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In the past 15 years, stretchable electronic circuits have emerged as a new technology in the domain of assembly, interconnections, and sensor circuit technologies. In the meantime, a wide variety of processes using many different materials have been explored in this new field. In the current contribution, we present an approach inspired by conventional rigid and flexible printed circuit board (PCB) technology. Similar to PCBs, standard packaged, rigid components are assembled on copper contact pads using lead-free solder reflow processes. Stretchability is obtained by shaping the copper tracks as horseshoe-shaped meanders. Elastic materials, predominantly polydimethylsiloxanes, are used to embed the conductors and the components, thus serving as a circuit carrier. We describe mechanical modeling, aimed at optimizing the build-up toward maximum mechanical reliability of the structures. Details on the production process, reliability assessment, and a number of functional demonstrators are described.

Ultra-Thin Chip Packaging (UTCP) technology enables the integration of off-the-shelf integrated circuits into thin substrates such as different kinds of circuit boards. This technology has been created at CMST during many years of intensive study. In this technology the thinned chip is embedded into polyimide and a copper fan-out is created to form the connections between the chip and the circuit board. This technology is compatible with standard circuit board manufacturing technology, which makes it an attractive solution to create miniaturized packages for electronics. In this paper the reliability of UTCP structure was studied using adhesion peel testing. The different solutions for constructing the material layers inside a UTCP were compared for their adhesion. Special test structures were manufactured for testing. The tested material layers consisted of polyimide, benzocyclobutene (BCB) and adhesion promoter for BCB. Altogether four different constructions were tested and the peel testing was done to half of the samples right after manufacturing and to the other half after exposure in 85/85 –environment for 500h. The results show difference in adhesion after exposure to humidity. Test structures with BCB suffer from loss of adhesion in PI – BCB interface after exposure to moisture. This result addresses that the structure is more reliable when the amount of different materials and interfaces is decreased.

It was announced by the EWEA (European wind energy association) that key areas for wind energy research should focus on : - Increasing the reliability, accessibility and efficiency of wind turbines - Optimizing the maintenance and installation of offshore turbines and their substructures - Improving the design and layout of wind farms - Demonstrating large wind turbine prototypes and large, interconnected offshore wind farms - New methods of grid management to allow high levels of wind power in the system - Expansion of education schemes and better training facilities We believe that the envisaged results of the SBO-project the ‘self sensing composites’ (2013 – 2016, IWT-contract 120024), we can offer added value in the first two key areas summarized above. Reliability and efficiency increase can only be reached when a thorough knowledge exists of the fabrication process together with redesigns of currently existing wind turbine blades using their online loading information. Optimizing maintenance planning and maintenance procedures should definitely be supported by online measurement techniques which are able to determine the integrity of the blade. If a life time extension is possible by having the knowledge through in-situ monitoring this will definitely increase the efficiency of the blades in terms of investment cost. In this scenario, the wind energy industry could switch from reactive maintenance (run-to-failure) and preventive maintenance (following the wind turbine manufacturer’s manual) to maintenance. In this presentation, the current progress in the SSC-project on the online monitoring of the cure degree of composites during production is shown. Two strategies are followed, firstly, the embedding of a deformable network of capacitances in the composite material, which can later on be used to sense the ageing effects of composites in operation, and secondly, the installation of a network of capacitances in the mould of the composite part. However, measuring only the cure-degree during the production cycle is however not sufficient to fully define the initial state of a composite part, and therefore these sensor networks will be combined with measurements using embedded optical fibres. Defining the onset of residual strain build-up is in this matter seen as a key factor. At the end of the presentation, some applications in which sensor networks are becoming reality are shown.

This contribution provides an overview of CMST's technologies on flexible ultrathin chip packaging and elastic electronic and sensor circuit technologies and applications.

Conventionally electronics circuits are produced by assembly of packaged components on flat, rigid printed circuit boards. However there is a growing demand for 2.5 dimensional (2.5D free form surfaces) or even full 3D electronics. Applications for such circuits are found in a vast range of fields: such circuits are desirable for comfort and ergonomic reasons (e.g. in wearable or implantable circuits), to respond to design and aesthetics considerations (e.g. 2.5D or 3D light sources), for ecological reasons (more efficient materials usage, less CO2 emissions in automotive applications) etc. In this contribution we will present two technologies, under development at our laboratory, which open the way to industrial production of randomly deformable circuits. These technologies are : • The ultra-thin chip package (UTCP) technology, by which bare Si (or other anorganic semiconductor based) chips are thinned down to a thickness of 20-30µm, embedded in a stack of spin-on polyimide layers, and provided with a fan-out metallization, resulting in an extremely miniaturized, lightweight and flexible chip package with a total thickness below 100µm • A number of technologies for dynamically stretchable (i.e. elastic) circuits, which are based on the interconnection of individual components or component islands with meander shaped thin-film or Printed Circuit Board (PCB) based metal wirings and embedding in elastic polymers like PDMS (silicone rubbers) or PU (polyurethanes). Process flow details, reliability data and applications for these technologies will be presented.

Thinning down ICs is a well-known approach to reduce the volume of chip packages. In this work ICs are thinned down to 30um, followed by a package procedure in polyimide with copper fan out, which allows their embedding in adhesives used for laminating flexible printed circuit boards (PCBs). In this way the chip does not consume PCB area, hence other circuit components can be assembled on top or at the bottom of the chip, enabling extreme circuit miniaturization. Furthermore, our ultra-thin chip package (UTCP) is highly flexible, enabling flexible electronic circuits without large rigid chip packages. Spin-on photo-definable polyimide precursors are used to build an interposer which can be embedded later in the flexible PCB. The chip is fixed in between three polyimide layers using BCB as adhesive. The central polyimide layer forms a cavity for the chip, the top layer of polyimide is exposed and developed to fabricate vias contacting the chip. An 8um thick copper layer is deposited and patterned using lithography and etching to form the fan-out, essential to match the fine IC pitch to the larger PCB pitch. The final chip package is about 75um thick, and is easily embedded using only small adaptations of the standard flexible PCB fabrication process. Last year, both the UTCP concept and the embedding in a flexible PCB were optimized in order to obtain a very high yield. Three types of chips were UTCP-packaged and embedded in a flexible PCB: two types of microcontrollers (MSP430F1611 and a proprietary digital signal processor) and an RF-chip. The yield of the tested UTCPs ranges in between 65% (proprietary IC) and 85% (MSP430F1611). The performance of the RF-chips can only be tested after embedding in a flexible substrate. Although the testing is still ongoing, 95% of the embedded UTCPs are fully functional after embedding.

A novel method for fabricating multimode PDMS waveguides is presented. This process is based on a hot-embossing technique and generates high quality optical waveguides without a substantial residual layer after embossing. Furthermore, the process allows for low-cost fabrication since it relies on a replication technique and additionally only commercially available materials are used. The measured propagation loss is smaller than 0.24dB/cm and can be further reduced by improving the master mould quality.

This paper describes the HOP system. It consists of a wireless module built up by multiple nodes and a base station. The nodes detect acceleration of e.g. human movement. At a rate of 100 Hertz the base station collects the acceleration samples. The data can be acquired in real-time software like Pure Data and Max/MSP. The data can be used to analyze and/or sonify movement.

The Musical Synchrotron is a software interface that connects wireless motion sensors to a real-time interactive environment (Pure Data, Max/MSP). In addition to the measurement of movement, the system provides audio playback and visual feedback. The Musical Synchrotron outputs a score with the degree in which synchronization with the presented music is successful. The interface has been used to measure how people move in response to music. The system was used for experiments at public events.