75

Printed Electronics on Flexible Substrate Using Inkjet TechnologyShinichi Nishi, Kazuo Asano, Daisuke Ishibashi, Akiko Kitami and Kumiko Furuno

KonicaMinolta IJ Technologies, Inc., Tokyo 191-8511, Japan

(Received August 5, 2009; accepted November 9, 2009)

Abstract

Inkjet technology is believed to be suitable for the mask-less production of various electronic devices. Printed electronics

using inkjet technologies have major benefits as shown below: 1) direct printing of metal patterns on large and/or flexible

substrates is possible, 2) the waste of coating materials, which are usually expensive, is minimized. In this paper, the

jetting characteristics of Ag nano particle dispersed ink with shear mode piezo print heads are described. A 2.7 pl droplet

can be ejected with a minimum line width of 70 μm. For applications with flexible substrates, the inkjet patterning of

electrodes for PCBs and thin film coating on PET substrates are described using a line-head module.

Keywords: Inkjet, Printed Electronics, Flexible Substrate, Ag Nano Ink, Line-Head Module

1. IntroductionRecently inkjet technologies have been applied to indus-

trial fields. In Figure 1, examples of applied inkjet technol-

ogies are classified. Printing mode is classified as shuttle

type or single pass type, indicated on the upper and lower

halves, respectively. Application is classified as normal

printing of images and/or characters and industrial print-

ing of lines and/or patterns, indicated on the right and left

sides, respectively. In the electronics field, inkjet pro-

cesses have been used for printed electronics and display

manufacturing. Examples of those applications are PCBs,

Ceramic circuits, IC packages, LCDs, PDPs, OLEDs, and

Solar Cells.

For those applications, many kinds of substrates are

used: rigid or flexible, sheet or roll, and smooth or rugged

surfaces. Recent attentions have been focused on flexible

substrates, especially plastic sheets, such as PI (polyim-

ide), PET (polyethylene terephthalate), and PEN (polyeth-

ylene naphthalate). The electronic devices and modules

using the above plastic substrates are suitable for light-

weight or handy products, and 3R policy (Reduce, Reuse,

Recycle) in respect of environment requirements.

We have developed a shear-mode piezo inkjet print head

which is operated most effectively and has contributed to

the new applications in the industrial field.

The structural model of the piezo actuator of the print

head is shown in Figure 2.[1]

2. Inkjet Technology for Printed ElectronicsInkjet technology is a remarkable process whereby a

small droplet of special liquid can be placed at a requested

point with high accuracy and at a precise volume. The com-

bination of those droplets makes a line pattern or area pat-

tern freely according to a digital image designed previously.

Utilizing the merits of inkjet technology, printed elec-

tronics has become a focus because of the features

described below:[2] 1) direct drawing without a mask on a

large substrate, 2) manufacturing of low volumes of many

Fig. 1 Industrial printing fields using inkjet technologies. Fig. 2 Shear-mode piezo actuator.

Nishi et al.: Printed Electronics on Flexible Substrate Using Inkjet (1/4)

76

Transactions of The Japan Institute of Electronics Packaging Vol. 2, No. 1, 2009

kinds of devices 3) real-time production by digital design

4) low levels of loss in coated materials.

We have established targets for developing a shear-

mode piezo print head.

1) The many kinds of inks which are used in printed

electronics must be compatible with the material-compo-

nents used in constructing a print head. These include

aqueous inks which are acidic or basic, organic solvent

based inks which are various ethers, esters, ketones, and

aromatic hydrocarbons, as well as special nitrogen contain-

ing cyclic compounds. We have developed a newly synthe-

sized epoxy glue, engineering plastics for the ink manifold,

and a passivation layer for the electrode of the piezo mate-

rial, which contact the ink fluid directly.

2) A highly sensitive piezo material, PZT (Pb(Zr,Ti)O3

ceramics), was selected and relatively high viscous ink

should be ejected at low driving voltages, below 20 V.[3]

3) Good directionality of the ejected droplets is required

for highly precise patterning. We achieved an average angle

deviation of droplets less than 0.1 degree as 1σ.

