ag NP ink Paper [52][57] Ag NP ink PI ICA Ag flake ink and Ag [50] PEN and PET NP ink [51] Ag NP ink Paper ACA [58] Cu foil PI more suitable than ICAs when mounting components in leadless packages such as ball grid array [56]. Related works addressing assembling SMD components onto flexible circuits using conductive adhesives are summarised in Table 2.3. 2.5.3 Other SMTs Special SMTs have also been developed for assembling components on e-textiles with conductive threads as conductors. In [53], it is shown how to sew SMD component terminals directly on land pads using threads containing metal wires. A terminal-to-land-pad direct contacting technique with a help of a non-conductive adhesive is proposed in [59]. A special solder paste is proposed in [60], containing Sn-Bi solder balls and epoxy resin. The solder paste is later used in [61], where components are soldered on circuits made from screen-printing Ag flake ink onto paper and cloth substrates. The land pads of components are plated with an extra Sn-Cu-Ni layer in order to assist the soldering process. In [52], it shows an SMT that uses an Ag NP ink as ICA to mount components on Ag NP ink tracks. The Ag NP ink was automatically deposited using an inkjet printer.

2.6 Large-area electronics One type of large-area electronic device made from non-conventional materials found in the market is displays, where a good example is the LG rollable TV (OLED-based) that has an area around 2.7 m2 (65 inches in diagonal). An- other type of large-area commercial electronics is flexible solar cells, which can easily cover 1 m2 per unit. Flexible solar cells can be connected in parallel so that the area is multiplied and large-area energy harvesting is achieved.

13 Chapter 2. Materials and Technologies of Flexible Circuits

Flexible solar cells can be deployed on uneven surfaces, where it is difficult to use rigid solar cells. A large-area pressure sensing matrix is proposed in [4] with an area of 23.5 × 21.5 cm2 using a PET substrate and an Ag flake ink. The matrix is composed of 16 resistive sensing points and is integrated into wheelchairs to monitor pressure distribution. An interesting concept for constructing large-area electronics is presented in [21], where pressure sensor nodes are equilateral trian- gles or hexagons. The sensor nodes are constructed on a PET substrate, comprising one programmable capacitance-to-digital converter (an SMD component) and up to 10 capacitive sensing points (printed sensors). Many triangle- or hexagon-shaped nodes are subsequently combined edge-to-edge to form a large-area pressure sensing surface for smart table applications. Both [21] and [4] require external devices for the power supply and read-out electronics. Antennas systems with large structures can be achieved when implementing antenna arrays. A multi-input multi-output (MIMO) antenna array, comprising 100 antennas and operating at 3.7 GHz, with an area of 160 × 110 cm2 on rigid PCB is reported in [62]. An antenna array for 2G/3G/4G base stations is proposed in [63], which is implemented using rigid materials with an area of 16.0 × 13.4 cm2. A hexagon-shaped WLAN antenna array containing three Landstorfer and three Yagi antennas is implemented on a rigid PCB with an area of 478 cm2 is presented in [64]. Large-area antenna arrays can also be implemented by arranging multiple separated smaller-area antenna elements made by rigid PCBs into a matrix, as presented in [65]. When using flexible materials, the concept of “large area” is more related to large-area manufacturing/patterning instead of the dimension of one integrated antenna system [66]. For example, the antennas of HF and UHF RFID tags can be manufactured on large-area flexible substrates with high speeds onto which RFID tag ICs are subsequently attached. To the best of the author’s knowledge, there are no reports on integrated large-area antenna arrays/systems fabricated from non-conventional flexible materials.

3 Problem Formulation and Objective

3.1 Problem formulation The thesis is divided into three sections with corresponding research questions (RQs) formulated. The interrelations between the research problems are presented in Figure 3.1. 3.1.1 Materials and manufacturing Standard flexible PCBs are commonly based on PI films that are resistant to high temperatures and with Cu as conductive material, but they are both expensive and environmentally hazardous. This is especially problematic when implementing large-area electronic systems. Therefore, one interesting option is to seek solutions and technologies using non-conventional materials and printed electronics. The DC resistance of conductive-ink-based tracks cannot be neglected and will directly result in a waste of power, which is problematic for high-current circuits as well as high-frequency structures. For metal foils, this issue is generally avoided due to their low resistivity (< 1 × 10-7 Ω-m). Therefore, it is important to find a manufacturing method for conductive inks that achieves a DC resistance that does not lead to excessive signal/power losses. Although many NP-ink-based tracks with low resistivity have been presented, a common issue is that they have not achieved sufficient dry film thickness to have a low DC resistance. In addition, there is a lack of a systematic characterisation to prove whether conductive-ink-based tracks are suitable for high-current applications. The following research question is identified in this section: RQ 1. How to achieve low DC resistance conductor tracks when using conductive inks? What are the limits when conductive-ink-based tracks are used in high-current applications? 3.1.2 Surface mounting techniques Si-based SMD components, such as microcontrollers, can be used to implement electronic systems with advanced functionalities. If non-conventional materials are used, the SMTs used for Cu conductors on FR-4 laminates might not be applicable. These SMTs might require high temperatures that deforms substrates such as PET and paper or might contain incorrect solder content that does not work with conductors such as Al which has a dense surface ox- idation layer. Thus, novel SMTs need to be developed in accordance to the

15 Chapter 3. Problem Formulation and Objective

RQ 1 RQ 2 RQ 3

Flexible circuits made Assembling Large-area from non-conventional components electronics and flexible materials (SMTs) systems

Figure 3.1 Interrelations of the research questions. properties of the used materials in a flexible circuit. In addition, for the sake of volume productions, the SMTs should be able to be performed in existing automatic assembling lines. The following research question is identified in this section: RQ 2. How to assemble SMD components onto flexible circuits made from non-conventional materials? How to assess the performance and quality of developed SMTs? Can the SMTs be automated to assist large-volume productions? 3.1.3 Large-area electronic systems Hybrid electronics gives an opportunity for large-area electronic systems made from non-conventional materials to be capable of integrating both analogue and digital signals. Such opportunities also cover high-frequency signals, such as in microwave frequency bands (> 300 MHz), which in turn enables non-conventional materials to be used in realising large-area integrated antenna systems. By doing so, it enriches the utilizations of non-conventional materials to be more than manufacturing RFID tags. Due to the non-conventionalities of the materials and the SMTs, it is important to study their effects on HF and microwave signals. When circuits use non-bulk metal materials as conductors, e.g. conductive inks, a decrease in performance, e.g. low efficiencies and high attenuations, is expected due to the resistivity of the inks. Signal attenuations due to losses in circuits, conductors, substrates, and the contact resistance of SMD components will decide the maximum area of the constructed circuits. RQ 3. What are the limits for large-area electronic systems that are implemented from non-conventional materials? What are the performance limitations of HF structures, e.g. antennas and microwave transmission lines, when constructed from non-conventional materials?

