screen-pad printing for electrode patterning on curvy surfaces

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1 3 Microsyst Technol DOI 10.1007/s00542-014-2396-0 TECHNICAL PAPER Screen-pad printing for electrode patterning on curvy surfaces Ken-ichi Nomura · Yasuyuki Kusaka · Hirobumi Ushijima · Kazuro Nagase · Hiroaki Ikedo Received: 9 December 2014 / Accepted: 10 December 2014 © Springer-Verlag Berlin Heidelberg 2014 devices can be used for wearable applications (Yao and Zhu 2014; Honda et al. 2014; Yeo et al. 2014; Jung et al. 2014; Pang et al. 2013). For example, medical patients can wear these devices; their vital signs can be easily monitored without discomfort. Currently, these devices are fabricated on a flexible two-dimensional substrate, and then shaped three-dimensionally for use. Unfortunately, these devices can only be bent to simple shapes such as cylinders. It is difficult to bend these devices so that they smoothly fit complicated shapes. This type of application requires much more three-dimensionally complex devices. One of the best ways to overcome this problem is direct pattern formation on a complex-curved surface. From this viewpoint, the conventional pad printing technique (Fig. 1a) is interesting because it has the potential to form patterns on such a surface (Lee et al. 2010; Krebs 2009; Hahne et al. 2001). This technique uses a soft silicone pad. First, the grooves of the metal gravure plate are filled with ink using a doctor blade. Next, the ink in the grooves is transferred to the silicone pad. Lastly, the ink is transferred to a final substrate. Owing to the softness of the pad, the ink can be placed on a curved surface. However, this method still has some problems. Thin patterns are obtained by pad printing, typically less than 3 μm (Lee et al. 2010). Fur- thermore, it is difficult to form large patterns because the ink is removed from the grooves during doctoring. These facts decrease its range of applications. To address these issues, we have developed a new print- ing technique, called “screen-pad printing.” We analyzed the potential for pattern formation on curvy surfaces with this new technique by observing the results of printing objects with various shapes. Further, we studied the impact of this method by comparing patterns obtained from con- ventional gravure-based pad printing with patterns obtained using the developed screen-pad printing method. Abstract Pad printing is a simple but effective method for fabricating electrodes onto complex curved surfaces. In this method, ink is picked up from the gravure plate by the soft pad, after which the ink pattern is transcription- ally formed from the pad to the final substrate. However, this printing method has some problems; primarily, it is difficult to form thick and large patterns. To address these limitations, we developed a new printing technique called “screen-pad printing.” In this technique, ink is first screen- printed onto a blanket made of silicone resin, after which the ink pattern is picked up by the soft pad, and the pat- tern on the pad is transferred to the substrate. In this paper, we describe the features of the developed screen-pad print- ing technique when forming patterns on objects that have complex surfaces. Further, the impact of our new method is analyzed by comparing the experimental results of screen- pad printing with results obtained from conventional pad printing. 1 Introduction High form-factor, flexible electronic devices are fascinating in part because they can improve our quality of life. Such K. Nomura (*) · Y. Kusaka · H. Ushijima Flexible Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan e-mail: [email protected] K. Nagase · H. Ikedo Mino Group Co., Ltd., 8-2 Kamita, Minami-cho, Gujo, Gifu 501-4101, Japan

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Page 1: Screen-pad printing for electrode patterning on curvy surfaces

1 3

Microsyst TechnolDOI 10.1007/s00542-014-2396-0

TECHNICAL PAPER

Screen-pad printing for electrode patterning on curvy surfaces

Ken-ichi Nomura · Yasuyuki Kusaka · Hirobumi Ushijima · Kazuro Nagase · Hiroaki Ikedo

Received: 9 December 2014 / Accepted: 10 December 2014 © Springer-Verlag Berlin Heidelberg 2014

devices can be used for wearable applications (Yao and Zhu 2014; Honda et al. 2014; Yeo et al. 2014; Jung et al. 2014; Pang et al. 2013). For example, medical patients can wear these devices; their vital signs can be easily monitored without discomfort. Currently, these devices are fabricated on a flexible two-dimensional substrate, and then shaped three-dimensionally for use. Unfortunately, these devices can only be bent to simple shapes such as cylinders. It is difficult to bend these devices so that they smoothly fit complicated shapes. This type of application requires much more three-dimensionally complex devices.

One of the best ways to overcome this problem is direct pattern formation on a complex-curved surface. From this viewpoint, the conventional pad printing technique (Fig. 1a) is interesting because it has the potential to form patterns on such a surface (Lee et al. 2010; Krebs 2009; Hahne et al. 2001). This technique uses a soft silicone pad. First, the grooves of the metal gravure plate are filled with ink using a doctor blade. Next, the ink in the grooves is transferred to the silicone pad. Lastly, the ink is transferred to a final substrate. Owing to the softness of the pad, the ink can be placed on a curved surface. However, this method still has some problems. Thin patterns are obtained by pad printing, typically less than 3 μm (Lee et al. 2010). Fur-thermore, it is difficult to form large patterns because the ink is removed from the grooves during doctoring. These facts decrease its range of applications.

