2011 13th

Electronics Packaging Technology Conference

Processing and Characterization of Flexographic Printed Conductive Grid Lok Boon Keng*, Wai Lai Lai, Lu Chee Wai Albert, Budiman Salam

Singapore Institute of Manufacturing Technology (SIMTech), Large Area Processing Programme

71 Nanyang Drive, Singapore – 638075

*Corresponding Author: Tel: +65- 67938998, Fax: +65- 67922779, Email: bklok@simtech.a-star.edu.sg

Abstract

In this paper, the conductive grids were flexographic

printed with solvent based conductive ink. Elastomeric plate

with 45° grid of 30µm line and pitch of 0.4mm was prepared.

Surface resistance and optical transmittance of the printed film

were characterized by two-electrode method and UV-VIS-NIR

spectrophotometer respectively. At a comparable transmission

level (83% measured at 600nm, without substrate), the sheet

resistance obtained (11 ohm/square) by printed grid was about

50% lower than that of high conductivity ITO film (24

ohm/square).

Background

ITO (Indium-doped Tin Oxide) film is widely used in

lighting, displays, photovoltaics and touch sensors as

transparent electrodes. Post-processing such as

photolithography and etching is usually required to create

patterns on the film surface. ITO is known to be brittle and is

less flexible [1, 2]. Hence it is not suitable for devices that

require extensive bending and rolling. Two broad approaches

for improving the conductivity of transparent conductor have

been reported: advanced material development (conductive

polymer, carbon nanotube, conductive nano-fiber [2]) and

advanced manufacturing in patterning of conductive grids [3].

Photographic process of diffusion transfer reversal of silver

grid was reported as an alternative technique. [4]

Flexographic printing is one of the potential roll-to-roll

patterning technologies that can be adopted to perform

additive deposition of functional materials. Flexography (often

abbreviated to flexo) is a type of graphic printing process

which utilizes a flexible elastomeric plate. It is an improved

version of letterpress or rubber stamp that can be used for

printing on almost any type of substrate including plastic,

metallic films and paper. It is also widely used for printing on

the non-porous substrates required for various types of food

packaging. A typical flexo printing roller assembly is

illustrated as in Figure 1. The flexo setup consists of an inking

tray or chamber (where ink is stored), anilox roller (which

captures accurate ink volume in the arrays of micro-

cavities/cells), plate roller (which receives ink from anilox

roller onto raised surfaces) and impression roller (where the

ink transfer occurred when the plate contacts the substrate).

Plate

Roller

Impression

Roller

Anilox

Inking tray

Substrate

Plate

Roller

Impression

Roller

Anilox

Inking tray

Substrate

(a)

(b)

Figure 1 (a) Schematic diagram of a flexographic printing roller assembly; (b)

a typical printing stage on a roll-to-roll flexographic printer

The flexible elastomeric plate making process is similar to

photolithography process in semiconductor and electronics

industry. The plate has a layer of light sensitive polymer on a

flexible carrier. The plate making process has advanced from

conventional negative film exposure technique to laser direct

writing process. With such advancement in elastomeric plate

for pattern transfer and ink development, the patternability of

fine features has become promising.

Experimental Details

Pre-treated Polyethylene terephthalate (PET) substrate

from Toyobo was used in this study. The PET was a biaxially

oriented film which was pretreated to have better smoothness

and transparency. The flexographic printing was carried on

IGT F1-UV printability tester (Figure 2). The tester comprises

of anilox roller, doctor blade, plate roller and print roller.

Figure 2 IGT printability tester F1-UV

A printing plate was made of photopolymer. Test pattern

was transferred on to the plate via direct laser writing followed

by developing. Test pattern was 45° grids of 30µm lines and

pitch of 0.4mm (Figure 3). Printing plate was attached to plate

cylinder with double sided adhesive mounting tape.