16 3.2. Objective

3.2 Objective The objective of this thesis is to provide a systematic approach and general guidelines on how to implement large-area electronic devices that comprise both digital and analogue signals using non-conventional flexible materials and flexible circuit technologies. The goals of this thesis are detailed as follow:  Identify non-conventional materials and fabrication techniques that can be used in implementing electronic systems. A sub-goal is to investigate fabricating methods that can achieve a relatively low DC resistance in conductive-ink-based tracks. Characteriza- tions are required to determine if the tracks are suitable for high- current applications.  Develop and evaluate novel SMTs to efficiently assemble Si-based SMD components in accordance with the used flexible circuit materials. The purpose of developing SMTs is to construct hybrid electronic systems.  Implement and characterise large-area antenna systems integrating both digital and RF signals for industrial RFID readers as proofs of concepts. Subsequently, calculations are performed to es- timate the maximum areas and limitations of the antenna systems. The two antenna systems are listed as follow: o An UHF RFID reader antenna system that operates at 867 MHz o An HF RFID reader antenna system that operates at 13.56 MHz

4 Papers Included in This Thesis

A short summary of each paper and how the included papers support this thesis are provided. The complete versions of the papers are attached in Part B.

4.1 Paper I – Soldering Surface Mount Components on Screen-Printed Ag Patterns on Paper and Polyimide Substrates for Hybrid Printed Electronics This paper is dedicated to address RQ2. This paper describes two new SMTs to for soldering standard SMD components directly onto flexible circuits fabricated by screen-printing an Ag flake ink onto a cardboard paper substrate (high surface roughness) and a PI substrate. A low-temperature solder paste (LTSP) was used and the soldering was performed using a hot air station and a reflow soldering oven. The results showed that the components soldered on the Ag ink tracks with cardboard paper substrate achieved low contact resistance (< 240 mΩ). The hot air soldering gave 10.4, 6.5, and 2.7 N for respectively 1206, 0805 and small outline integrated circuit (SOIC) packages, which are about 60% higher than reflow oven soldering and about 10% to soldering on rigid PCBs. However, the contact resistance and bonding strength were not satisfactory on the PI substrate. An electronic system level demonstration was also given, where a NFC tag with RH sensing functionality was fabricated using the flake ink and cardboard substrate. It showed that the NFC tag can interact with NFC-equipped smart phones. The methods described in this paper showed potentials to form hybrid electronic devices, which was further utilised in Paper VIII, where an RFID reader antenna system that works on the same frequency (13.56 MHz) was implemented.

4.2 Paper II – Soldering Surface Mount Components onto Inkjet Printed Conductors on Paper Substrate Using Industrial Processes This paper is dedicated to address RQ2. This paper describes an SMT to directly solder standard SMD components onto flexible circuits fabricated by inkjet-printing an Ag NP ink onto a photo

19 Chapter 4. Papers Included in This Thesis paper substrate using an industrial process. The soldering process was carried out in a reflow soldering oven using an LTSP. The results showed that components soldered onto this type of flexible circuits suffered from unstable contact resistance and high failure rates. The Ag NP ink cracked frequently during the soldering process due to a thin Ag NP ink layer (~700 nm) plus a thermal expansion coefficient mismatch among the Ag NP ink, the LTSP, and the coating layer of the photo paper. The methods presented in this paper are therefore not recommended to fabricate hybrid electronic circuits.

4.3 Paper III – Enabling Paper-Based Flexible Circuits with Aluminium and Copper Conductors This paper is dedicated to address RQ2. This paper presents two types of high-performance paper-based flexible circuits. To overcome the non-negligible resistivity in tracks based on conductive inks, the proposed flexible circuits used Cu or Al foils as conductors. The flexible circuits were manufactured by laminating pre-pattered Cu and Al foils onto bleached Kraft paper using a high-speed roll-to-roll process (hereafter denoted as Cu-paper and Al-paper). Three SMTs were used to assemble standard SMT components onto the manufactured circuits: an ICA, an ACA, and an LTSP. Cu-paper circuits achieved satisfactory results during the characterisations. A two-point circuit bending test showed that they can withstand 9,000 bending cycles without obvious change in their electrical properties. The ICA, ACA and LTSP have achieved low contact resistance (30-440 mΩ) and component bonding strengths of 20%, 28% and 34%, respectively, compared to soldering on rigid PCBs. However, although Al-paper circuits achieved similar component bonding strength, they withstood fewer bending cycles (2,000) and suffered from unstable contact resistance (0.53-4.12 Ω) and significant failure rates (10-20%) due to the relatively high edge and surface roughness on the Al foil. As demonstrators, NFC tags that have the same functionality and designs as Paper I were fabricated and tested to be functional. The circuits presented in this paper also showed potential to be used in automatic component mounting processes. With the low resistivity in conductors, the proposed circuits can compete with conventional rigid PCBs as well as flexible circuits made from Cu and PI.

