Influencing Parameters in Droplet Formation for DoD Printing of

Influencing Parameters in Droplet Formation for DoD Printing of
CICMT 2008
Influencing Parameters in Droplet Formation
for DoD Printing of Conductive Inks
Dominik Cibis, Klaus Krüger
Institute of Automation Technology, Helmut-Schmidt-University/University of the German Armed Forces,
Holstenhofweg 85, 22043 Hamburg, Germany
Dominik Cibis: Phone +49/(0)40/6541-2740, Fax +49/(0)40/6541-2004, E-Mail: [email protected]
Klaus Krüger: Phone +49/(0)40/6541-2722, Fax +49/(0)40/6541-2004, E-Mail: [email protected]
Abstract
The inkjet-printing principle is becoming more and more important for new applications besides conventional
graphic printing. In most cases, the nozzle diameter is the limiting factor when printing colloidal inks that contain
metal particles with different particle size distributions.
When printing conductive silver lines, high resolution on the substrate is desirable, meaning: Small width of the lines
and sharp edges. The volume and thus the diameter of the droplet and consequently the spot size on the substrate
does not only depend on the nozzle diameter of the print head. To a great extent, the droplet volume and velocity can
be controlled by the piezo signal. By varying piezo voltage and piezo pulse duration, the volume of a 100 µm nozzle
droplet can be modified in a range from 103 to 295 pL which results in droplet diameters from 58 to 82 µm.
Furthermore, the stability of the printing process depends on the energy provided to the droplet by means of the
piezo signal.
As a starting point, this article deals with a study that shows how the droplet’s volume and velocity depend on the
piezo parameters voltage and pulse length. This is done for micro feeding systems with three different nozzle
diameters: 50, 70 and 100 µm. It can be proven that the different volume ranges are overlapping, so that the droplet
volume generated with a 100 µm nozzle can be smaller than the volume of a droplet generated with a 50 µm nozzle.
However, the use of a 100 µm nozzle minimizes the risk of nozzle clogging.
Furthermore, the influence of the medium’s viscosity on the printing process is investigated by increasing the nozzle
temperature to lower the ink’s viscosity. When formulating a drop-on-demand-ink, adjustment of the surface tension
might be difficult. The addition of new components often changes the ink’s surface tension. Changes occurring in the
droplet’s characterisic diagram when dispensing inks of different surface tensions are shown in a final study.
Key words: drop-on-demand, ink-jet printing, piezo control signal, viscosity, surface tension
______________________________________________________________________________________________
sary to avoid sedimentation and agglomeration of
metal particles [2]. Often the ink viscosity and surface
tension are modified in the ink production process.
Print parameters such as piezo voltage, pulse duration
or nozzle temperature have to be adjusted to the ink to
make it printable.
This article shows the influences of these essential
parameters for stable droplet formation. Besides, the
velocity v in the moment of impact on the substrate or
the droplet volume V play an important role when
regarding line widths and thicknesses of conductors.
Furthermore, the mass transportation in a droplet, as
well as the ink’s viscosity and surface tension
influence the conductors’ sharpness of edges. Nozzle
heating and a variation of pulse energy and pulse
duration are indispensable means to control Volume
and velocity of a droplet.
1. Introduction
Over the last years, printing techniques like ink-jet
printing are interesting alternatives to conventional
photolithography or screen-printing for the production of electronic devices [1]. The advantages are not
only the flexibility and reproducibility of the droplet
formation and the possibility to deposit material on
nearly any desired location on top of a substrate, but
also the opportunity of saving material, e.g. silver or
gold, when printing conductive paths. The method
works contactless, droplets can be generated at frequencies of up to tens of kHz.
On the basis of an ink-jettable colloidal silver ink,
lines of different widths and thicknesses are printed
sucessfully at the Institute of Automation Technology
(IfA). Before printing, the formulation of such an ink
is of special interest. Stabilizing additives are neces-
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CICMT 2008
overcome the stabilizing influence of surface tension.
A small droplet of pure fluid tends to be spherical due
to the large surface to volume ratio [4]. In the
experiments done here, the ambient atmosphere is
kept constant so that the droplet will stay spherical
after its formation process has ended. The corresponding volume can be calculated as the volume of a
sphere with radius r.
2. Experimental details
The print head system that is used for the experiments
is a piezo-based ink-jet print head [3]. In this study,
three print heads of the same type are used, the nozzle
diameters of the heads are 50, 70 and 100 µm. Each
DoD print head is inserted in the drop watcher whose
setup can be seen in figure 1. The droplet formation
process is observed and evaluated, and photos are
taken at certain points in time to measure radius and
velocity of the generated droplets.
