Research project Characterization methods for determining the sintering properties of printed inks containing micro- and nanoparticles and their influence on the homogeneity of conductivity and reliability – SIMONE (IGF Project 20928 N)
Driven by digitalisation, the demand for circuit carriers is increasingly accelerating, for example in the automotive sector (control, autonomous driving or infotainment) or for antennas for IoT or mobile phone applications. Parallel to this, the requirements profile in the respective application fields is also increasing and diversifying, whereby conventional photolithographic production in particular is slowing down this trend due to technical limitations and the many sequential production steps. Consequently, additive manufacturing processes are particularly interesting for medium-sized companies, as they have a high degree of technical flexibility and thus facilitate the development and production of innovative products, even for smaller companies. In addition, there are a number of economic advantages, such as shortened delivery times or lower production costs – especially for small series.
For the production of additively manufactured conductive structures on circuit carriers, inks or pastes containing micro- and nanoparticles are applied by means of aerosol, piezo or dispensing processes. In order to remove water or solvent components and to compact the particles contained, sintering is carried out in a downstream step to form a continuously conductive structure. In this context, an effect that has hardly been discussed and in particular little researched so far is the inhomogeneous sintering across the cross-section of the line, as can be seen in Figure 1. The most obvious hypothesis to explain the different sintering is a suboptimal or too high or too low energy input per time, respectively, which only leads to a sintering of the uppermost layer and thus to a kind of closure of the remaining material. Publications refer to this effect as skinning, which leads to different conductivities within the same structure.
The analysis of the state of research and development leads to the conclusion that a correct electrical characterisation of this effect is not possible or only possible to a limited extent with the measuring methods available so far. The measurement method to be developed plays a key role in improving existing products and processes and at the same time solves recognised technical and economic problems on the part of industry by creating the basis for a standardised characterisation of sintered structures.
Figure 1: FIB section and SEM image of inhomogeneous sintering of an aerosol jet printed structure after convection; Source: LHFT (FAU), FAPS (FAU)
Novel approach to a solution
The basis of the process is the skin effect known in high-frequency technology, which describes the penetration depth of electromagnetic alternating fields in metals that decreases with frequency and thus allows a connection to be established between electrical transmission properties and the conductivity profile of sintered lines. Figure 2 shows the schematic representation of the skin effect in printed structures.
Figure 2: Schematic representation of the skin effect at 10 MHz and 1 GHz on printed and sintered structures; Source: LHFT (FAU), FAPS (FAU)
This makes it possible to characterise different inks and the effects of different parameters during the sintering process using appropriate measurement technology. The expected measurement frequencies are in the three-digit MHz range, depending on the DC conductivity. If additional measurements at higher frequencies are added, the penetration depth is so low that conclusions can be drawn about the surface roughness of the sintered structure. In this context, the sometimes poor reliability is one of the biggest challenges of printed electronic products, which manifests itself, for example, in insufficient adhesion, cracking or bubble formation. A measurement method that enables the characterisation of the sintering properties in this area in particular provides the tools for optimising the mechanical properties of sintered structures in existing products.
The project is being carried out by the research centres Chair of High Frequency Technology (LHFT) and the Chair of Manufacturing Automation and Production Systems (FAPS) at the Friedrich-Alexander University Erlangen-Nuremberg (FAU). On the industry side, the project is supported by the companies Adphos digital printing GmbH, Contag AG, Diener elektronic GmbH + Co. KG, Dr. Hönle AG, KSG-Leiterplatten GmbH, LPKF Laser & Electronics AG, merconics GmbH & Co. KG, Neotech AMT, SEHO Systems GmbH and Zollner Elektronik AG.
The IGF Project 20928 N of the Research Association for Mechatronic Integrated Devices 3-D MID is funded by the Federal Ministry for Economic Affairs and Energy via the AiF as part of the programme for the promotion of joint industrial research (IGF) on the basis of a resolution of the German Bundestag.
Further information on the project can be found here.