20 4.4. Paper IV - Flexible Circuits Based on Aluminium Conductor and Nonwoven Substrate

4.4 Paper IV - Flexible Circuits Based on Aluminium Con- ductor and Nonwoven Substrate This paper is dedicated to address RQ2. This paper is similar to Paper III and presents a new type of high-performance e-textile that uses Al foil as conductor and nonwoven fabric as substrate (hereafter denoted as Al-nonwoven). This type of e-textile has the same manufacturing methods as the flexible circuits presented in Paper III. The ICA and ACA were used to assemble SMD components. The Al-nonwoven circuits showed unstable contact resistance (0.64-4.06 Ω) with 10-50% higher failure rates than the Al-paper circuits presented in Paper III. The reason of this was similar to the Al-paper circuits. In addition, the component bonding strength achieved by the ICA and the ACA were respectively 6% and 26% compared to rigid PCBs. However, further improvements on circuit cleaning methods/instruments are likely to achieve better results.

4.5 Paper V – Low-Resistance Screen-Printed Nanoparti- cle Ink Tracks for High-Current Applications This paper is dedicated to address RQ1. This paper presents low DC resistance tracks fabricated by screen-printing an Ag NP ink and investigates the possibilities of the tracks to be used in high- current applications. The Ag NP ink is printed onto: three types of photo papers, one PI, one PET, and one coated PET. Studies were given if substrate coating materials and multi-layer printing can be used to reduce the DC resistance of printed Ag NP ink tracks. It was found that applying a coating layer on substrates can reduce the DC resistance due to coating materials can assist ink deposition, which increases the dry film thickness of the ink. When one layer of ink was applied, average of 3.6 µm and 7.3 µm dry film thickness were achieved on respectively un-coated and coated substrates, which are significantly higher than inkjet-printed NP ink tracks (~ 1 µm). In addition, it was found that applying thick ink through sten- cils used to deposit solder pastes (wet film thickness 100 µm), as well as repeated print-sinter processes, have caused the ink cannot be completely sintered. A DC current carrying capacity test, according to international test standards, was performed to the printed tracks. It showed that the tracks can carry 2.0-3.5 A DC current on 5.0 mm wide tracks depending on the substrate and the achieved ink dry film thickness. Despite the current carrying capacity of

21 Chapter 4. Papers Included in This Thesis the tracks presented in this paper cannot be compared to conventional Cu- based rigid PCBs, the results presented in this paper open possibilities for conductive-ink-based tracks printed onto flexible substrates to be used in high-current electronics. It further enriches the applications of printed electronic technologies.

4.6 Paper VI – Flexible Circuits and Materials for Large- Area RFID Reader Antenna Systems This paper is dedicated to address RQ2 and RQ3. Paper VI is divided into two parts. The first part describes fabrication methods for constructing double-layer circuits using non-conventional and flexible materials. The top layer of the circuits is patterned Al foil with a PI substrate, and the bottom layer is Al foil. The dielectric layer sandwiched between the top and bottom layer is a 3.0 mm thick polyethylene foam. This paper also examines the possibilities to use Al as the main conductor material of an electronic system instead of Cu. With a special solder flux, an SMT was successfully developed to manually solder components onto Al conductors that are well-known to have a thick oxide layer that prevents the use of conventional SMTs normally used for Cu conductors. The SMT achieved a component bonding strength that was 48% that of soldering on rigid PCBs, and achieved low contact resistance (110 mΩ in average). Another SMT used an ICA, which achieved a sufficiently low contact resistance (190 mΩ in average) with a component bonding strength 23% to rigid PCBs. The second part of Paper V shows the possibilities for using non-conventional and flexible materials to construct large-area electronic devices. As a demonstrator, a large-area UHF RFID reader antenna system was implemented, which has an area of 1.2 m × 0.6 m and operates at 867 MHz (EU standard). The antenna system was designed into a three-tier single-pole- four-throw (SP4T) switching network with microstrip antennas as loads, and thus has the capacity to be further expanded. The antenna system cooperates together with a commercial UHF RFID reader via a 50 Ω coaxial cable. The antenna system can monitor passive RFID tags above its surface at least as high as 2.0 m, with a minimum system input power 15 dBm without obvious RFID interrogation dead zones. Based on the measured RFID interrogation zones, the antenna system showed potential to perform large-area RFID tag monitoring.

22 4.7. Paper VII – UHF RFID Shelf Reader Antennas for Object Classification and Distance Estimation for Non-Tagged RFID Objects

Based on the characterisations performed with the antenna system, a large number of calculations were made in order to study the limitations of the antenna system in terms of the maximum number of microstrip antenna elements and the maximum antenna system areas. The calculations suggested that the antenna system can be expanded to cover an area over 1,000 m2 or monitor over 10,000 antenna elements with a single RF input port.

4.7 Paper VII – UHF RFID Shelf Reader Antennas for Ob- ject Classification and Distance Estimation for Non- Tagged RFID Objects This paper presents some non-conventional applications of the microstrip antenna element presented in Paper VI. The purpose of this paper was to perform estimations on the electrical properties and distances of non-tagged RFID objects that are placed in the near field of the microstrip antenna element. The principle of achieving this is that objects with different electrical properties placed on top a microstrip antenna behave as superstrates that have in- fluences on: resonant frequency, the magnitude of return loss at the resonant frequency, bandwidth, and the magnitude of return loss at non-resonant frequencies. These parameters can be in turn monitored by a network analyser. The results of this paper showed that the microstrip antenna element are capable to perform such functions. With the antenna system presented in Paper VI, it is also possible to perform estimations on the electrical properties and distances of non-tagged RFID objects over a large area. However, it requires industrial RFID readers to be equipped with network analyser functions, which is a limit for this method.

4.8 Paper VIII – A Paper-Based Screen Printed HF RFID Reader Antenna System This paper is dedicated to address RQ1 and RQ3. This paper presents an HF (13.56 MHz) RFID reader antenna system. The antenna system was fabricated by screen-printing an Ag flake ink onto a photo paper substrate. The SMT presented in Paper I was used in this paper to mount SMD components. A method to achieve a low DC resistance in the Ag-ink tracks was developed, where an Ag flake ink was printed six times. The dry film thickness of the tracks were measured to be 42 µm and the DC resistance of the tracks

23 Chapter 4. Papers Included in This Thesis dropped to ~15% compared to the tracks that were printed with only one layer of Ag ink. The antenna system was designed into a one-tier SP4T switching network with four identical loop antenna elements that were matched to 50 Ω as loads. The results showed that the loop antennas achieved a Q factor of 6. The results also showed that the antenna system could perform passive RFID tag monitoring and positioning above its surface without dead zones up to 6 cm (maximum input power 33 dBm). A parametrical study was also carried out to study the effect of loop antenna DC resistance, where the performance of the screen printed reader antenna element is compared with those made of Cu and different Ag ink thicknesses. This comparison was given in Table VI of Paper VIII. Despite the low-cost and low-waste advantages of the chosen materials, the results showed that the screen printed antennas had disadvantages in performance due to the non- negligible resistivity in the Ag flake ink.

