Functional Printing

Flatbed screen printing and stencil printing have been used for decades in many fields of technical printing. Specific application examples are the metallization of silicon solar cells, the printing of solder paste for the SMT assembly of printed circuit boards or the printing of antennas used in vehicles. The peculiarity of including even very fine structures with a high layer thickness on a wide variety of substrates means that screen and stencil printing belongs to various other processes. At Fraunhofer ISE, fine line contact fingers with a width of 19 µm will be screen printed on silicon solar cells.

Dispensing is used in many areas of industrial production as a method for precise dosing and precise positioning for the application of fluids.

Dispensing requires high-precision manufacture of the nozzles, fast and micrometer-precise control of the dispensing head over the substrate and intensive rheological optimization of the fluids used. Using dispensing, very homogeneous structures with almost perfect shape and uniformity can be created, for example ideal conductor structures with the finest line width and minimal deviations. A specialty is the multi-nozzle dispensing, which combines the advantages of the dispensing process in terms of precision with a high throughput. In the multi-nozzle dispensing process, the finest contact fingers e.g. for the front-side metallization of silicon solar cells with perfect geometry and high throughput can be transferred highly parallel. However, the process also offers great potential for other applications. Multi-nozzle dispensing was developed at Fraunhofer ISE using CFD simulations and has been commercialized by Highline Technology - a spin-off of Fraunhofer ISE - since 2020.

Rotary printing processes such as flexographic printing, rotary screen printing and gravure printing have been used in a large number of industrial applications in the graphic and technical field for decades. Rotary printing processes are unique in terms of speed, precision and flexibility. Usually rotary printing processes are used to print roll-to-roll.

Whereas printing on non-flexible piece goods such as e.g. Silicon solar cells, printed circuit boards, electronic components or chip cards is a lot more complex than roll-to-roll printing.  

Coating such substrates at high printing speeds requires a high-precision transport system and precise alignment of the rotary printing units.

Such a technology was developed by Fraunhofer ISE in cooperation with project partners as part of the "Rock-Star" project. The innovative rotary printing demonstration system in the PV-TEC – Photovoltaic Technology Evaluation Center of Fraunhofer ISE enables the high-precision transfer of the finest structures in flexographic and rotary screen printing with high throughput. In addition to the metallization of silicon solar cells, this technology offers great potential for a wide variety of applications in the field of functional printing.

Inkjet is a digital printing process that enables the contactless and extremely precise application of drops of all kinds. Fraunhofer ISE has many years of experience in the development of printing processes, for example with inks based on nano-scale metal particles, wax-like etching masks (also UV-curing) and dopants. To make sure the desired substrate can be wetted by the ink, Fraunhofer ISE offers the complete portfolio for characterizing media with regard to viscosity and surface tension. By using inkjet printers in glove boxes, the environment can be adjusted in terms of wetting. The Fraunhofer ISE also offers plasma-assisted preliminary processes for the activation of surfaces.

The FlexTrail process is currently in development and enables the precise application of media of different viscosities with an extremely precise, minimal structure width. In this way, conductor tracks with a minimum structure width of less than 10 µm could already be printed stably on smooth substrates.

The printing of functional fluids not only requires high-precision printing and coating processes, but also suitable drying and sintering processes. Various highly specialized drying and sintering technologies are available at Fraunhofer ISE. In addition to classic continuous dryers based on IR radiation and convection, high-temperature sintering processes (contact firing) as well as innovative processes such as vertical-cavity surface-emitting laser (VSCEL) and intense pulsed light (IPL) can be realized in PV-TEC.

The analysis of the geometric and electrical quality of functional structures such as conductor paths require special characterization procedures as well as corresponding and know-how for the correct implementation of measurements and assessment of the results.

At Fraunhofer ISE there are a number of special characterization methods for analyzing the printing process and the quality of the printed functional structures, e.g. 3D microscopy, scanning electron microscopy and high-speed cameras for direct optical printing process analysis are available. In addition, the flow behavior of relevant print media can be examined using ultra-modern rheology measuring devices and then modeled. Finally, based on these models, entire printing processes can be fully automated optimized within the framework of CFD simulation.