4) According to the required line width, which is

proportional to the droplet volume, and the thickness, we

developed 4 types of print heads which can eject droplets

of different volumes, 42 pl, 14 pl, 6 pl, and 4 pl, respec-

tively. In Table 1, the line-up of print heads is shown.

3. Experiment3.1 Inkjet head

We used the 42 pl head for large area pattern drawing

and the 4 pl head for fine patterns.

3.2 InkWe used various kinds of sample inks supplied by many

ink manufacturers. Ag nano-metal particles dispersed inks

were used as oil-based or organic solvent-based inks.

3. 3 Direct printing systemWe tested the jetting properties of various kinds of metal

dispersed inks using a drop-watcher system. Metal

electrode patterns were obtained using a direct printing

system easily operated for experimental research as

shown in Figure 3. The system is constructed from 1) an

XY stage which scans the print head as the X axis and the

media on the table as the Y axis, 2) a memory board and

head driving board which generate the waveform of pulse

ejecting droplets as required for the drawing patterns, 3) a

PC and software which control the electric boards and XY

stage movement, 4) an ink storage syringe which supplies

ink to the print head by a static pressure difference.

4. Results and Discussion4.1 Inkjet ejection of Ag nano particle dispersed ink

For stable ejection of nano-metal particle dispersed ink,

it is necessary that the aggregation and precipitation of

particles be inhibited under all conditions and that the vis-

cosity of the ink used not be increased at the meniscus at

the liquid-air boundary near a nozzle of the print head. The

preferable viscosity of ink is from 5 to 15 mPa sec.

The properties of the ink and printing conditions are

listed below;

1) Ag nano particle dispersed ink:

viscosity at 20°C 14 mPa sec

density 1.8 g/cm3

metal contents 55 wt%

solvent tetradecane

2) print head: 512 nozzles

droplet volume 2.7 pl

droplet weight 4.8 ng

driving frequency 1.6 kHz

applied voltage 14 V

drive pulse unit width 4.1 μsec

Table 1 Various print heads with different numbers ofnozzles and droplet volumes.

SharedWall

NozzleNumber

NozzleDensity

[npi]

Usable Ink DropVolume

[pl]

EjectingFrequency

[kHz]

L

512 360

Solvent-based 42 7.6

M Ink 14 12.8

S AggressiveSolvent Ink

4 20

IndependentWall

NozzleNumber

NozzleDensity

[npi]

Usable Ink DropVolume

[pl]

EjectingFrequency

[kHz]

M 256 180 Aqueous Ink 15 15

M 128 90 Aqueous Ink 15 15

S 128 90 AggressiveSolvent Ink

6 20–40

Fig. 3 A view of the whole direct printing system.

77

The ejected droplets were monitored by a drop-watcher

apparatus. The results are shown in Figure 4. Stable ejec-

tion was observed for a long period at room temperature.

The ejection characteristics of the Ag nano particle dis-

persed ink with 4 pl print head are shown in Figures 5 and 6.

In Figure 5, a linear relationship was observed between

the applied voltage which generates the drop-ejecting

acoustic power from the piezo walls and the drop velocity

of the ejected drop measured 1.3 mm from the nozzle

plate. This linearity proves the stable ejection of the Ag

nano particle dispersed liquid by the piezo print head. In

the drawing pattern experiment, the drop velocity was 6

m/sec at 14 V applied to the print head used.

In Figure 6, the linear relationship was also observed

between the drop velocity and the drop weight. The slope

of the line represents the sensitivity of drop size to velocity

depending on ejection frequency. A low frequency seems

to result in relatively small drop generation.

According to the above results, the drop weight, i.e., the

drop volume, can be controlled mainly by the applied

voltage.

The dot size on the OHP sheet was 42 μm diameter as

shown in Figure 7.