5 Methodology

The work presented in this thesis followed the flowchart shown in Figure 5.1 and had three major steps – identifying materials and fabricating flexible circuits, developing SMTs, and implementing large-area electronics systems. Necessary evaluations were performed corresponding to each step.

5.1 Materials and fabrication methods The electrical properties of the materials, e.g. dielectric constant and loss tangent, were characterised for implementing electronic systems in microwave frequency bands (> 300 MHz). An open microstrip quarter wavelength stub resonator method [67] was used in Paper VI to measure the dielectric constant and tangent loss of a foam that was used as the substrate of microstrip antenna, which further assisted in the finite element method (FEM) simulations. The materials used with the corresponding circuit fabrication methods and paper number are summarized in Table 5.1. Both subtractive and additive techniques were used as fabrication methods. Subtractive techniques were applied when Cu and Al foils are patterned, the details of which are described in Paper III, IV and VI. It needs to be mentioned that a high-speed roll-to-roll transfer laminating method was performed to fix the foils onto bleached paper and nonwoven fabric substrates, which is described in both Paper III and IV. Additive techniques were used to fabricate paper- and conductive-ink- based circuits, where screen printing was used for Paper I, V and VIII and inkjet-printing was used for Paper II. The following flexible circuit characterizations have been used in to the fabricated circuits: - Repeated circuit bending tests (two-point) - Edge roughness inspections - Surface inspections A two-point circuit bending test was used in order to evaluate the quality of the fabricated flexible circuits. During the bending tests, a 4-wire resistance measurement setup was applied to study the relationship between conductor material sheet resistance and bending cycles. The edges and surfaces of the conductor materials were examined under a microscope as another quality evaluation. The details of the two characterisations can be found in Section 2.4.1, Paper III.

25 Chapter 5. Methodology

Implementation Characterisation Identifying Non- Study materials Material conventional properties flexible materials RQ1

Fabricating Flexible circuit flexible circuit characterisations

For hybrid electronic systems

Developing SMT RQ2 SMTs chracterisations

Used in Implementing Electronic Antenna system large-area system characterisations antenna systems Support discussions in RQ3 Antenna system limitations

Figure 5.1 Flowchart of methodology. Table 5.1 List of flexible circuit types. Flexible circuit type Fabrication Paper number (Substrate + conductor material) method Cardboard paper + Ag flake ink Screen printing I Photo paper + Ag flake ink Screen printing VIII Photo paper + Ag NP ink Inkjet printing II Photo paper + Ag NP ink Screen printing V Bleached paper + Al foil * Chemical etching III Bleached paper + Cu foil * Chemical etching III Nonwoven + Al foil * Chemical etching IV PI + Al foil Chemical etching VI, VII

* Circuit patterned by chemical etching; however a transfer roll-to-roll laminating is needed.

26 5.2. Surface mounting techniques

Two methods to implement vertical interconnect accesses (VIAs) were proposed when double-layered circuits are needed. One was to connect conductive traces to the top and bottom layers using PCB rivets, which is described in Paper I, III, and IV. Another method was to insert a straight metal connector through the traces on different layers as described in Section 2.5, Paper VI, which was used when soft materials that cannot withstand riveting were used. 5.1.1 DC current carrying capacity test A DC current carrying capacity test was performed with Ag NP ink tracks screen-printed on: photo paper, PET, coated PET, and PI substrates. The test was conducted according to international standards [68], with the setup presented in Fig. 1, Paper V. A contactless temperature measurement was performed together with the current carrying capacity test with the help of a FLIR i7 thermal camera. With this test, a relationship between DC input current and ink track temperature increment is given, and the temperature coefficient of the ink track resistance can subsequently be calculated.

5.2 Surface mounting techniques SMTs were developed in order to assemble SMD components onto the flexible circuits listed in Table 5.1. A general objective for the developed SMTs was that they should be able to be applied or upgraded to industrial/automated processes. In addition, to increase the efficiency of the SMTs, SMD components were mounted directly onto flexible circuits. A summary of the developed SMTs and the corresponding types of circuits is given in Table 5.2. Except for soldering components on Al-PI circuits, all of the developed SMTs were performed under low-temperature conditions, which was to prevent damages to the PET, paper, and nonwoven fabric substrates. The following characterisations were performed on the developed SMTs: - Contact resistance measurements - Component bonding strength measurements - Cross-sectional inspections of mounting joints - Assembled circuit bending tests - Electronic system demonstrators: NFC tags with RH sensing The contact resistance measurement was initially performed by the 2-wire method (Papers I and II) and then with the 4-wire method (Paper III and IV). These methods were incorporated with a digital multimeter (HMC8012, Rhode & Schwarz).

