Transmission line printed using silver nanoparticle ink on FR-4 and polyimide substrates
© The Author(s) 2016
Received: 9 June 2016
Accepted: 25 August 2016
Published: 10 September 2016
In this paper, nano-silver ink-jet printed transmission lines were fabricated to investigate RF performance on both flame retardant (FR-4) and polyimide (PI) as rigid and flexible substrates. The transmission lines were printed by using the ink-jet printer with velocity of 3.5 mm/s and were sintered in a convection oven with 250 °C. The RF performance of transmission lines was simulated and measured at low frequencies. The transmission loss is measured to be 0.22 dB@1 GHz and 0.32 dB@1 GHz, respectively, for the FR-4 and PI substrates, respectively. The return loss has over 16 dB for FR-4 substrate and over 12 dB for PI substrate. The RF performance of transmission line was investigated and discussed in regard to an influence by two substrates. The measured RF performance of fabricated transmission lines results in the possibility that flexible device is explored in low frequencies application.
In recent years, printed electronics has received fast growing interest as alternative manufacturing method, because conventional processes, which are required etching and deposition processes, generate large amount of toxic chemical wastes during lift-off and electroplating process [1–4]. Therefore, many researchers started to pay attention on printed electronics that the etching or deposition processes are not required. In addition, printed electronics technology has the striking advantages such as simple process, low-cost, reduction of waste and capability of various substrates, compared with conventional processes.
Printed electronics device is fabricated by several printing methods including the screen printing, gravure printing, and ink-jet printing [5–8]. First, screen printing was widely used from long ago because it had simple printing tools which included stencil and squeegee. However, screen printing is not suitable for fragile substrate due to inevitable contact of stencil with substrate and is not inexpensive because it accompanies large amount of ink. Furthermore, this printing method has a lower resolution than other printing methods. In gravure printing, ink was directly transferred through engraved roll. For this reason, it easily applies to roll-to-roll (R2R) process. Gravure printing has benefit of higher throughput because this printing method have advantages such as printable large area and fast printing speed by continuous process. However, a developed R2R printing equipment can be used by ink of one-type and printed structure can be damaged by contact of substrate with roll. Ink-jet printing technology creates a flexible electronics without contact of nozzle with substrate, which produces single droplet contained metal nano-particle through nozzle. This printing method offers relatively lower throughput than other printing methods, because ink-jet printer restricts the number of nozzles and directly draws a circuits of electronics device. However, the resolution is relatively higher than other printing methods, and various substrates are broadly utilized in order to use benefits of ink-jet printing technology. In addition, ink-jet printing provides distinguished advantage such as drop-on-demand printing, no wastage of ink, various functional inks, and non-contact printing. For this reason, research on ink-jet printing technology is required. In particular, RF performance of printed structure has to be investigated in regard to type of substrate because flexible device is fabricated on flexible substrate rather than rigid substrate.
In this paper, we investigated the influence of the rigid and flexible substrate about ink-jet printed transmission lines. The transmission lines are fabricated using ink-jet printing technology on rigid substrate (FR-4) and flexible substrate (PI). The RF performance of transmission lines is simulated and measured in the range from 300 kHz to 1 GHz. The measured RF performance agrees well with tendency of simulation results and has potential in radio frequency identification (RFID) and smart label applications by simple and low-cost inkjet printing.
The ink, DGP 40LT-15C (Advanced Nano Product Co., Korea), was used to print transmission lines. The ink was composed of 30.50 wt% silver nanoparticles with average size of approximately 20 nm. The specific resistance of the ink was 11–12 μΩcm, and the viscosity was 17.3 cPs at 100 rpm. The printing system was composed of a printhead equipped with nozzles of diameter of 19 μm, XY stage motorized with a positioning accuracy of ±2 μm, and alignment system. The ink was ejected using ink-jet printer, Dimatrix DMP-2831 (Fujifilm Dimatix Inc., USA), with drop-on-demand from the substrate above 2 mm. The substrates were patterned by FluoroCarbon (FC) solution to make the structure with high aspect ratio. The FC solution was composed of FC-722 and FC-40 (3M Co., Korea), which the spin-coated surface by the solution was known to have higher hydrophobicity. We used FR4 and PI substrate with the thickness of 1.6 mm and 200 μm, respectively, as rigid and flexible substrates, respectively. The SubMiniature version A (SMA) connectors with gap of 0.8 and 1.6 mm were used to measure RF performance of transmission lines.
Design and fabrication process
Design parameters of the transmission lines
Transmission line #1
Transmission line #2
Height of substrate (h)
Length of substrate (l)
Width of silver (w)
Thickness of silver (t)
Results and discussion
In order to resolve the parasitic capacitance by the air bubbles between the PI substrate and copper tape, the transmission line with the ground layer of the silver paste is additionally fabricated, as shown in Fig. 3c. The dimension and material property of the transmission line is same to transmission line #2 except for the length of transmission line, as shown in Table 1. The length is 91 mm, which is transmission line with the ground layer of silver paste. Although the transmission line for the ground layer of copper tape and silver paste were fabricated with different length, however the loss was measured and calculated per unit length to compare the proposed transmission lines. Figure 4c shows the RF performance of transmission lines with ground layer using the copper tape and silver paste. The transmission loss is measured to be 0.22 and 0.32 dB/cm at 1 GHz, respectively. Although the ground layer of silver paste is used for improvement of RF performance, transmission loss is increase than previous transmission line because the silver paste has the lower conductivity than copper tape. Consequently, the results show that the conductivity of ground is important factor at RF performance of transmission line.
Comparison of RF characteristic of fabricated transmission lines
Transmission loss (dB/cm)
Return loss (dB)
In this study, we fabricated the ink-jet printed transmission line on FR-4 and PI substrates and measured the RF performance of transmission line. The transmission loss of transmission line on FR-4 and PI substrate is 0.12 and 0.22 dB/cm at 1 GHz, respectively. The transmission losses have a negligible difference even if additional transmission loss occurs by the ground layer at the transmission line on PI substrate. Therefore, the fabricated transmission lines show the possibility that flexible device can be used as a transmission line in UHF applications. Furthermore, we believe that the transmission line is alternative for conventional transmission line by low-cost, simple fabrication process in electronics device required performance of moderate level at low frequencies.
JMK conceived the idea and supervised the project. JMK, SHL, KYS and SMS discussed the design and the fabrication process of the ink-jet printed transmission line. SMS, YSL and HLK performed the measurement of RF performance and analysis of the results. JMK and SMS drafted the manuscript. All authors read and approved the final manuscript.
This work was supported by research funds from the Korea Institute of Industrial Technology (KITECH).
The authors declare that they have no competing interests.
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