Flexible electronics is a rapidly growing technology for a multitude of applications. Wearables and flexible displays are some application examples. Various technologies and processes are used to produce flexible electronics. An important aspect to be considered when developing these systems is their reliability, especially with regard to repeated bending. In this paper, the frequently used methods for investigating the bending reliability of flexible electronics are presented. This is done to provide an overview of the types of tests that can be performed to investigate the bending reliability. Furthermore, it is shown which devices are developed and optimized to gain more knowledge about the behavior of flexible systems under bending. Both static and dynamic bending test methods are presented.

We have investigated the influence of C₆₀-fullerenes as a dopant in thin layers of phthalocyanines (Pc). We expect that C₆₀ acts as an electron acceptor. Charge transfer of an electron from the first excited state of the Pc to C₆₀ occurs, thus leading to an enhanced number of free holes, which are the majority charge carriers in phthalocyanine. The observation of luminescence in co-evaporated layers of phthalocyanines and C₆₀ shows a fluorescence quenching depending on the amount of fullerenes. This shows that the admixture of C₆₀ reduces direct recombination of electron–hole pairs and thereby increases the charge-generation efficiency. The effect on photoconductivity is established by electro-optical measurements, which show an improved current–voltage characteristic of C₆₀-doped phthalocyanine layers.

Nowadays, digital printing technologies such as inkjet and aerosol jet printing are gaining more importance since they have proven to be suitable for the assembly of complex microsystems. This also applies to medical technology applications like hearing aids where patient-specific solutions are required. However, assembly is more challenging than with conventional printed circuit boards in terms of material compatibility between substrate, interconnect material and printed ink. This paper describes how aerosol jet printing of nano metal inks and subsequent assembly processes are utilized to connect electrical components on 3D substrates fabricated by Digital Light Processing (DLP). Conventional assembly technologies such as soldering and conductive adhesive bonding were investigated and characterized. For this purpose, curing methods and substrate pretreatments for different inks were optimized. Furthermore, the usage of electroless plating on printed metal tracks for improved solderability was investigated. Finally, a 3D ear mold substrate was used to build up a technology demonstrator by means of conductive adhesives.

Substrate materials for printed circuit boards must meet ever-increasing requirements to keep up with electronics technology development. Especially in the field of high-frequency applications such as radar and cellular broadcasting, low permittivity and the dielectric loss factor are key material parameters. In this work, the dielectric properties of a high-temperature, thermoplastic PEEK/PEI blend system are investigated at frequencies of 5 and 10 GHz under dried and ambient conditions. This material blend, modified with a suitable filler system, is capable of being used in the laser direct structuring (LDS) process. It is revealed that the degree of crystallinity of neat PEEK has a notable influence on the dielectric properties, as well as the PEEK phase structure in the blend system developed through annealing. This phenomenon can in turn be exploited to minimize permittivity values at 30 to 40 wt.-% PEI in the blend, even taking into account the water uptake present in thermoplastics. The dielectric loss follows a linear mixing rule over the blend range, which proved to be true also for PEEK/PEI LDS compounds.

Microstructuring techniques, such as laser direct writing, enable the integration of microstructures into conventional polymer lens systems and may be used to generate advanced functionality. Hybrid polymer lenses combining multiple functions such as diffraction and refraction in a single component become possible. In this paper, a process chain to enable encapsulated and aligned optical systems with advanced functionality in a cost-efficient way is presented. Within a surface diameter of 30 mm, diffractive optical microstructures are integrated in an optical system based on two conventional polymer lenses. To ensure precise alignment between the lens surfaces and the microstructure, resist-coated ultra-precision-turned brass substrates are structured via laser direct writing, and the resulting master structures with a height of less than 0.002 mm are replicated into metallic nickel plates via electroforming. The functionality of the lens system is demonstrated through the production of a zero refractive element. This approach provides a cost-efficient and highly accurate method for producing complicated optical systems with integrated alignment and advanced functionality.

The assembly of passive components on flexible electronics is essential for the functionalization of circuits. For this purpose, adhesive bonding technology by isotropic conductive adhesive (ICA) is increasingly used in addition to soldering processes. Nevertheless, a comparative study, especially for bending characterization, is not available. In this paper, soldering and conductive adhesive bonding of 0603 and 0402 components on flexible polyimide substrates is compared using the design of experiments methods (DoE), considering failure for shear strength and bending behavior. Various solder pastes and conductive adhesives are used. Process variation also includes curing and soldering profiles, respectively, amount of adhesive, and final surface metallization. Samples created with conductive adhesive H20E, a large amount of adhesive, and a faster curing profile could achieve the highest shear strength. In the bending characterization using adhesive bonding, samples on immersion silver surface finish withstood more cycles to failure than samples on bare copper surface. In comparison, the samples soldered to bare copper surface finish withstood more cycles to failure than the soldered samples on immersion silver surface finish.