27 Chapter 5. Methodology

Table 5.2 List of the developed SMTs. Paper SMT Description Applied flexible circuit number Cardboard paper + Ag flake ink I Low temper- Reflow and Photo paper + Ag flake ink VIII ature solder hot air sol- Photo paper + Ag NP ink II paste (LTSP) dering Bleached paper + Cu foil III Isotropic Bleached paper + Al foil III conductive Convection Bleached paper + Cu foil III adhesive, oven curing Nonwoven + Al foil IV Ag-based (ICA) PI + Al foil VI Anisotropic conductive Automatic Bleached paper + Al foil III adhesive, Ni- component Bleached paper + Cu foil III Au-based mounting Nonwoven + Al foil IV (ACA) Soldering Manual sol- with special dering, only PI + Al foil VI flux for Al for PI-Al cir- conductor cuits The setup used for the component bonding strength tests is presented in Section 2.2.1, Paper I. The setup was developed following the guidelines given in international standards [69-71], which performs component bonding tests with two-terminal leadless components (shear force) and gull-wing leads components (45˚ pull force). The central piece of equipment in the setup is a dynamometer Lutron FG-6020SD that logs force data every 10 ms. The cross-sectional inspection to the mounting joints was performed using a TESCAN MAIA3 GMU 2016 scanning electron microscopy (SEM). A plastic embedding and sample grinding process was required before using the SEM in order to reveal the cross sections of the mounting joints, which is described in Section 2.2.2, Paper I. Assembled circuit bending tests were carried out in order to determine the flexibilities of the circuits. Components were assembled on flexible circuits and subsequently bent on the surface of cylinders with different diameters, the details of which are given in Section 2.2.3, Paper I.

28 5.3. Large-area RFID reader antenna systems

In order to demonstrate that the proposed SMTs and flexible materials can be used to construct functional electronic systems, NFC tags with RH sensing functionality were fabricated. The NFC tags can be found in Paper I-IV, with design details given in Section 3.6, Paper I. The NFC tags presented in the included papers have identical schematics and components, but with different materials and SMTs.

5.3 Large-area RFID reader antenna systems Two antenna systems, operating at HF (13.56 MHz) and UHF (867 MHz), were implemented for industrial RFID readers. The purpose of implementing the two antenna systems was to demonstrate how the materials and techniques previously described could be used to construct large-area electronic systems, comprising analogue and digital signals. The topology used for both antenna systems was an SP4T switching network, as shown in Figure 5.2. The switching functionality was realised using I2C addressed SP4T multiplexers (MUXs) controlled by exterior control units, the central component of which was an SP4T RF/analogue switch. The loads of the SP4T network were antenna elements, and transmission lines were used for interconnections between different MUX tiers, and between the MUXs and antenna elements. The operational details of the antenna systems are respectively given in Section 3, Paper VI, and Section III, Paper VIII. For the antenna elements and transmission lines used for the UHF antenna system, 3D FEM simulations using ANSYS HFSS were performed based on the parameters of the chosen materials. The SP4T MUX for the UHF antenna system was also simulated using ANSYS Circuit Designer using the S-param- eter files of the used RF switch which is available from its manufacturer’s website. The antenna system was implemented after the simulations. For the HF antenna system, a direct experimental approach was chosen, and simulations were not performed. One advantage of the multiplexing topology is that it only requires one input 푡표푡푎푙 port. The total number of antenna elements 푁푎푛푡 of a multiplexing topology can be calculated from: 푡표푡푎푙 푁푡푖푒푟 푁푎푛푡 = 푁푀푈푋,푐ℎ푎푛푛푒푙 (5.1)

where NMUX,channel is the number of MUX output channels, e.g. 4 in Figure 5.2, and Ntier is the total tier number of MUX, e.g. 3 in Figure 5.2.

29 Chapter 5. Methodology

1st tier multiplexer 2nd tier multiplexers 3rd tier multiplexers (MUX) (MUX) (MUX) 2 2 2 I C addressed I C addressed αTXline I C addressed More tiers Load Load TX line Load Load attuenation Load Load αMUX αTXline Load = antenna element Load Load Input power Pin,ANT

Input power Pin Load αTXline Load αTXline αMUX RFID reader αTXline Load Load Multiplxer Load insertion loss Load αMUX αMUX Load Load αTXline Load Load Load Load Load αMUX αTXline

Figure 5.2 A three-tier single-pole-4-throw switching network. Another advantage is that all channels have similar characteristics, meaning that all antenna elements have the same amount of input power, and the input power of every antenna element Pin,ant in Figure 5.2 is:

푃𝑖푛,푎푛푡 = 푃𝑖푛 + 퐿푇푋푙𝑖푛푒훼푇푋푙𝑖푛푒 + 푁푡𝑖푒푟훼푀푈푋 (5.2)

where Pin is the antenna system input power, i.e. RFID reader output power, LTXline is the total length of transmission lines, αTXline is the insertion loss of transmission lines per unit length, and αMUX is the insertion loss per MUX. Despite the similarities in the topology, the two antenna systems have many differences, which comparison is detailed in Table 5.3. The following characterisations/measurements were performed on the antenna systems: - Antenna element return loss (S11) - Antenna element Q factor (for the HF antenna system only) - MUX return loss (S11) and insertion loss (S21) for all channels - Transmission line return loss (S11) and insertion loss (S21) (For the UHF antenna system only) - Passive RFID tag read range measurements - Passive RFID tag interrogation zone measurements The return losses (S11) and insertion losses (S21) mentioned above were measured using a 2-port Agilent E5070B network analyser. These measurements are standard procedures with guidelines given in Agilent user manuals.

30 5.3. Large-area RFID reader antenna systems

Table 5.3 A comparison between the UHF and HF RFID reader antenna systems. UHF antenna system HF antenna system Al-PI foil, polyethylene Ag flake ink and photo Material foam, and Al foil paper substrate Double-layer circuit Construction Single layer circuit board board Resonant frequency 867 MHz 13.56 MHz Antenna element Rectangular microstrip Loop antenna (imped- type patch antenna ance matched to 50 Ω) SP4T network 3-tier 1-tier Microstrip transmission Transmission line Not applied line Circuit area 1.2 m × 0.6 m 13.5 cm × 13.5 cm The passive RFID tag read ranges and interrogation zones were measured with industrial RFID reader units with different antenna system input power. The measurements for the two antenna systems are described in Section 3.2.2, Paper VI, and Section III.C, Paper VIII, respectively. The results of the characterizations were used in discussing the limitations of the two antenna systems in terms of: how many antenna elements can be included in an antenna system and how large of area can an antenna system be constructed to cover.