To address these issues, we have developed a new print-ing technique, called “screen-pad printing.” We analyzed the potential for pattern formation on curvy surfaces with this new technique by observing the results of printing objects with various shapes. Further, we studied the impact of this method by comparing patterns obtained from con-ventional gravure-based pad printing with patterns obtained using the developed screen-pad printing method.

Abstract Pad printing is a simple but effective method for fabricating electrodes onto complex curved surfaces. In this method, ink is picked up from the gravure plate by the soft pad, after which the ink pattern is transcription-ally formed from the pad to the final substrate. However, this printing method has some problems; primarily, it is difficult to form thick and large patterns. To address these limitations, we developed a new printing technique called “screen-pad printing.” In this technique, ink is first screen-printed onto a blanket made of silicone resin, after which the ink pattern is picked up by the soft pad, and the pat-tern on the pad is transferred to the substrate. In this paper, we describe the features of the developed screen-pad print-ing technique when forming patterns on objects that have complex surfaces. Further, the impact of our new method is analyzed by comparing the experimental results of screen-pad printing with results obtained from conventional pad printing.

1 Introduction

High form-factor, flexible electronic devices are fascinating in part because they can improve our quality of life. Such

K. Nomura (*) · Y. Kusaka · H. Ushijima Flexible Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japane-mail: [email protected]

K. Nagase · H. Ikedo Mino Group Co., Ltd., 8-2 Kamita, Minami-cho, Gujo, Gifu 501-4101, Japan

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2 Experimental details

2.1 Procedure of screen-pad printing

Figure 1b is a schematic explaining the screen-pad printing process. This printing technique is based on screen-offset printing, which involves screen printing to a silicone blan-ket and transcriptional formation of the patterns from the blanket to the final substrate (Nomura et al. 2014a, b). On the other hand, in screen-pad printing, the pattern screen-printed on the blanket is picked up by the pad, after which the pattern on the pad is transferred to the final substrate.

Compared to conventional pad printing shown in Fig. 1a, the inking methods used before ink is picked up by the pad are different; this difference determines the superiority of screen-pad printing, as described later.

2.2 Materials

In this study, Ag ink [MCP6111/03 (trial product), Mino Group] was used as the conductive ink for screen-pad print-ing. The ink includes flake-shaped Ag of a few microme-ters. For conventional gravure-based pad printing, other Ag ink (MP301-Pad, Mino Group), also containing flaked Ag, was used. Screen masks with a polyester mesh were used for screen printing to the silicone blanket. A sheet made of ~2-mm-thick polydimethylsiloxane (PDMS) (Shin-Etsu Chemical) was used as the silicone blanket. Note that a stainless plate was adhesively placed under the blanket for reinforcement. The pad material was room-temperature-vulcanizing silicone rubber. In addition to observation using conventional optical microscopy, we also used scan-ning electron microscopy (SEM, Keyence, VHX-D510).

3 Results and discussion

First, we formed patterns on a concave curved glass sub-strate by screen-pad printing. Figure 2a shows an image of printed Ag patterns with a width of 100 μm, and its enlarged image obtained by optical microscopy is shown in Fig. 2b. Prior to the formation of the Ag patterns, a resin film including black dye was coated on the Ag-printed area

Soft padDoctor blade

Groove

Conductive ink

Gravure plate Final substrate

gravure-based pad printing

(i) Doctoring (ii) Picking up ink (iii) Transfer to a substrate

Silicone blanket

Squeegee

Conductive ink

Screen maskSoft pad

(a) Conventional

(b) Developed screen-pad printing

Final substrate(i) Screen printing

to a silicone blanket(ii) Picking up ink (iii) Transfer to a substrate

Fig. 1 Schematic explaining conventional pad printing (a) and screen-pad printing (b)

Fig. 2 a Image of 100-μm-width screen-pad printed patterns, formed on a concave curved surface. b Enlarged image of the pattern shown in (a)

Fig. 3 Cross-sectional SEM image of the patterns formed on a con-vex curved acrylic plate, obtained by screen-pad printing

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so that the printed patterns could be easily observed. The patterns were found to be properly formed on the substrate by the screen-pad printing process; we expect it will be dif-ficult for conventional printing methods to form patterns on such a complex curved surface. Figure 3 shows a cross-sectional SEM image of the screen-pad-printed patterns formed on the convex curved surface of an acrylic plate. We could clearly see that Ag was formed along the curved surface, and its thickness was ~6 μm; such a thickness is difficult to obtain by conventional gravure-based pad print-ing. Figure 4 shows simple demonstration of the continuity check of the electrode pattern formed on the convex curved surface of a polycarbonate plate. It can be seen that the pat-tern has low resistance, indicating its conduction. Note that the printed electrode shown in Fig. 4 was not thermally treated; if we anneal it, much lower resistance must be obtained.

