Author: Thibaut Soulestin, PhD; Senior Application Engineer, Henkel Adhesive Technologies - Printed Electronics; thibaut.soulestin@henkel.com
Henkel Adhesive Technologies holds leading market positions worldwide in the industrial and consumer business. As a global leader in the adhesives, sealants, and functional coatings markets, Henkel has developed a large material portfolio of LOCTITE® conductive inks and coatings suitable for printing electronic circuitry, that is thin, lightweight and flexible. The LOCTITE® Printed Electronics portfolio offers more than 100 different material solutions of which more than 20 are silver inks.
Driven by the megatrend of digitalization, printed electronics offer a complementary solution to traditional electronic circuitry, opening new opportunities for electronics integration and miniaturization across industries. To achieve surface integrability, with limited visibility to the human eye, the printing of fine line structures has become increasingly important. To achieve high-performing fine line printed circuitry, it requires conductive inks having fine silver particles and adapted viscosity e.g. LOCTITE® ECI 1006.
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1. LOCTITE® ECI 1006
Among the large range of Henkel silver inks, LOCTITE® ECI 1006 is specifically developed for fine-line printing, targeting a line width below 150 µm (Figure 1). Fine lines are of particular interest for the development of transparent conductive surfaces, amongst others enabling human-machine interface (HMI) solutions. More precisely transparent capacitive touch sensors enable backlighting for HMI.
The better long-term reliability of LOCTITE® ECI 1006 can be an advantage compared to alternative materials on the market, e.g. PEDOT: PSS inks. In addition, LOCTITE® ECI 1006 can heat up to 150°C, without performance degradation, allowing rapid heating. This makes it an interesting material solution for visible or radar-transparent heaters with printed fine lines. The heater power can be adjusted by the design of the fine lines. De-icing of advanced driving assistance systems, ADAS, is a good application example.
LOCTITE® ECI 1006 is specifically formulated with fine silver particles to avoid clogging of the screen and provide homogeneous electrical contact, even for very fine lines below 50 µm width. This ink also shows high viscosity and thixotropic index to reduce the spreading of the ink.
Figure 1. Example of a 1 kg LOCTITE® silver ink
The dry ink layer shows good adhesion onto a large range of substrates after 1000 hours at 85°C and 85 %RH, even on difficult substrates such as copper, or ITO. Beyond good adhesion on ITO LOCTITE® ECI 1006 offers low contact resistance, making it particularly suitable for busbars on transparent ITO films.
To meet the growing demand for hybrid electronics and components attached onto flexible substrates, this ink is compatible with low-temperature solder Sn42Bi58 paste; and electrically conductive adhesives such as Henkel LOCTITE® ABLESTIK CE 3104WXL or LOCTITE® ABLESTIK 2030SC.
Table 1. Typical Properties of LOCTITE® ECI 1006
2. Printing Trials with LOCTITE® ECI 1006
2.1 Equipment
Stainless steel screens with thin wires and high mesh count are mandatory to achieve good printing quality. It is the most important parameter. For a test study, a 640-15 stainless steel mesh from Asada was selected. Thermotropic liquid crystal polymer monofilament mesh, also known as V-screenTM, are a good alternative.
Figure 2 shows the test design with line width from 100 µm down to 30 µm width with different spacing and orientation to see the influence of print direction and ink spreading.
Figure 2. Fine line test screen design
A thin and low roughness emulsion over mesh, EOM, is also required to keep the good printing definition, below 10 µm with a roughness below 2 µm, is recommended. A thicker EOM will impede the ink release, resulting is pinholes or line breaks. The emulsion AZOCOL® Z 177 FL with release agent KIWOMIX® RA 1750. EOM 8 µm from Kissel+Wolf. The release agent improves the paste releases from the screen (Figure 3).
Figure 3. Emulsion AZOCOL® Z 177 FL with release agent KIWOMIX® RA 1750. EOM 8 µm. After printing, the paste is fully released from the screen.
A Carbon S 75° 8 mm squeegee from RKS was chosen to provide constant printing quality over the entire trial without mechanical changes of the squeegee behaviour.
Standard untreated polyester substrate, CUS5 125 µm thick from Mc Dermid, was selected. The substrate surface energy can play a significant role in the printing quality, especially for the ink spreading after printing. Using a standard substrate demonstrates that with the right ink and printing equipment, fine silver lines, below 50 µm, are already possible.