6 Discussion

Based on the results presented in the included papers, the following topics are discussed in this chapter: - Materials and manufacturing - Qualities of the developed SMTs - Limitations of the RFID reader antenna systems - Social and ethical considerations

6.1 Materials and manufacturing 6.1.1 Materials Paper substrates were commonly used in the work for this thesis. Despite having many advantages, e.g. ultra-low-cost, environmental friendliness, and re- cyclability, paper-based flexible circuits are not suitable for use in high-temperature processes or environments. Another limitation is that paper can absorb water from the surrounding environment, which can lead to changes in the dielectric constant and electrical isolation. Polymer aqua-resist coatings can be used if paper-based circuits are used in high RH environments. Circuits based on nonwoven fabric can be used in wearable applications. Although satisfactory results were not achieved when mounting components onto the Al-nonwoven circuits, this does not rule out that other material combinations might not work, e.g. Cu foil and nonwoven. Compared to paper, the temperature endurance of nonwoven fabric is lower, which is why LTSP was not used. In addition, it was found in that nonwoven fabric has a tendency to shrink, which leads to a misrecognition of land pads by machine-vision-based chip bonders. This in turn affects ACA depositions and component placements during automated mounting processes. 6.1.2 Manufacturing Both subtractive and additive manufacturing techniques were performed, including screen printing, inkjet printing, and chemical etching. High speed fabrication methods, such as sheet-to-sheet and roll-to-roll, were also applied to some of the material combinations (see Table 5.1). Two approaches are given in the papers for achieving a low DC resistance in the printed Ag ink tracks. One was presented in Paper VIII, where six layers of ink were screen-printed on a loop antenna, which reduced the DC resistance of the loop antenna from 25.4 Ω to 3.8 Ω. Despite the result, it needs to be pointed out that this methods has a relatively low printing accuracy, and

33 Chapter 6. Discussion thus cannot be applied to fine features or conductor tracks with small clear- ances. Another method was presented in Paper V, where a screen-printable Ag NP ink was used, which had the advantages of low resistivity and greater dry film thickness than inkjet Ag NP inks. A DC current carrying capacity 2.2 - 3.5 A on 5.0 mm wide tracks was achieved depending on the substrate and achieved dry film thickness. However, a limit factor for this method is the sintering technique. A higher current carrying capacity and lower track resistance would be achieved if effective sintering techniques were applied. By doing so, the ink applied through the repeated print-sinter process and the stencil printing can be correctly sintered. Thus, the current carrying capacity of the Ag NP ink tracks can be one step closer to bulk Cu.

6.2 Qualities of the developed SMTs 6.2.1 Contact resistance High contact resistance means that there are both DC and RF signal losses when a current is going through the mounting joint. For RF signals, this loss can directly increase the insertion loss of an RFIC. Having this in mind, some SMTs developed (see Table 5.2) showed results that are sufficiently low, which are considered not to contribute obvious signal/power losses. In con- trast, some SMTs suffered from high contact resistance with significant failure rates, and thus are not considered suitable for implementing hybrid electronic systems. An overall summary on contact resistance is given in Table 6.1. 6.2.2 Component bonding strength The component bonding strengths measured from the developed SMTs are compared to rigid PCBs as shown Table 6.1. It can be concluded that the developed SMTs have achieved lower strength than rigid PCBs (Table 6, Paper I). This indicates that the environmental durability of the assembled flexible circuits is lower than rigid PCBs. Together with the properties of the chosen non-conventional materials, it is not advised to use the flexible circuits in harsh environments such as high pressure, high temperature, or high mechanical stress. However, it is proven that, except for Ag NP ink with photo paper substrates, the assembled circuits can be used in applications that requires flexibility.

34 6.2. Qualities of the developed SMTs

compared to

13% 28% 48% 19% 20% 23% 31% 34% 26%

100%

Not availableNot

Component bonding

strength

soldering on rigid PCBs

)

he SMTs developed in this thesis. in this developed he SMTs

90 60 70

440

240

- - - -

110 190

60 30 40

210

5100 high failure with rate 4060 high failure with rate 530 high failure with rate

25300 high failure with rate

1470 high failure with rate

- -

Contact Contact resistance

320

640

1390 1410

2200

ink ink

Ag NP

PI + Al foil PI + Al foil

Nonwoven + Al foil Nonwoven + Al foil

Material combination

board paper Ag+ flake

Bleached paper + Al foil Bleached paper + Al foil

Bleached paper + Cu foil Bleached paper + Cu foil Bleached paper + Cu foil

Photo paperPhoto +

Achieved contact resistance and component bonding strength of t of strength bonding component and resistance contact Achieved

Card

Table Table

ICA

ACA

LTSP

Cu + FR4

techniq

Manual soldering

Surface mounting

35 Chapter 6. Discussion

Using the LTSP and ICA as examples, it is seen that the same SMT did not achieve similar component bonding strengths on every type of circuit. In combination with the failure modes described in the included papers, it is possible that the lamination strength between different conductors and substrates plays a role in this. For example, the lamination between Cu foil and bleached paper is stronger than that between Al foil and nonwoven fabric. Using Cu- paper circuits as an example, the ACA achieved the highest bonding strength, which is due to the fact that the ACA has a larger adhesion area than other SMTs. The next strongest bonding strength was for LTSP, followed by the ICA. 6.2.3 Automatic component assembling Several developed SMTs show potential to be used in automated component assembly lines. A good example is shown in Papers III and IV where components can be mounted using an automatic chip bonder (for the ACA). It needs to pointed out that due to the limitation of the experimental equipment, some steps were performed manually, e.g. solder paste (or ICA) depositions and component placements. However, these steps can be easily upgraded to automated processes by using commercial screen printers and pick-and-place machines.

6.3 RFID reader antenna systems 6.3.1 Antenna performance when using conductive inks The influence of DC resistance on the Q-factor of loop antennas were presented in Section IV.D of Paper VIII. Several loop antennas are fabricated with identical geometries, but with different dry film thicknesses (Ag flake ink), which resulted in different DC resistances. Compared to a loop antenna fabricated on a rigid PCB with a Cu conductor, the performance of the Ag-ink- based loop antennas is significantly lower, which the Q factor of the loop antennas droped from 44.7 (0.34 Ω DC resistance in the loop antenna) to 6.0 (3.8 Ω DC resistance in the loop antenna). The Q factor could even drop further down to 1.5 for the loop antenna with 25.4 Ω DC resistance. 6.3.2 Antenna system limitations The following aspects are discussed with regards to the UHF RFID reader antenna system: - What is the limit of the area of the antenna system? - What is the maximum number of the microstrip antenna elements that can be integrated in the antenna system?

