Thomas Löher

Cyclic endurance reliability of stretchable electronic substrates

Abstract Stretchable electronic circuit boards have been developed based on three different technologies. Such substrates serve to connect rigid interposers or electronic components. The conducting traces have a meandering shape and consist of Cu-foil or screen-printed Ag-paste. These conducting traces are attached to or embedded in polyurethane, polydimethylsiloxane, or breathable non-woven stretchable substrate material. The long-term endurance behavior of this novel type of boards is studied by cyclic elongation at strain ranges of up to 20% and monitoring the electrical connectivity. The main failure mode in the Cu-foil based technologies is fatigue of the conducting traces and can be described in terms of the Manson–Coffin relation. Indications for high-cycle fatigue were found. The screen-printed conductors on non-woven substrates fail by breaking of the connection between the metallic grains. The application areas are electronic monitoring systems that need to be placed directly on the skin, or conformable systems for curved surfaces.

Stretchable electronic systems

The variety and applications of electronic systems have undergone a considerable evolution during the past century. Paramount milestones have been the introduction of the printed wiring board, which allowed for a rationalized assembly of electronic systems and the fabrication of integrated circuits with still ongoing increase of functional density due to miniaturization. Additionally sensors, micro electro mechanical systems, displays, and light emitting diodes have greatly added application potentials to electronic systems. Miniaturization is still one of the drivers for development in electronic components. On the other hand, however, conformity of the electronic systems to some give shape or space is increasingly requested for future applications. At present a great number of electronic appliances e.g. in mobile communication and medical electronics are manufactured to be foldable by using flexible printed wiring boards for assembly. The flexible printed wiring board restricts the bending to be only around on axis at a time. For a conformity to a truly three dimensional shape the wiring substrate needs to be stretchable to a certain extend. The development of stretchable wiring boards is the aim of the European project STELLA (stretchable electronics for large area applications). In the present paper process technologies for the efficient fabrication of stretchable wiring substrates using printing technology and/or photo structuring will be described. The conductive wiring which has to be as stretchable as the substrate matrix is designed as a meander and was optimized using FEM modelling. The electric contact formation is effected either by soldering techniques using low melting solder alloys or with anisotropic conductive adhesive. The interface between components assembled on the stretchable board and the wiring is mechanically the most fragile point of the wiring. Approaches for the reinforcement of the vicinity of the assembled components and first results are presented.

Stretchable Circuit Board Technology and Application

An innovative technology for the mass production ofstretchable printed circuit boards (SCBs) will bepresented in this paper. This technology makes itpossible for the first time to really integrate fine pitch,high performance electronic circuits easily into textilesand so may be the building block for a totally newgeneration of wearable electronic systems. Anoverview of the technology will be given andsubsequently a real system using SCB technology ispresented.

Stretchable electronic systems for wearable and textile applications

Assembly of electronic components on rigid and/or flexible printed circuit boards is today the customary way to fabricate electronic systems in stationary, mobile and automotive applications. On the other hand, many of the demands from emerging application fields like wearable and textile electronics cannot be met if with standard technologies. These fields have therefore become mayor drivers for the development of novel technologies. Among these dasiastretchable electronicspsila have attracted much attention recently. Especially for textile applications the potential of the electronic system to comply with the body shape and movement will considerably improve the user comfort. In this paper we will present a cost effective technology for the realization of stretchable systems by common printed circuit board techniques like lamination, lithography, etching and micro via technology with polyurethane as a stretchable matrix/substrate material. Mastering of the adhesion between materials and the transitions region from stretchable to non-stretchable parts of the system are crucial for the mechanical performance and robustness. Technical approaches and the obtained results to tackle these issues will be presented. After a complete embedding of the components/interconnections the systems can be firmly attached to textile or non-woven cloth, which can be subsequently integrated into garments. The described process technology bears the potential for large scale roll to roll processing. Reliability aspects for stretchable electronic systems are so far not standardized and will be discussed briefly. Electrical and mechanical functionality of test vehicles subjected to multiple stretch and mild washing cycles will be presented. A functional electronic demonstrator with embedded passives, a micro controller, and LEDs which was realized with this technology will be shown.