A solvent based silver nano-particle was used in this

experiment. The ink was selected for thin profile of deposition

and it can be cured in a shorter time. Ink was dispensed on

anilox roller. Ink was leveled and well distributed to the

individual cell on the anilox before ink was transferred to

flexo plate. In this experiment, the print speed and the anilox

volume was fixed at 0.3m/s (i.e. 18m/min) and 1.8BCM

(Billion Cubic Microns). The print force and anilox were set at

Impression

roller

Inking

unit

Anilox

Plate Roller

978-1-4577-1982-0/11/$26.00 ©2011 IEEE

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2011 13th

Electronics Packaging Technology Conference

10, 30N and 50, 100N respectively. The printed film was

cured in oven for 10min under 110°C.

The profile of printed film was obtained by Veeco white

light interferometry system. The transmission characteristic of

the printed film was measured on UV-VIS-NIR

spectrophotometer. The printed samples were scanned from

300nm to 2000nm. The sheet resistance of the film was

measured according to the setup in Figure 3. Screen printed

silver paste of 10 X 20 mm was used as electrodes. Area of

coverage of conductive grids was 20 X 20mm.

A

g

A

g

20 X 20mm

R

Figure 3 Measurement of equivalent sheet resistance of printed conductive

grid

Effects of Impact Forces

Figure 4 Optical image of flexographic printed grid

Optical image of flexo printed grid is shown in Figure 4.

There are two impact forces involved in flexo printing: anilox

force (Fa) and print force (Fp). Anilox force is the force

exerted on the flexo plate when anilox is brought in contact

with print plate to transfer the ink in the anilox cavity to the

protruding surface of flexo plate. Print force is required when

flexo plate is to transfer the ink on the patterned surface onto

substrate.

It was observed that lower print force had caused less

spreading of ink (Table 1). It was about thrice the variation in

printed line width when print force increased 3 times. The

effect of anilox force was more prominent when the print

force was set to low. When anilox force was doubled, the line

width increased 25% at low Fp and 6% at high Fp. (Figure 5)

Table 1 Average line width (µm) and standard deviation of

printed lines under different printing conditions

Fp (N)

Fa (N)

Low

10

High

30

Low 50 32.09±1.89 42.42±5.40

High 100 40.71±2.68 44.94±5.61

20

25

30

35

40

45

50

55

50 100

Anilox Force (N)L

ine W

idth

(µ

m)

10 30

Print Force

Figure 5 The effect of print force and anilox force on printed line width

Ink transfer typically can be split into three phases:

contact, immobilization and splitting [5]. The increase in line

width could be resulted from two aspects: more inks could

have picked up from the anilox roller when anilox force was

increased and hence more inks were transferred to substrate;

and inks had spread more when higher contact force exerted.

When the contact force increases, the flexo plate as an

elastomeric material deforms if the force exerts on it larger

than a critical value. A hollow effect in the centre of the lines

could be observed while the contact force is excessively large.

This is mainly caused by the deformation of the plate (Figure

6) - majority of the ink has been squeezed to the edges of the

patterned features on the plate [6].

Actual

Deformation

Flexo

Plate

Substrate Ink

Actual

Deformation

Flexo

Plate

Substrate Ink

Figure 6 Ink transfer at the contact of raised surface of flexo plate and

substrate. Excessive contact force will drive the inks to the edges

518

2011 13th

Electronics Packaging Technology Conference

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 10 20 30 40 50 60 70 80 90 100

Length (µm)

Th

ick

nes

s (µ

m)

Figure 7 Typical profile of a flexographic printed line

Figure 8 Profile of the printed gird under low anilox force and high print

force

Flexo plate

(a) (b)

Figure 9 (a) Image of a patterned flexo plate with grid protrusion; (b) images

of anilox cells/cavities before inking and after ink transfer. The inks in the

cavities transferred the protrusion surface of plate while in contact.

Optical Transmission

PET from Toyobo was used in this experiment. Pretreated

PET Melinex 506 from DuPont Teijin was measured as

reference. It was shown in Figure 10 that both PET have

transmission of 90% in the visible light spectrum (390 to

750nm). The low sheet resistance ITO coated film with

24ohm/square absorbed more green light compared to blue

and red and resulted into not uniform transmission

characteristic in the visible light spectrum. Comparatively the

PEDOT:PSS coated PET with 31ohm/square exhibited more

uniform transmission in blue, green light and less transparent

to orange and red light. Similar observation was reported by

Yang and Nishii [7, 8].

