Evaluation of screen printed silver trace performance and long-term reliability against environmental stress on a low surface energy substrate

Otherwise attractive substrate materials for printed electronics may have such surface characteristics that make patterning challenging. This article focuses on the printability and performance characterization of conductive patterns on a low surface energy substrate. Surface characteristics of a hydrophobic polyphenylene ether (PPE) substrate and the effects of surface modification using chemical and physical pre-treatments were studied. In addition, silver ink performance and its reliability on this substrate were evaluated. The surface was characterized by surface energy measurements and surface profile analysis. Screen-printed test patterns were characterized to evaluate print quality and electrical and mechanical performance. A further inspection of substrate-ink interactions was conducted using environmental reliability tests. It was observed that ink adhesion could be significantly promoted by choosing a suitable surface pre-treatment method. Low sheet resistances were obtained, and thus, suitable inks for further characterization were found. In addition, it was observed that environmental stress has a significant impact on ink-substrate interactions.acceptedVersionPeer reviewe

Studies of Low Surface Energy Materials for Printed Electronics Applications

Printed electronics have gained steadily increasing attention due to the attractive simplicity and straightforwardness of the processes. The processes are also compatible with various flexible and stretchable materials. Thus, they can be used to create lightweight, unobtrusive, and conformable devices. However, there are certain challenges related to material compatibility. For example, hydrophobic materials with low surface energies tend to repel liquid coatings, prohibiting straightforward manufacturing. Therefore, substrate materials with beneficial properties like chemical inertness, elasticity, optical transparency, or robustness may be neglected because processing can become exhaustively complicated. In this thesis, two water-repelling polymers, a poly(phenylene ether) (PPE) based polymer blend and poly(dimethylsiloxane) (PDMS) were studied. Furthermore, their applicability in printed electronics was investigated. These studies began with surface characterization. In addition, conductive tracks were printed and evaluated on PPE and PDMS substrates by screen printing silver (Ag) flake inks and inkjet printing silver nanoparticle (Ag NP) inks, respectively. The performance of the screen-printed Ag patterns was evaluated on both the native and surface-treated substrates, and the endurance against environmental stress was studied in accelerated aging tests. After developing an inkjet printable PDMS ink, alternative approaches for multilayer fabrication on this substrate were proposed. First, direct layer-by-layer inkjet deposition of conductive and dielectric layers on PDMS was studied. In the second approach, conductive and dielectric layers were printed separately on mesh-like electrospun poly(vinyl alcohol) (PVA) nanomesh substrates to form self-standing layers, which were sandwiched to create capacitive pressure sensor elements. The results indicate that conductive tracks can be printed on both substrates relatively straightforwardly. Highly conductive lines could be screen printed with only a single pass, the resulting sheet resistance being only 8-10 mΩ·□-1. However, a comprehensive understanding of ink composition and printing parameters is required to minimize the prints' surface roughness and edge roughness. In addition, as new screens are needed for each new pattern, prototyping with this method is time-consuming. Finally, the environmental reliability of the screen printed patterns was heavily dependent on the used materials, and the results emphasize the need for proper encapsulation. In comparison, prototyping was relatively easy by inkjet printing the functional materials directly on PDMS, and sheet resistances below 0.5 Ω·□-1 were obtained, even though print thickness was less than 2 μm. Although the sheet resistance of the porous, PVA-based conductive layers was higher (3 Ω·□-1) despite the high layer thickness of 10 μm, the obtained adhesion between the ink and the substrate was excellent. Intermediate surface treatments and curing steps of the direct layer-by-layer multilayer deposition approach made the printed patterns susceptible to deformation already during the fabrication process, requiring a new patterning strategy and curing condition optimization to prevent the defects. The integrated PVA layer made the self-standing dielectric easy to handle despite its low thickness. The capacitive sensors had linear sensitivity of up to 4 Mpa-1 with low hysteresis (< 8.5 %), remained functional over 2000 cycles, and were capable of sensing physical interactions with the surroundings. It was shown that simple processes could be used to print electronics even on hydrophobic substrates. Moreover, multilayered device configurations can be prototyped using just one material printer, either by a direct, layer-by-layer deposition or using separately printed functional layers to build devices, such as sensors. The presented results provide a reference point to further studies, which should determine the full potential and usability of the proposed materials and methods in complex printed electronic applications where both circuits and sensor elements are used in various device configurations