Using the direct printing system with the 42 pl print

head, a sample print of an Ag electrode pattern for a

printed circuit board was obtained on PI film and cured at

150°C for one hour as shown in Figure 8. An enlargement

of the pattern of Figure 8 is shown in Figure 9. The mini-

mum line width was 70 μm when drawn one droplet wide

on a PI sheet treated with N2 plasma.

In further experiments with 4 pl print heads, we

observed a 30 μm wide Ag pattern on a fluorine-polymer

layer coating on a PET sheet. The layer made a large con-

tact angle with the Ag dispersed ink, over 80 degrees.

These results indicate that the surface energy of the pat-

terned substrate is the most important factor in determin-

ing the line width.

The diameter of a 42 pl drop is 43 μm, that of a 4 pl drop

is 20 μm, and that of a 1 pl drop is 12 μm. This relationship

means that the line width is not only determined by drop

volume but drop diameter. Furthermore, a drop will

Fig. 4 Ejected droplets monitored by drop- watcher.

Fig. 5 Relationship between applied voltage and velocity ofejected drop.

Fig. 6 Relationship between velocity of ejected drop andweight of the drop.

Fig. 7 Ejected dots on OHP sheet for IJ printing.

Nishi et al.: Printed Electronics on Flexible Substrate Using Inkjet (3/4)

78

Transactions of The Japan Institute of Electronics Packaging Vol. 2, No. 1, 2009

spread on contact with the surface of the substrate depend-

ing on the surface energy of the substrate. A non-wetting

surface makes small dots and a fine pattern. We estimate

that a 1 pl print head can make a fine pattern with lines

approximately 20 μm wide.

4.2 Inkjet printing on flexible substrateWe studied constructing an inkjet coating system for

flexible sheets, usually called a web coating apparatus.

One example was an apparatus for PET roll film in which

a line-head module with 12 heads was installed. The mod-

ule shown in Figure 10 had 6144 nozzles in total and was

216 mm wide. The nozzle pitch was 720 npi (nozzles per

inch), or 35 μm. The droplet volume was 14 pl.

Ejecting droplets at 12 kHz, the coating speed was 400

mm/sec. When a uniform coating of PI film is required, 14

pl droplets make a film 5 μm thick wet, which results in a

thickness 0.5 μm dried if the density of the ink is 10 vol%.

Using the line-head module shown in Figure 10, an example

of the obtained optical anti-reflection film on TAC (Triacetyl

cellulose) film showed good uniformity of thickness, 0.5 μm

± 2%.

5. ConclusionsInkjet technology is suitable for the mask-less produc-

tion of various electronic devices. The shear-mode piezo

print head and Ag nano particle dispersed ink make fine

electrode patterns by direct drawing. A line-head module is

usable for coating flexible substrates.

AcknowledgmentsThe authors are grateful for supplies of Ag nano particle

dispersed inks by ULVAC, Inc., Harima Chemicals, Inc.,

and Cabot Specialty Chemicals, Inc.

References

[1] K. Komatsu, M. Ueda, S. Uraki, H. Arakawa and T.

Uno, “Development of New Inkjet Head for the

Display Panel Industry”, KonicaMinolta Technology

Report, 3, 129–132 (2006).

[2] S. Nishi, “Direct Metal Patterning for Printable

Electronics by Inkjet Technology”, Proceedings of the

IMAPS 2006, TP63, San Diego, CA, 8–13 (2006).

[3] S. Nishi, “Direct Patterning for Printable Electronics by

Inkjet Technology”, Proceedings of the IMAPS/ACerS

CICMT 2007, WA1, 183, Denver, CO, 23–26 (2007).

[4] S. Nishi, “Printable Electronics on Flexible Substrate by

Inkjet Technology”, Proceedings of the International

Conference on Electronics Packaging, 289–293 (2009).

Fig. 8 A sample of printed electrode pattern on a PCB.

Fig. 9 An enlargement of the printed electrode of the PCBpattern from Figure 8.

Fig.10 A line-head module constructed with 12 heads.