Rolf Aschenbrenner

Fully untegrated EKG shirt based on embroidered electrical interconnections with conductive yarn and miniaturized flexible electronics

A T-shirt has been developed that measures an EKG signal. This work is different to other research in the field as it focuses more on advanced interconnection and integration technologies for electronics in textiles rather than on the EKG shirt as functionality. It is the first application using an interconnection technology based on embroidery of conductive yarn that has been developed recently and published in T. Linz et al. (2005)

New assembly technologies for textile transponder systems

Passive transponder labels are available in a variety of types mainly for logistic purposes which require higher functionality and/or higher reliability than the conventional barcodes allow. Smart labels for textiles either for production, logistics or professional laundry have to meet different requirements from these applications. They have to be ultrathin and very reliable under conditions unusual for electronics (washing, high pressure,. . .). Therefore the realization of textile transponders has become possible only recently with the development of thin silicon chips. Such chips are flexible and can overcome high mechanical load. They can he easily encapsulated and can resist the extreme conditions of a washing process. In order to achieve high readout distances (0.5 ... 2 m) the antennas have to enclose an area of > 6 an2, The reliability of flexible substrates in this size during washing procedures is not suficient. Therefore textile based antennas have to be produced. The transponder ICs have to he assembled to conductive yarn. The module also needs encapsulation to protect the IC from humidity, chemicals etc.. This requires new technologies especially as the dimensions and tolerances between microelectronic components and textiles differ by orders of magnitude. The development of these technologies will be the hasis for the realization of textile based wearable electronic assistants which will be a key component of future ubiquitous computing. This paper presents a concept to realize textile based transponders, the electrical interconnections to conductive yarn and first experimental results.

Design, simulation and technological realization of a reliable packaging concept for high power laser bars

We introduce a new packaging concept for high power laser bars using Au(80)Sn(20)-solder on different heatsinks e.g. CVD-diamond and CuW. First the optimal thermal concept and design with the lowest possible thermal resistance is described. The thermal simulations were done by an inhouse made computer program based on FFT (Fast-Fourier-Transformation). The results of the 3D- and 2D-FEM-simulations for the thermomechanical design show a solution with low bonding stress. Design rules based on the discussion of these results including the technological limitations are described, which were used to define different alternative packaging concepts. The technological realization of these concepts is described and presented. The electro-optical characterization of such bonded laser bars gave first indications about maximum power, thermal resistance, bonding stress and reliability.

Reliability study of flip chip on FR4 interconnections with ACA

This report presents the results of the evaluation of anisotropic conductive adhesives for flip chip applications. Samples consist of bumped test chips mounted on rigid substrates with pitches down to 150 /spl mu/m. The dies were prepared with different bumps-electroless Ni, mechanical and electroplated Au. Several commercially available adhesives were selected to be investigated in this study. A detailed thermo-mechanical analysis was used to characterize the materials according to their physical properties. This kind of analysis method has also been used to optimize the curing profile, i.e. to shorten the cure time. The influence of process parameters like pre-baking, cure temperature and heating ramps on the properties and appearance of the adhesive layer has been investigated. We found also a strong influence of the bump metallurgy and shape onto the reliability of the electrical interconnection. The reliability evaluation was performed with special regard to the degradation of the adhesives and weakening of the adhesive joints. The electrical and mechanical performance of the adhesive bonds were studied by evaluating initial contact resistance and mechanical adhesion as a function of temperature and humidity. The reliability results of Flip Chip interconnections on FR4 were compared to those on flexible substrates obtained in former studies.

Reliability Potential Of Epoxy Based Encapsulants For Automotive Applications

Automotive under-the-hood electronics often have to withstand temperatures up to 175 °C in combination with harsh environment conditions. This paper gives an overview about the reliability potential of epoxy based encapsulants for automotive applications. Therefore the resistance of epoxy molding compound against typical automotive fluids at temperature of use is analyzed. Six epoxy molding compounds available on the market, showing high temperature automotive potential have been carefully selected to undergo a media resistance testing. For this purpose a dedicated mold tool has been designed and manufactured to prepare these encapsulants for material testing. Thermo-mechanical, mechanical and thermal properties had been determined in initial state directly after molding and after storage in aggressive fluids typical for automotive applications as e.g. gas oil, automatic transmission fluid (ATF), brake fluid or accumulator acid at the respective temperature of use.

