Cicor Group runs a development lab for printed electronics at its headquarters in Bronschofen, Switzerland. In one particular application for a customer, the company compared several different printing technologies, and opted for aerosol jet printing from Optomec.
The company procured a five-axis system and its advantages were clear. It is flexible enough to handle a range of substrates and materials, as well as various shapes, inks, and print resolutions. Also, the system has low risk of clogged heads offering another advantage over other systems.
Author
Karl-Heinz Fritz, VP Development and Technology, Cicor Group
Inks
An aerosol surrounded by a sheath gas is printed onto the surface of the workpiece. The ink is atomized using ultrasound or pneumatic devices and a mixture of nitrogen and ink droplets is generated. This mixture is passed on to the print head with ink-specific parameters. In the actual print head, the ink jet is formed and focused in a gas stream of nitrogen. This technology makes it possible to focus the ink jet very precisely, achieving consistent printing results with variable distances to the substrate surface between 1 and 10 millimeters. If the topography or structure heights vary within this range, it is not necessary to move the print head or the workpiece in the z-axis.
There are several types of inks available, including metal nanoparticle, metal flake, non-metallic conductive, resistive, dielectric, and piezo materials.
In order to make the right ink selection, it is necessary to have a precise specification of the product requirements, such as the desired conductivity or adhesion to the substrate. Currently, the most commonly used inks are based on silver nanoparticles, they offer the best balance between printability, material cost and layer properties, such as adhesion and conductivity/resistance.
Curing
To achieve the final properties of a printed structure, the inks must be cured. The most common way is using batch or continuous ovens. Typical curing temperatures of nanoparticle inks start at about 248°F (120°C), depending on the metal used. During this curing process, organic components, e.g. solvents, are removed from the printed layer and the nanoparticles merge, leading to the formation of a conductive path. Conductivity usually increases with higher curing temperature and longer time. Curing methods include infrared radiation, intense pulsed light, and ultraviolet curing mainly used for dielectric inks.
Substrates
The moderate curing temperatures of the inks used enable printing on a wide variety of different materials. The selection of suitable materials depends on the specified layer properties. The temperature and solvent resistance of the substrates used provide crucial information needed for the selection of suitable inks. Cicor has successfully printed on plastics (PEEK, LCP, ABS, polycarbonate, POM, PBT), silicone, polyimide films, glass, metals, parylene, and paper.
Soldering to Printed Silver
In addition to mechanical technologies such as connectors or spring contacts, it is also possible to solder components onto printed silver structures.
Due to the structure of the silver layer, processes with standard solders are not suitable. This is due to the fact that the conductive structure consists of a mixture of silver particles and polymer binders. The silver particles dissolve in standard solders due to the high temperatures, which can lead to an uncontrolled reduction in the layer thickness. In addition, the thermal stress starts the decomposition of the polymer binders present in the layer.
Pre-treatment of the printed solder surfaces is required in order to remove these binders on the surface and make pure silver accessible for the soldering process. The soldering process is carried out with low-temperature solders, to ensure that a sufficiently thick functional silver layer remains after the assembly process.
The aerosol jet printing technology used by Cicor offers significant advantages over comparable methods in terms of flexibility, resolution and ease of maintenance. The use of this technology opens up possibilities that were previously impossible or could only be realized with a great deal of effort. If emphasis is placed on the selection of suitable material combinations in the early stages of a project and design rules are followed, this technology is suitable for producing highly reliable and cost-effective products. Its 3D capability is a plus, and its versatility in materials handling make aerosol jet printing a valuable process for today's electronics manufacturing.