Manuel Seckel

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 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.

Large area processes for 3D shaped electronics

Conformable electronic systems consisting of laterally distributed electronic components (typically sensors, actuators or LEDs) have attracted considerable interest during the last years. By using different technology approaches considerable elasticity, repeated stretchability and conformability of such systems has been shown by research groups. Contrary to the expression of interest by many potential industrial manufactures of stretchable electronics, the adaptation of related technologies into fabrication environments is lagging. Among the reasons for the reluctance with respect to industrialization are concerns with respect to material used in deformable electronics (silicones), reliability issues (repeated stretchability), and (initial) cost. In this paper an approach for “single cycle deformable” - electronic systems and some of its application scenarios will be presented. Based on a previously developed technology for stretchable electronics using thermoplastic polyurethane as the matrix material, an approach for three dimensionally shaped electronic systems was developed. In order to fabricate a stable self-supported structure the stretchable system is attached to a thermoplastic polymer sheet (typically polycarbonate) with a thickness between 200 and 800 μm prior to being 3D-deformed by thermo-forming. Potential applications are any kind of products (consumer electronics, automotive, household appliances), where a need to integrate sensors and actuators into ergonomically or aesthetically 3D-shaped surfaces is identified. The fabrication of deformable electronics is a process fully compatible with a typical printed circuit board manufacturing and electronics assembly line. Also the used materials are well compliant with wet-chemical processes used during the processing. Rather complex electronic systems with a number of components like distributed sensors or actuators can be assembled in a conventional way on a flat electronic panel. The low temperature solder SnBi is used for the electronics assembly. Mounted components are typically fixed additionally by an underfiller, so that during the thermoforming process when the solder eventually melts components are not released from the contact pads. The stretchable electronics is subsequently fixed to a stiff thermoplasitc support sheet prior to being 3D-deformed by a thermoforming process.

Stretchable and deformable electronic systems in thermoplastic matrix materials

Deformable of electronic systems consisting of laterally distributed electronic components (typically sensors, actuators or LEDs) has attracted considerable attention during the last decade. By using different technology approaches considerable elasticity, repeated stretchability and conformability of such systems has been shown by number of research groups. Contrary to the expression of interest by many potential industrial manufactures of stretchable electronics, the adaptation of related technologies into industrial fabrication environments is lagging. Among the reasons for the reluctance to industrialize are concerns with respect to material used in deformable electronics (silicones), reliability issues (repeated stretchability), and (initial) cost. In this paper an approach for “single cycle deformable”-electronic systems and some of its application scenarios will be presented, which is very close to established printed circuit board technologies. Based on a previously developed technology for repeatedly stretchable electronics -using thermoplastic polyurethane as the matrix material, see figure 1- an approach for three dimensionally deformable electronic systems is formed. In order to fabricate a stable self-supported structure the stretchable system is attached to a thermoplastic polymer sheet (typically polycarbonate or a polycarbonate/Acrylonitrile butadiene styrene blend) with a thickness between 200 and 800 μm prior to being 3D-deformed by thermo-compression. Potential applications are any kind of products (consumer electronics, automotive, household appliances), where a need to integrate sensors and actuators into ergonomically or aesthetically 3D-shaped surfaces is identified. The fabrication of deformable electronics is a process fully compatible with a typical printed circuit board manufacturing and electronics assembly line. Also the used materials are well compliant with wet chemical processes used during the processing. Rather complex electronic systems with a number of components like distributed sensors or actuators can be assembled in a conventional way on a flat electronic panel. The low temperature solder SnBi is used for the electronics assembly. Mounted components are typically fixed additionally by a under filler, so that during the thermoforming process when the solder eventually melts components are not released from the contact pads. The stretchable electronics is subsequently fixed to a stiff thermoplasitc support sheet prior to being 3D-deformed by a thermoforming process. Manufacturing aspects and results will be discussed in the presentation.