36 6.3. RFID reader antenna systems

The essence of these discussions is to, based on the characterizations listed in Chapter 5.3, theoretically calculate the limit of the SP4T switching network shown in Figure 5.2 in terms of area and number of antenna elements, which mathematical principle of the calculations is given in (6.1). 푠푒푛푠𝑖푡𝑖푣𝑖푡푦 푃푡푎𝑔 ≤ 푃푟,푡푎𝑔 = 푃𝑖푛 + 퐿푇푋푙𝑖푛푒훼푇푋푙𝑖푛푒 + 푁푡𝑖푒푟훼푀푈푋 + 훼2 (6.1)

where α2 is the attenuation (range dependent) between the input of a microstrip antenna element and the input of a tag’s antenna, obtained from Table 푠푒푛푠𝑖푡𝑖푣𝑖푡푦 9, Paper VI; 푃푟,푡푎𝑔 is the received power from the tag antenna; 푃푡푎𝑔 is the minimum required power of the tag to be functional. The maximum attenuation of the SP4T switching network can be reached by altering both the total transmission lines length (LTXline) and the network tier numbers (Ntier), so that an RFID tag can receive its minimum required power. Calculations aimed for two objectives were performed: maximum antenna system areas and maximum number of antenna elements. The details of the calculations are given in Section 5, Paper VI, with calculated results given in Table 11-12, Paper VI. The calculations showed that the UHF RFID reader antenna system can cover an area over 1000 m2 or monitor over 10000 microstrip antenna elements. In addition, if antenna systems with ultra large areas (over 1000 m2) are to be implemented, the attenuations in digital signals also need to be character- ized, even with metal foil conductors. There can be significant voltage drops for transmitting digital signals over long distances, and in this case, digital signal amplifiers or repeaters are needed. Although the materials used are flexible, the UHF RFID reader antenna system does not have the same characteristic. This is due to the antenna system was constructed in a double layer circuit where the materials are piled on top of each other and cannot stretch. In addition, the microstrip antenna elements are sensitive to bending because they have a relatively low bandwidth. Simi- lar conclusion can be made regarding the HF antenna system, which although the used SMT is able to resist bending, the loop antenna elements might suffer from shifts in antenna characteristics, e.g. loop inductance and resonant frequency. 6.3.3 Potential applications

6.3.3.1 Large-area RFID tag monitoring Because the microstrip antenna elements used in the UHF reader antenna system interact with RFID tags via electromagnetic waves, the UHF antenna sys-

37 Chapter 6. Discussion tem is capable of reading RFID tags up to 4 m away. Therefore, it is an advantage to use such types of antenna systems when monitoring RFID tags over a large volume, such as in a storage room. Such an antenna system can be deployed under the floor of a room with necessary protections. However, for the HF reader antenna system, due to its inductive coupling operating principle, its read range was limited to around 11 cm. Considering the fact that the HF antenna system has only one tier of MUXs, the antenna system read range would be further reduced if more than one MUX tier level were to be applied. Therefore, the HF antenna system can be used for monitoring RFID tags on a 2D surface within a few centimetres above the surface of the antenna system, such as a smart table. 6.3.3.2 RFID tag positioning A Cell of Origin RFID positioning principle [72] can be applied based on the measured RFID tag interrogation zones. From the results presented in Paper VI, it can be seen that the interrogation zones for the UHF reader antenna system were relatively large compared to the distance between the antenna elements. This caused to the interrogation zones formed by adjacent antenna elements to mostly overlap with each other, which led to low positioning accuracy when a tag could be read by multiple antenna elements instead of only one. Compared to the UHF reader antenna system, the measured interrogation zones for the HF reader antenna system were much more confined, e.g. circles with a radius less than 8 cm. Reasonable interrogation zones could be formed in the HF antenna system by adjusting the input power and the tags’ heights (see Section III.C, paper VIII). These interrogation zones eliminated RFID interrogation dead zones and did not cause large overlap areas. Despite the disadvantages such as the HF antenna system being unable to monitor tags at long ranges and the fact that it might suffer from conductor loss because of its Ag-ink-based conductors, the HF antenna system does have an advantage in RFID tag positioning accuracy. A non-conventional use of RFID reader antennas is shown in Paper VII, where a microstrip antenna element, used in the UHF antenna system, showed potential to perform classification and distance estimation on non- RFID tagged objects. However, it needs to be pointed out that the proposed concept requires RFID readers to be equipped with network analyser functionalities. This is a limiting factor for this concept to be widely used.

38 6.4. Social and ethical considerations

6.4 Social and ethical considerations 6.4.1 Environmental impact There are some designs in this thesis that use non-environmentally friendly and hazardous materials. Such materials include the soldering flux used for mounting components onto Al tracks, the screen-printed Ag flake ink, the inkjet-printed Ag NP ink, solvents such as acetone and methyl ethyl ketone, and any hazardous materials that are used during the material fabrication processes. In order to protect the environment, such materials must be used in laboratories that provide good ventilations and professional equipment, and the leftover materials and reaction residues must be disposed of properly. 6.4.2 Ethics when using RFID techniques The designs of the RFID reader antenna systems need to adapt to both global and local RFID standards and regulations. The fabricated antenna systems should not violate the data privacy of other users, e.g. reading and copying other users’ IDs. The antenna systems also cannot have effective radiated power greater than what is stated in the RFID standards. In addition, electromagnetic interferences with other electronic devices should be strictly limited. 6.4.3 Social benefit This thesis has provided a solution towards square metre-level large-area electronic devices. By using these non-conventional PCB materials, the system costs and time consumptions will significantly decrease. Such a solution can be applied to any electronic devices that require large areas.

