Jan Vanfleteren

Design of metal interconnects for stretchable electronic circuits

In this work, the design of flexible and stretchable interconnections is presented. These interconnections are done by embedding sinuous electroplated metallic wires in a stretchable substrate material. A silicone material was chosen as substrate because of its low stiffness and high elongation before break. Common metal conductors used in the electronic industry have very limited elastic ranges; therefore a metallization design is crucial to allow stretchability of the conductors going up to 100%. Different configurations were simulated and compared among them and based on these results, a horseshoe like shape was suggested. This design allows a large deformation with the minimum stress concentration. Moreover, the damage in the metal is significantly reduced by applying narrow metallization schemes. In this way, each conductor track has been split in four parallel lines of 15 mum and 15 mum space in order to improve the mechanical performance without limiting the electrical characteristics. Compared with the single copper or gold trace, the calculated stress was reduced up to 10 times.

Design and Fabrication of Elastic Interconnections for Stretchable Electronic Circuits

For biomedical and textile applications, the comfort of the user will be enhanced if the electronic circuits are not only flexible but also elastic. This letter reveals a simple moulded-interconnect-device technology for the construction of elastic point-to-point interconnections, based on 2-D spring-shaped metallic tracks, which are embedded in a highly elastic silicone film. Metal interconnections of 3-cm long were constructed with an initial resistance of about 3Omega , which did not significantly increase (<5%) when stretched. A stretchability above 100% in one direction has been demonstrated.

Adhesion enhancement by a dielectric barrier discharge of PDMS used for flexible and stretchable electronics

Currently, there is a strong tendency to replace rigid electronic assemblies by mechanically flexible and stretchable equivalents. This emerging technology can be applied for biomedical electronics, such as implantable devices and electronics on skin. In the first step of the production process of stretchable electronics, electronic interconnections and components are encapsulated into a thin layer of polydimethylsiloxane (PDMS). Afterwards, the electronic structures are completely embedded by placing another PDMS layer on top. It is very important that the metals inside the electronic circuit do not leak out in order to obtain a highly biocompatible system. Therefore, an excellent adhesion between the 2 PDMS layers is of great importance. However, PDMS has a very low surface energy, resulting in poor adhesion properties. Therefore, in this paper, PDMS films are plasma treated with a dielectric barrier discharge (DBD) operating in air at medium pressure (5.0 kPa). Contact angle and XPS measurements reveal that plasma treatment increases the hydrophilicity of the PDMS films due to the incorporation of silanol groups at the expense of methyl groups. T-peel tests show that plasma treatment rapidly imparts adhesion enhancement, but only when both PDMS layers are plasma treated. Results also reveal that it is very important to bond the plasma-treated PDMS films immediately after treatment. In this case, an excellent adhesion is maintained several days after treatment. The ageing behaviour of the plasma-treated PDMS films is also studied in detail: contact angle measurements show that the contact angle increases during storage in air and angle-resolved XPS reveals that this hydrophobic recovery is due to the migration of low molar mass PDMS species to the surface.

Design of an Implantable Slot Dipole Conformal Flexible Antenna for Biomedical Applications

We present a flexible folded slot dipole implantable antenna operating in the Industrial, Scientific, and Medical (ISM) band (2.4-2.4835 GHz) for biomedical applications. To make the designed antenna suitable for implantation, it is embedded in biocompatible Polydimethylsiloxane (PDMS). The antenna was tested by immersing it in a phantom liquid, imitating the electrical properties of the human muscle tissue. A study of the sensitivity of the antenna performance as a function of the dielectric parameters of the environment in which it is immersed was performed. Simulations and measurements in planar and bent state demonstrate that the antenna covers the complete ISM band. In addition, Specific Absorption Rate (SAR) measurements indicate that the antenna meets the required safety regulations.

Design and Manufacturing of Stretchable High-Frequency Interconnects

The increasing number of biomedical applications for electronic systems have led to the need for stretchable electronics in order to significantly enhance the comfort of the user. This paper describes the design and manufacturing process of new stretchable high-frequency interconnects with meander-shaped conductors in a coplanar waveguide topology. The novel interconnects are produced based on laser-ablation of a copper foil, which is then embedded in a highly stretchable bio-compatible silicone material. Measurements on prototypes of the designed stretchable high-frequency interconnects revealed a maximal magnitude of -14 dB for the reflection coefficient and a minimal magnitude of -4 dB for the transmission coefficient in the frequency band up to 3 GHz. The influence of stretch on the performance of the high-frequency interconnects was analyzed using a stretch testing machine. The results showed that nor the magnitude, neither the phase of the transmission coefficient was influenced by elongations up to 20%.