0

10

20

30

40

50

60

70

80

90

100

200 300 400 500 600 700 800 900 1000

Wavelength (nm)

Tra

nsm

issio

n %

PET_TYB PET_506

ITO/PET_24ohm PEDOT/PET_31ohm

Grid #1 Grid #2

Grid #3 Grid #4

Figure 10 The transmission plot included two types of PET from different

suppliers: Toyobo and DuPont; transparent conductive film: ITO sputtered

PET and PEDOT:PSS coated PET; four printed samples (#1 to #4) with

different line width resulted from different print conditions

Depending on the width of the printed lines, printed grids

generally reduced the transmission of the visible light but in a

uniform manner. Assume that the printed area which was

covered by silver particles were opaque to light and the

incident light were vertical to the film surface, the opacity of

the film per unit area was calculated based on the geometric

calculation of the printed area. The data obtained was

compared to the transmission data obtained by

spectrophotometer (Figure 10). As the least square estimator

was approximately 0.85, the transmission level of the printed

grid could be estimated by a simple geometric calculation

(Figure 11)

y = 1.0274x

R2 = 0.8474

50

55

60

65

70

75

80

85

70 71 72 73 74 75 76 77 78 79 80

Transmission by Geometric Calculation

Tra

nsm

itta

nce w

/o S

ub

str

ate

(measu

red

@600n

m)

Figure 11 Correlation of measured transmission @ 600nm on

spectrophotometer and transmission calculated based on geometric coverage

of printed lines

Electrical measurement

The equivalent sheet resistance of printed film was

measured as per described in previous section Figure 3. The

sheet resistance in a grid largely depends on the cross-

sectional area and width of the lines. It was difficult to

determine accurate thickness of the printed line in an area due

to the uneven cross-sectional distribution of transferred ink

(Figure 7). Although the effective transferred volume of the

conductive inks might be varied under different printing

conditions, the sheet resistance of the printed grid was found

inversely proportion to the width of the printed film. When the

line width of printed grids increased, the sheet resistance

improved (Figure 8). The transmittance of the printed grids

Anilox Cell after

Ink Transfer

Anilox Cell

before Inking

Ink

Ink transferred

to substrate

Anilox Cell after

Ink Transfer

Anilox Cell

before Inking

Ink

Ink transferred

to substrate

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2011 13th

Electronics Packaging Technology Conference

without substrate showed well correlation to the equivalent

sheet resistance (Figure 13).

50

55

60

65

70

75

80

85

90

95

100

20 25 30 35 40 45 50

Average Line Width (µm)

Tra

nsm

itta

nce w

ith

ou

t S

ub

str

ate

(%)

0

5

10

15

20

25

Sh

eet

Resis

tan

ce (

oh

m/s

qu

are

)Ag Ag

20 X 20mm

R

Figure 12 Equivalent sheet resistance and transmittance of the printed grids

with offsetting the substrate were plotted against average printed line width

y = 2.2901x + 57.421

R2 = 0.9647

50.0

60.0

70.0

80.0

90.0

100.0

4 5 6 7 8 9 10 11 12

Sheet Resistance (ohm/square)

Tra

ns

mit

tan

ce

wit

ho

ut

Su

bs

tra

te (

%)

Figure 13 Transmittance of the printed grids without substrate was plotted

against equivalent sheet resistance

Conclusions

Flexographic printing of conductive grid was studied with

an ink printability tester. At a comparable transmission level

(83% measured at 600nm, without substrate), the sheet

resistance obtained (11 ohm/square) by printed grids was

about 50% lower than the high conductivity ITO film (24

ohm/square) whereas the coated PEDOT:PSS film was about

63% transmittance with 31ohm/square. Further verification of

the results on industrial printer is to be carried out. Further

study of ink volume transferred and process simulation is

essential to achieve accurate prediction of sheet resistance

required.

Acknowledgments

The authors would like to thank Singapore Institute of

Manufacturing Technology (SIMTech) and the Science and

Engineering Research Council of the Agency for Science,

Technology and Research (SERC, A*Star) of Singapore for

the project funding and IGT Singapore for supporting the

printability tester F1-UV.

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