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.

Current loadability of ICA for flip chip applications

Miniaturization is a key issue to achieve advanced performance of electronic devices and to decrease the overall cost of an electronic package. In this respect the flip chip technology provides excellent capabilities to meet the demands of recent and future products. Although solder joining is still the most common technology a great deal of research work has been conducted on conductive adhesives. They are successfully used as die attach, display and hybrid assembly. Progress with its application to flip chip devices has still to be made. Recently, the current carrying capability of isotropic conductive adhesive (ICA) has been a matter of great interest. Predominantly, corresponding characteristics of anisotropic conductive adhesives (ACAs) have been studied by several groups. Though interest, mainly on part of the automotive industry is shown, little is known about ICAs high current behaviour. This paper is part of a broad study focused on the current capabilities of isotropic conductive adhesives. Tests have been carried out on flip chip assemblies. The contact resistivities depending on different DC loads and increased environmental temperatures have been monitored. In-situ measurements have been carried out with an automated measuring system. TGA measurements were made to find out about degradation temperatures of the used materials. The results contribute to the following discussion about the failure mechanisms due to high current loads.The first part of the study focuses on fundamental investigations performed on printed ICA test structures (epoxy vs. thermoplastic resin). Measurings of the conductivity during cure have been made. The results have been related with DSC/TMA data. As well electrical performance of the cured adhesives under elevated temperature/humidity and high current loads have been determined. In the second part of the study first results obtained from high current tests on Flip Chip packages are presented. Limitations and mechanisms of failure are discussed.

Stackable packages with integrated components

A technology for the integration of thin chips into build-up layers of organic substrates is under development. In order to improve the process, laser technology has been introduced for via drilling and interconnect structuring. Basic reliability tests of embedded chips were performed. For the realization of integrated resistors, the deposition of ultra-thin electroless Ni layers is used. Also here, laser ablation has been implemented for structuring. Finally, an improved and simplified concept for the realization of stackable chip packages is presented.

Ultrathin assemblies on flexible substrates

Miniaturization enforcement of electronic modules in complex as well as low cost applications is the driving force to integrate flip chip technology in common surface mount device processes. The use of flexible substrates enables a large variety of geometric possibilities including folding and bending. On the one hand there are numerous low cost applications for this technology such as smart cards and smart labels, on the other hand flexible substrates offer a wide potential for highly complex folded packages and 3D modules. Thinned silicon chips with subsequent very thin bumps mounted on flexible substrates open up new dimensions in packaging technologies. They can be integrated even in thin products, e.g. documents or stacked to low profile 3D modules. Beside the geometrical aspects the parasitic influences of silicon material can be eliminated by thinning of the wafers. This advantage is of great interest for applications in the GHz-area for example for GPS modules

Ultrathin soldered flip chip interconnections on flexible substrates

Flip chip assembly of silicon ICs on flexible substrates has gained more interest in the last years. On the one hand there are numerous low cost applications for this technology such as smart cards and smart labels, on the other hand flexible substrates offer a wide potential for highly complex folded packages and 3D modules. Using conventional flip chip assembly processes the stand-off is too high compared to the dimensions of chip and substrate. Until now solder application by stencil printing is only used successfully down to 150 /spl mu/m pitch with bump heights larger than 70 /spl mu/m due to the solder paste and stencil features. Deposition technologies for smaller solder volumes together with new assembly processes have to be developed and qualified to realize suitable contact heights. With the immersion soldering process a cost effective maskless bumping process for thin solder layers has been developed. Thermode bonding has been investigated as a promising fast flip chip technology for thin soldered contacts on flexible substrates. In this paper the process development for two different solder materials in combination with noflow underfiller materials will be presented for flip chip contacts of less than 10 /spl mu/m height. The reliability of thin solder joints is a key issue. Therefore, the failure mechanisms and the ageing behaviour were studied. The intermetallic phase formation has a larger influence because the intermetallics consume the majority of the solder alloy. The effect of intermetallic growth for the different solder materials as well as the impact of the small stand-off during ageing are investigated in this paper. Special emphasis is put on failures resulting from the joint geometry of ultrathin contacts. The promising results of a reliability test program consisting of thermal cycling, temperature/humidity testing and multiple reflow tests are discussed.