Polyimide-Enhanced Stretchable Interconnects: Design, Fabrication, and Characterization

This paper discusses the optimization of a stretchable electrical interconnection between integrated circuits in terms of stretchability and fatigue lifetime. The interconnection is based on Cu stripes embedded in a polyimide-enhanced (PI-enhanced) layer. Design-of-experiment (DOE) methods and finite-element modeling were used to obtain an optimal design and to define design guidelines, concerning both stripe and layer dimensions and material selection. Stretchable interconnects with a PI-enhanced layer were fabricated based on the optimized design parameters and tested. In situ experimental observations did validate the optimal design. Statistical analysis indicated that the PI width plays the most important role among the different design parameters. By increasing the PI width, the plastic strain in the Cu stripes is reduced, and thus, the stretchability and fatigue lifetime of the system is increased. The experimental results demonstrate that the PI-enhanced stretchable interconnect enables elongations up to 250% without Cu rupture. This maximum elongation is two times larger than the one in samples without PI enhancement . Moreover, the fatigue life at 30% elongation is 470 times higher.

Polycrystalline CdSe films for thin film transistors

Abstract In this paper X-ray, ESCA, TEM and electrical measurements on evaporated CdSe films, used in thin film transistors (TFT), are reported. Special attention has been paid to semiconductor films obtained from recrystallized mixtures of CdSe and 1–2% In 2 Se 3 . Such films might be represented as (3CdSe) x (In 2 Se 3 ) 1- x . Doping the CdSe evaporation source with In yields 20 μm self-aligned TFTs with excellent characteristics: electron mobility in the evaporated thin films increases from 20–50 cm 2 /V · s for undoped films to more than 100 cm 2 /V · s for doped films. DC stability behaviour is also improved: the TFT current drop after 180 s is reduced from 30% to less than 5%.

Electro-conductive adhesives for high density package and flip-chip interconnections

Abstract Dispensable isotropic conductive adhesives (ICA) and snap-curing anisotropic conductive adhesives (ACA) are developed through the EC funded Brite EuRam project DACTEL #BE95-1503. They show very promising capabilities for high-density applications when compared to benchmark electro-conductive adhesives. As first high-density application, assemblies of ceramic and plastic ball grid array/land grid array (LGA) on FR4 with DAC3-102/14 ICA are realized. Mixed assemblies solder/ICA show poor results, especially during aging. Full polymer LGA assemblies are built successfully. Daisy chains with hundreds of transitions component/substrate present resistances as low as 4 Ω. After comparison with benchmark products, CLGAs show themselves to be particularly reliable under moisture conditioning. Secondly, flip-chip assemblies on board, of medium sized chips bumped with electroless NiAu and using DAC2-143/02 ACA, are performed. Contact resistances as low as 10 mΩ are produced. For this application, reliability results are succinct. Finally, flip-chip assemblies on glass of slim chips with NiAu bump pitch down to 80 μm, by means of the newly developed DAC2-143/02 ACA, are demonstrated. The material shows better performances than a benchmark anisotropic conductive film, where measurements reveal contact resistances lower than the sheet resistance of the transparent indium tin oxide metallization used in display applications. Thermal cycling and temperature storage reveal good behavior of the ACA paste.

UTCP: A Novel Polyimide-Based Ultra-Thin Chip Packaging Technology

Flexible materials, today, are being used already as base substrates for electronic assembly. A lot of mounted components could be integrated in flexible polyimide (PI) substrates. Very interesting advantages of integrating components into the flex are compactness and enhanced flexibility; not only the interconnection but also the components themselves can be mechanically flexible. This paper describes a PI-based embedding technology for integrating very thin silicon chips in between two spin-on PI layers, the ultra-thin chip package (UTCP). This paper discusses the different process steps in the UTCP production and also presents the interconnection test results realized with this technology.

Stretchable Electronics Technology for Large Area Applications: Fabrication and Mechanical Characterization

The development and mechanical characterization of a novel technology for stretchable electronics is presented, which can be used for the realization of wearable textile electronics and biomedical implants. The stretchable devices consist of rigid or flexible component islands interconnected with stretchable meander-shaped copper conductors embedded in a stretchable polymer, polydemethylsiloxane. The technology uses standard printed circuit board manufacturing steps and liquid injection molding techniques to achieve a robust and reliable product. The conductors in the device are designed to accommodate strains up to 10-15%. Spin-on photo-definable polyimide as mechanical support for the stretchable interconnects and the functional flexible islands are introduced. By use of polyimide, the reliability of the stretchable interconnects, the straight interconnects on the flexible islands and the transitions between the stretchable and nonstretchable parts are improved. Long-term endurance behavior of the stretchable interconnects is studied by cyclic elongation at strain ranges of up to 20% while monitoring the electrical connectivity. Its shown that the lifetime of the polyimide supported interconnects is at least two times better compared to the nonsupported.