7 Conclusion and Future Work

7.1 Conclusion This thesis has presented methods and guidelines for how to construct large- area electronic systems, comprising both digital and analogue signals, from non-conventional and flexible materials. The conclusions are made in accordance with the research questions identified in Chapter 3.

RQ 1: How to achieve low DC resistance in the conductor tracks when using conductive inks? What are the limits when conductive-ink-based tracks are used in high- current applications? Two methods were developed to obtain a relatively low DC resistance in conductive-ink-based tracks. One was to screen-print six layers of an Ag flake ink onto a paper substrate, where a thickness of 42 µm was achieved. The other method was to screen-print an Ag NP ink that achieved a significantly higher film thickness (3.5-8.7 µm depending on that substrate) than inkjet-printed Ag NP ink tracks (~ 1 µm). To further reduce the DC resistance, Cu and Al foils were also used. DC current carrying capacity tests were performed with the screen-printed Ag NP ink tracks. A maximum 2.2 - 3.5 A can be conducted on 5.0 mm wide tracks depending on the substrate material as well as achieved dry film thickness. Due to the non-negligible ink resistivity, the current carrying capacity of these tracks is obviously lower than metal foils of the same dimensions. From a materials perspective, several material combinations other than Cu- PI were used. Cu and Al foils and Ag-based conductive inks were used as conductor materials, while paper, nonwoven fabric, and PI were used as substrate materials. Both additive and subtractive manufacturing methods were applied.

RQ 2: How to assemble SMD components onto flexible circuits made from non-conventional materials? How to assess the performance and quality of developed SMTs? Can the SMTs be automated to assist large-volume productions? Novel SMTs were identified and developed, including using LTSP, ICA, and ACA to mount SMD components onto the fabricated flexible circuits. It was shown that SMTs using the ACA could be used in automatic assembly lines. SMTs based on ICA and LTSP can also be upgraded to automated component mounting processes so long as the necessary machinery is provided. System- atic characterisations were performed to examine the qualities of the SMTs,

41 Chapter 7. Conclusion and Future Work where a common conclusion is that these SMTs have obviously lower component bonding strength (6-48%) compared to rigid PCBs. On the other hand, the measured contact resistance were sufficiently low for most of the SMTs, and thus did not contribute significantly signal/power losses.

RQ 3: What are the limits for large-area electronic systems that are implemented from non-conventional materials? What are the performance limitations of high-frequency structures, e.g. antennas and microwave transmission lines, when constructed from non-conventional materials? As demonstrators, two RFID reader antenna systems that integrate both RF and digital signals were fabricated, which operated at respectively 13.56 and 867 MHz. The antenna that operates at 867 MHz was fabricated from Al foil, PI substrate, and a polyethylene foam dielectric. It was arranged as an ex- pandable three-tier SP4T switching network with eight microstrip antennas as loads, which had an area of 1.2 m × 0.6 m. Based on the characterizations made for the antenna systems, a large number of calculations were made to study the limitations of the antenna system. The calculations showed that the proposed methodology provides the potential to further expand the demon- strated configuration to cover areas over 1,000 m2 or up to 64 antenna elements in a confined area. The antenna system is suitable for applications such as large-area RFID tag monitoring. The antenna system that operates at 13.56 MHz used screen-printed Ag flake ink as the conductor and photo paper as the substrate. It was arranged into a one-tier SP4T MUX with four loop antenna elements as loads. The characterisations of the antenna system showed that the antenna system had more confined and accurate RFID interrogation zones than the UHF antenna system, which suggests that it is suitable to be used in applications such as RFID tag positioning. A parametrical study was carried out to study the effect of loop antenna DC resistance. The study suggested that the Q-factor of screen- printed antenna elements is significantly lower than bulk metal antenna elements due to the DC resistance.

7.2 Future work In future work, the following investigations, among others, should be conducted if similar materials are to be widely used: In order to enable multi-layer circuits from the chosen materials and combinations, novel methods to implement VIAs need to be addressed in future works. Multi-layer circuits can significantly increase the complexity of circuits

42 7.3. Authors’ contributions

within the same area, where VIAs plays an important role. The VIAs should be implemented in an automated or semi-automated way for the sake of efficiency. In addition, similar to SMTs, the VIAs should have sufficiently low resistance and adequate mechanical strength. Improvements to the presented SMTs need to be made in order to enable the use of the components in other packages, e.g. ball grid array and thin quad flat no-lead. Together with the VIAs, the application of multiple component packages would help the material combinations presented in this these to compete with Cu-PI flexible circuits and rigid PCBs.

7.3 Authors’ contributions The list of authors and authors’ contributions to each paper in Part B are presented in Table 7.1 and Table 7.2.

43 Chapter 7. Conclusion and Future Work

Table 7.1 List of authors Paper XL1 JS2 HA3 TS4 VS5 LG6 RÖ7 JE8 TG9 AS10 I M11 C12 C C II C C M C C III M C C C C C C IV M C C C C C C V M C C VI M C C VII M C C VIII M C C C

1 Xiaotian Li, 2 Johan Sidén, 3 Henrik Andersson, 4 Thomas Schön, 5 Vincent Skerved, 6 Linnea Gyllner, 7 Robert Öhman, 8 Jeannette Eriksson, 9 Tove Genchel, 10 Anurak Sawatdee 11 Main author, 12 Co-author

Table 7.2 Authors’ contributions

Paper Author’s contribution

XL: Main author, idea, experiment, article writing I HA and JS: Idea, article writing and supervisions HA: Main author, experiments and results JS: Writing and results II VS: NFC tags design and software XL: NFC tags design LG: Experiments and results

XL: Main author, idea, experiment, article writing HA and JS: Idea article writing and supervisions III RÖ, JE and TG: Flexible circuit manufacture AS: ACA automatic component mounting

IV Same as Paper III

XL: Main author, idea, experiment, article writing V HA and JS: Idea, article writing and supervisions XL: Main author, implementation, characterisation, article writing VI JS and HA: Supervision, article writing, and idea

VII Same as Paper VI XL: Main author, experiments, analysis, and idea VIII JS and HA: Supervision, revision, and idea TS: Co-author, technical support, manufacturing

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Part B