Stretchable electronic systems: Realization and applications

Commonplace electronic appliances for consumer or industrial use are still mostly rigid or at maximum flexible entities. The flexibility of foldable units like laptops or cell phones is usually realized through flexible circuit board (FCB) interconnectors. Although flexibility allows for considerably enhanced degrees of freedom in design, it is not compatible with more complex three dimensional curvatures and dynamics thereof. In the past years a number or approaches to realize stretchable electronic circuits in order to reach beyond unidirectional bending or folding of electronics have been reported. In the frame of the European Project STELLA a particular fabrication technology for stretchable electronic systems has been developed at Technische Universitaet Berlin. This technology, termed ?stretchable circuit board? (SCB) technology, is derived from conventional printed circuit board manufacturing. Stretchability of the boards is enabled by (i) using polyurethane instead of FR4 or polyimide as a carrier material of the copper structures and (ii) a meandering design of the Cu interconnects between commercial (rigid) electronic components. Such boards can be (once) extended by up to 300% before fracture of the Cu interconnections. For repeated elongation/relaxation cycles elongations with a few percent are allowable in order reach high cycle numbers. Electronic components are assembled after local application of a solder mask and surface finish for solderability. The electronic interconnection is established using a low temperature solder alloy (SnBi, Tm=142?C). For protection and enhanced system robustness all components are subsequently encapsulated within a polyurethane capping. Systems thus realized can be readily attached to different kinds of surfaces. Most interesting for various application cases is the easy attachment to textile substrates by a simple lamination process. The field use case studies of stretchable systems in the frame of the STELLA are mostly sensor applications in the field of medical electronics like a breathing frequency monitor for babies, a shoe insole pressure sensor for diabetes patients, or a band aid inlay to measure pressure and humidity of an acute wound when pressure therapy is applied. The latter application will be described in more detail since different aspects of bio-medical applications can be explained with this example. Another emerging field of applications is textile electronics, where it has been proven, that stretchable electronics can serve a versatile building blocks for complex electronic systems integrated in textiles.

Highly integrated flexible electronic Circuits and Modules

Within the electronic circuit board industry flexible circuit still cover a small the market share, however, with the fastest growth rate. The technology is increasingly used in automotives and aerospace, in handheld mobile appliances and many medical devices like pace makers or hearing aids [1,2]. Over past years a European consortium of research institutes and industry has explored the future technological potential of flexible printed circuits in the framework of the project SHIFT. One aspect was to investigate the frontiers of flexible circuit fabrication with respect to minimum feasible line width and pitch using different manufacturing methods. Still further beyond today mainstream flex fabrication technologies were the developments to integrate active and passive components into the buildup layers of flex circuits. In this way extremely high integration of electronic systems and highest functional densities can potentially be realized. Techniques and results of these developments will be presented in this paper. Embedded components in order to comply with the thin buildups of flexible circuits should be very thin as well. To this aim components were be mechanically thinned to 20 ptm. A dicing by grinding technique was applied using etched separation grooves on the wafer. Two technologies for embedding of ultra thin components were developed. The first one is thin flip chip assembly on inner layers of the flex and embedding by subsequent lamination of build up layers. The gap between chip and substrate was in the order of a few microns using either low profile solder or anisotropic adhesive.

Stretchable electronics manufacturing and application

In the recent years the fabrication of electronic systems that can to a certain extent be stretched has attracted increasing attention. As general motivation such electronics will in contrast to conventional electronics be compliant with free form shapes, as for example the human body surface. A number of different successful approaches to realize such systems have been demonstrated. Thus far various aspects of stretchable electronics have been addressed ranging from stretchable Si detector arrays to large scale, low cost stretchable systems with integrated commercial components. Within the EU project STELLA the latter types of fabrication technologies have been developed. These technologies are presently further developed with focus on optical applications in the project PLACE-it. The rationale of both projects is the use of conventional printed circuit board technologies for the fabrication of stretchable wiring substrates onto which components are assembled and embedded. Stretchability is accounted for by a meandering layout of the Cu-interconnects between components and the rubber like polymeric substrate polyurethane, instead of polyimide which is used in conventional flex-prints. In the present paper basic features of the fabrication process for stretchable substrates will be discussed. Selected materials, process parameters and design aspects will be addressed with respect to selected target applications. Reliability requirements of stretchable systems are very much dependent on the application scenario. Besides obvious stretch-to-failure, cyclic stretching and bending tests, also robustness with respect to cleaning and washing is required. Comprehensive reliability standards are yet to be defined. In the present paper a brief overview of reliability assessment will be given. A few of the realized applications using stretchable electronics will be presented and discussed. The versatility of the fabrication technology and its products will be emphasized. The potential impact on different application fields will be highlighted, as for example electronic integration into textiles and in automotive applications.

