The aim of this project is to subject existing methods for measuring adhesive strength to a thorough investigation of the weak points, influencing variables and possible correlations between results. On the one hand, existing weak points are reduced through optimization. On the other hand, methods can be coordinated with regard to their requirements in relation to the various layer categories mentioned and the most suitable test method can be identified in each case. Certain test methods could turn out to be particularly suitable, for example, for individual layer thickness ranges, as a result of which less effort can be made in sample preparation and still comparable results can be achieved. At the end of the project, the user should be given a uniform, possibly category-specific recommendation for the application-oriented preparation, implementation and evaluation of adhesion measurements of printed layers on rigid and flexible substrates.
Picture: Hot-Pin-Pulltest; Source: Kuhn & FAPS, FAU
For the various printing processes used in connection with printed electronics (e.g. inkjet, aerosol and screen printing), different pastes and inks are used, which differ fundamentally in terms of viscosity, composition, structure and thickness of the hardened or sintered, electrically functional layers. In the case of polymer-bound pastes, for example, intermolecular interactions between the matrix polymer of the paste and the substrate are mainly responsible for the adhesion; the adhesion is similar to the mechanisms in adhesive connections. The layer thicknesses created with polymer-bound pastes are usually well over 10 μm (applied, for example, by means of screen printing or jetting). With low-viscosity inks, depending on the printing process, layer thicknesses of up to about 1 μm (e.g. inkjet printing) and 1 μm to about 10 μm (e.g. aerosol printing) are produced; the adhesion occurs between the metal particles, any residual functionalization and the substrate material .
Given the fact of the different categories of printed layers mentioned in printed electronics, there has so far been no consensus on the reliable and quantitative measurement of adhesive strength. In some cases, there are standards for testing the adhesive strength (e.g. cross-cut or forehead pull-off test), but it should be noted that there is a large amount of subjective leeway in sample preparation and evaluation of the results. Test methods such as the cross-cut or wallpaper test are also characterized by their ease of use, but (so far) only qualitative results have been achieved. In contrast, with the forehead pull or hot pin pull test, quantitative results are obtained after extensive sample preparation, but important influencing factors (penetration of adhesive into the layer, solderability of the layer) have not yet been systematically investigated.
The research project will contribute to increasing the acceptance of printed electronics on the market and thus the competitiveness of the printed electronics through a standardized and as practical as possible procedure for adhesion testing including the possibly necessary sample preparation against the background of the above-mentioned different categories of printed layers on rigid and flexible substrates to significantly increase the corresponding SMEs.
Benefits and economic importance for SMEs
Standardized test procedures are always at the beginning of a general quality assurance. The lack of uniform procedures and methods for testing the adhesion of printed, thin layers is therefore a hurdle that has so far hardly been overcome for the wider acceptance of printed electronics. With a fundamental analysis of existing methods and the development of the simplest possible method for the quantitative measurement of adhesive strength, the application potential of printed electronics could be significantly expanded. For SMEs, the benefits of the project are given by:
- Basic statements on the comparability of existing measurement methods through knowledge of the correlation between established methods
- Standardized test procedures accompanying the product development process (initially simple procedures for basic feasibility, later more complex but more precise procedures)
- Increase in competitiveness, since simpler and more cost-effective test methods can be used for customer-specific inquiries
- Provision of a stable knowledge base for the new and further development of printed mechatronic systems and thus the possibility of opening up new fields of application