LOCTITE® ECIs 1006 was used un-diluted and diluted with 2 wt% of DBE solvent, CAS number: 95481-62-2. The ink is mixed with a propeller mixer, at low speed, 300 rpm. Dilution up to 10 wt%, by 1 wt% increments, is possible to balance printability versus ink spreading.
Printing trials were carried out in Henkel Inspiration Center Düsseldorf, ICD in a standard lab environment. For reliable quality production, dust-controlled rooms or even clean rooms are highly recommended.
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2.2 Visual Inspection
Figure 4 shows the microscope images of 30 µm and 75 µm lines width with un-diluted and 2 wt% diluted ink. 75 µm lines with un-diluted ink have good edge quality with very low spreading. At 30 µm, due to the high ink viscosity and the screen mesh, the edges are wavy and line breaks are observed. 2 wt% dilution with DBE solvent improved the printability and much better line edges are observed without lines breaks. This improvement is coming with a larger ink spreading as observed for the 75 µm line width.
Figure 4. Left column: Un-diluted LOCTITE® ECI 1006 ink, Right column: 2 wt% diluted. Top row: 30 µm line width design, Bottom row: 75 µm line width design.
Line breaks may be caused by the rough surface of the mesh and high ink viscosity. But they can also result from external pollution such as dust. Figure 5 shows a line break of a 50 µl line width despite good printing quality. This break is likely due to screen blockage by external pollution. Thus, it is important to produce in a dust-controlled environment, and clean room conditions to avoid contaminations resulting in line breaks or shortcuts between lines with narrow spacing.
Figure 5. Microscope image of 50 µm line width printed with undiluted LOCTITE® ECI 1006.
The mesh structure is visible on the edges of the printed tracks but also on the surface. Figure 6 gives an overview of interferometer measurements on fines lines with undiluted LOCTITE® ECI 1006. At low width, below 30 µm, the ink is barely printed on the substrate. For the 50 µm lines, even if the edges are regular (Figure 5), the surface roughness is important with hills and valleys ranging from 1 µm up to 9 µm thick. This roughness is strongly related to the screen mesh but can also be mitigated by diluting the ink to favour ink flow.
Figure 6. Interferometer images of 20, 25, 30, 40, and 50 µm lines width printed with undiluted LOCTITE® ECI 1006.
Printed track surface homogeneity and edge regularity have a direct influence on the final printed track resistance. The valley and narrower sections create local higher resistive points resulting in higher track resistance. Figure 7 summarises the track resistance (R) normalized to the track shape (Nsq = length/width), called Square Resistance, in mOhm/sq. For standard silver ink printing with a line width in the 0.5 to 20 mm range, the square resistance on tracks on one print is constant, as the thickness is the same for all the tracks and the edges have no influence. For fine lines, the influence of edges and roughness is clearly visible. 20 µm tracks are not conductive. The square resistance for the 30 to 50 µm lines is above the expected value because of a narrower section of the tracks. As of 75 µm, the square resistance is constant indicating negligible influence of the edges and roughness. The square resistance of the tracks printed with undiluted ink is higher as the edge quality is lower. To achieve good quality 50 µm tracks, a dilution higher than 2 wt%, closer to 4 wt%, would be required.
Figure 7. Square resistance of undiluted and 2 wt% diluted LOCTITE® ECI 1006 for different line widths
Advanced microscopic equipment is thus not needed to check the printing quality of the tracks. By simply monitoring the square resistance, it is possible to assess the edge quality and even more thickness homogeneity.
Homogenous tracks are important for fine-line heater applications to avoid local hot spots leading to local burns and loss of heater performance.
3. Conclusion
Successful fine line printing is a subtle balance between ink, equipment, and processing. Different LOCTITE® inks can be used for fines lines such as LOCTITE® ECI 1010, LOCTITE ECI® 1011 or LOCTITE® ECI 1006. The later features the best combination of fine silver particles, adapted viscosity, good reliability, and adhesion to a wide range of substrates. Aside the ink, the screen is the most important parameter for successfully printing fine lines. Therefore, it is highly recommended to seek advice and alignment with your screen supplier before starting the printing. To ensure you achieve your printing goals, the Henkel Printed Electronics team can help you choose the right ink for your application requirements and introduce you to trusted industry partners. Connect with our printed electronics experts and learn more about Henkels established portfolio of conductive inks and coatings: printed.electronics@henkel.com.
4. Acknowledgements
Thank you; to Asada Mesh for supplying the stainless-steel mesh; Kissel+Wolf for supplying the screens with the emulsions, and RKS for the carbon squeegees. Thank you all for the in-depth and fruitful technical discussions.
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