Manufacturing Concepts for Stretchable Electronic Systems

Assembly of electronic components on rigid and/or flexible printed circuit boards is today the customary way to fabricate electronic systems in stationary, mobile and automotive applications. On the other hand, many of the demands from emerging application fields like wearable and textile electronics cannot be met if with standard technologies. These fields have therefore become mayor drivers for the development of novel technologies. Among these stretchable electronics have attracted much attention recently. Especially for textile applications the potential of the electronic system to comply with the body shape and movement will considerably improve the user comfort. This paper will present a cost effective technology for the realization of stretchable systems by common printed circuit board techniques with polyurethane as a stretchable matrix material. Mastering of the adhesion between materials and the transitions region from stretchable to non-stretchable parts of the system are crucial for the mechanical performance and robustness. Technical approaches and the obtained results to tackle these issues will be presented. The described process technology bears the potential for large scale roll to roll processing. Reliability aspects for stretchable electronic systems are so far not standardized and will be discussed briefly. Electrical and mechanical functionality of test vehicles subjected to multiple stretch and mild washing cycles will be presented. A functional electronic demonstrator with embedded passives, a micro controller, and LEDs which was realized with this technology will be shown.

Stretchable circuit board technology in textile applications

Todays electronic systems are based on an assembly of components onto rigid or flexible substrates, serving perfectly the needs of traditional product fields like automotive, computing or industry electronics. On the other hand, many of the demands from emerging applications like wearable and textile electronics cannot be met if standard technologies are used for their realization. These new fields have therefore become mayor drivers for the development of novel technologies. Among these `stretchable electronics have attracted strong attention. Especially for textile applications the potential of electronic systems to comply with the body shape and movement will improve the user comfort dramatically. A manufacturing technology for the realization of stretchable systems by common printed circuit board techniques, based on polyurethane as a stretchable matrix material has been developed. The stretchable circuit board technology has been used to realize a number of textile applications. As an example the realization of a fashion dress with integrated high brightness LEDs and movement sensing will be described.

Compact power electronic modules realized by PCB embedding technology

Power electronics packaging with a high level of compactness, robustness and versatility has recently become the focus of many technology development projects. In Europe main drivers for such efforts are e-mobility and green energy harvesting, which are promoted by European Commission and national authorities. One promising approach to realize a substantial level of size reduction and process flexibility in the packaging of power electronic systems is the embedding into the build-up layers of printed circuit boards. Conventional printed circuit board embedding -for low power applications- has meanwhile reached a considerable level of maturity and the number of products shipped per month are in the tens of millions. Using embedding technology for power electronic devices, however, is still challenging in many aspects. First of all for power applications the embedded system layout has to account for high currents and/or voltages and at the same time has to provide means to facilitate the power dissipation from the embedded components. PCB-substrates and modules therefore contain conductor traces with large cross sections and/or massive copper and ceramic structures in order to enable the required heat spreading. Such constructions are composed of materials with different CTEs. As a result considerable stresses prevails in the build-up layers. Since contrary to conventional PCBs the build-up of power modules are non-symmetrical a careful layout of embedded modules is necessary in order to avoid warpage of the package. Embedded power electronic components are semiconductor dies on the basis of silicon, silicon carbide and gallium nitride. The full area silver on the drain contact is compliant with the embedding approach. The gate and source contacts have to be reinforced by ten to fifteen micrometers copper. The required robustness and conductance of the electrical contact between chip and embedded substrate is achieved by back contact sintering using silver nanoparticles. Chips are mounted in a two-step process: they are placed in a high accuracy die bonder and pre-bonded, subsequently the contacts are sintered in a stack-lamination press at higher temperature and pressures. Front contacts between chips and the subsequent wiring layer are established by micro-via technology. During the embedding process power components are enclosed into a matrix of glass fabric and epoxy resin (prepreg). The choice of materials strongly depends on the foreseen use case. Subsequent definition of the conductor lines is an established printed circuit board process. A large variety of embedded power electronic modules has been realized so far. Size and performance of the systems differ accordingly: from modules with lateral dimensions of a few square millimeters containing two embedded components for low voltage and currents of up to 10 Amperes, to complex assemblies with 12 embedded semiconductors and a module area of several square decimeters for an operating Voltage of 600 V and a total power of up to 50 kW. The present paper will focus on recent developments in power electronic embedding using PCB technologies.