In our experiment, the patterns on the blanket were com-pletely picked up by the pad. Generally, organic solvents included in ink are absorbed into the silicone blanket (Lee et al. 2003). This creates a wet interface between the blan-ket and ink, whereas the interface between the ink and air becomes dry; such an ink is called “dry” ink (Kina et al. 2010). The viscosity of the dry ink is high because of the absence of the solvents. This can help prevent cohesion failure during transfer and ensures the complete transfer of the pattern from the blanket to the pad, facilitating the formation of thick patterns. On the other hand, if we use conventional pad printing, the ink maintains its original viscosity in the grooves because there is nothing to absorb the solvent in the ink. This causes cohesion failure of the ink, decreasing the thicknesses of the printed patterns.

Moreover, ink remaining in the gravure grooves gradu-ally dries and sticks to the groove surfaces. This results in the decrease of the apparent depth of the grooves and an increase in the thinness of the patterns formed.

We next compared large patterns of the order of cen-timeters, formed by conventional pad printing and screen-pad printing. In this experiment, the patterns were formed on polyethylene terephthalate (PET) films (HK-31WF, Higashiyama Film). These films were used “as-received” and highly adhesive treatment was performed on their surfaces. Figure 5a shows patterns sized 1.5 × 1.5 and 2 × 2 cm2 obtained by conventional pad printing. We observed that there was no ink at the center of the patterns. This is because the doctor blade is strongly compressed to remove the ink from the unnecessary (convex) part of the plate during doctoring. Unfortunately, this has a negative impact for the formation of large patterns because some of the ink in the grooves is also removed. In fact, the lack of a pattern at the center is inherently unavoidable in con-ventional gravure-based pad printing. Figure 5b shows a 3 × 6 cm2 pattern obtained by screen-pad-printing. This large pattern was found to be properly formed. The results shown in Figs. 2b and 5b imply that the screen-pad printing technique can accurately fabricate small or large patterns.

Fig. 4 Demonstration of the continuity check of the electrode pattern formed on a convex curved polycarbonate plate, obtained by screen-pad printing

(a) Conventional pad printing

Lack of the pattern

(b) Screen-pad printing

Fig. 5 Large printed patterns formed on PET films. a Square patterns of 1.5 × 1.5 and 2 × 2 cm2 obtained by conventional pad printing. b Rectangular pattern of 3 × 6 cm2 obtained by screen-pad printing

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In the screen-pad printing technique, screen printing is used to form patterns onto a blanket. Screen printing is a simple technique that lets the ink just fall down to the blan-ket through the open area of the screen mask. This, in turn, means that large patterns can be easily formed by screen printing.

Based on these results, our newly developed screen-pad printing technique seems to be very useful. Owing to its ability to form thick and large patterns, it should have a wider range of application compared to the conventional method. Another point to note is that because screen-pad printing is based on conventional screen printing, the machine can be easily constructed by simply adding a sili-cone blanket and a pad transfer system to a conventional screen printing machine. Based on these facts, we believe that screen-pad printing is quite valuable as a next-genera-tion fabrication technique, and contributes to the realization of forming various complex-shaped devices.

4 Conclusions

We have developed a new screen-pad printing method for the formation of electrode patterns on curvy surfaces. In this method, the inking technique used in conventional pad printing is changed, i.e., the inking in our method uses screen printing to a silicone blanket. This inking method makes it possible to obtain thick (t ~ 6 μm) and large (order of centimeters) patterns on complex curved surfaces, which are difficult to achieve using conventional gravure-based pad printing. Moreover, a screen-pad printing machine can be easily constructed by adding a silicone blanket and a pad transfer system to a conventional screen-printing machine. Thus, we believe that the developed screen-pad printing method is useful and contributes to the further development of various devices having complex shapes.

Acknowledgments The authors thank Drs. Ryosuke Mitsui and Shin-ichiro Nakajima of Japan Aviation Electronics Industry, Ltd. for their advice on the experiments. This research was financially

supported in part by Grant-in-Aid for Research Activity start-up (Pro-ject No. 26889075) from the Japan Society for the Promotion of Sci-ence (JSPS).

References

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Honda W, Harada S, Arie T, Akita S, Takei K (2014) Wearable, human-interactive, health-monitoring, wireless devices fab-ricated by macroscale printing techniques. Adv Funct Mater 24:3299–3304

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Kina O, Koutake M, Matsuoka K, Yase K (2010) Organic thin-film transistors fabricated by microcontact printing. Jpn J Appl Phys 49:01AB07

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