3. The print head’s characteristic diagrams
The piezo voltage and pulse duration are used to
control the droplet formation. For measurement of the
kinetic and surface energy, a Matlab program was
developed. Based on image processing, it calculates
the volume of spheric or rotation-symmetric bodies,
the droplet’s velocity as well as its kinetic and surface
energies.
The printing process is observed with a CCD camera
and a strobe lamp, which emits a flash at a certain
time after the raise of the piezo signal [5]. The piezo
voltage U and pulse duration time tP are the main
control parameters affecting the droplets’ velocity and
volume. Thus, different spot sizes on a substrate are
possible. As a result, different line widths and
resolutions of conductive paths are achievable.
Furthermore, the shape of the surface area of e.g.
conductive silver lines can be varied by changing the
droplets’ speed at the moment of impact. Due to that
and additionally in order to run the printer in a stable
range, a characteristic diagram should be measured
for each individual print head - ink combination.
Figure 1: The drop watcher system
The system is activated by means of a standard
voltage pulse. Standard means the signal shape does
not change, rise and dwell times of the pulse take
about 420 ns. Controllable parameters are pulse voltage and pulse duration. They influence the stability
of the droplet formation process and the energy
provided to a single droplet determines the droplet’s
velocity and its volume. Figure 2 depicts the droplet
formation process.
3.1. Measuring of volume and velocity
There are two types of characterictic diagrams:
Number one shows the change of the droplet volume
when changing tP and U, number two depicts the
achievable range of the corresponding droplet velocities. These characteristic diagrams are shown in
figures 3 and 4 and are given for the Microdrop print
head with a nozzle diameter of 100 µm.
To measure the volume of a droplet, the formation
process does not necessarily have to come to an end,
a filament that belongs to the droplet formation
process is permitted as well. The only restriction is
the displayable area of the CCD camera.
The velocity is calculated from the distance the
droplet has covered between two different strobe
delay times. In this case, calculation requires a
completed formation process. The solvent ethylene
glycol is used because it is printable very stably, that
means the droplet’s position at a certain strobe delay
does not change at all and the photo shows sharp
droplet edges.
Due to an overlay of 30 droplets on a photo (compare
figure 2), best results and best accuracy in volume
Figure 2: Droplet formation process
The shape and size of a fluid drop significantly
affects its motion as well as the associated heat and
mass transfer process. The drag force depends on the
drop shape and it is one of the factors that determines
the magnitude of the fluid velocity. The surface
tension force acts to minimize the surface energy and
therefore tends to keep a pure fluid drop spherical in
shape. The droplet will deform only when it is subject
to nonuniform or nonsymmetric forces, due to
pressure or temperature variations, such effects may
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CICMT 2008
300
V droplet in pL
250
200
140V
150V
160V
170V
150
180V
190V
200V
210V
100
220V
230V
240V
250V
50
5
10
15
20
25
30
35
40
45
50
55
t pulse in µs
Figure 3: Measured characteristic diagram of the droplet volume with medium ethylene glycole (100 µm nozzle)
7
140V
150V
6
160V
170V
180V
190V
5
200V
v droplet in m/s
210V
220V
4
230V
240V
250V
3
2
1
0
5
10
15
20
25
30
35
40
45
50
t pulse in µs
Figure 4: Measured characteristic diagram of the droplet velocity with medium ethylene glycole (100 µm nozzle)
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CICMT 2008
and velocity calculation are achieved when printing
pure solvent. Ethylene glycol has a surface tension of
46.8 mN/m and a viscosity of 17.4 mPas at 25 °C, so
the solvent is well printable at room temperature.
Heating of the print head nozzle is not necessary to
gain a stable droplet formation. Further investigations
prove that the droplet formation process is stable at
least over the immense number of 130 million droplets generated within the observation time of 36
hours.
The droplet volume, which is shown in figure 3,
reaches values in the range from 103 to 295 pL. This
means the 100 µm nozzle print head can generate
droplets with diameters from 58 to 82 µm when
printing ethylene glycole.
occur. Besides, the voltage ranges for stable droplet
formation differ.
Table 1: Volume and velocity ranges of different
nozzles with varying piezo voltage
Nozzle
Diameter
60 V
50 µm
110 V
120 V
Voltage Range
70 µm
Because of the periodic oscillation of the fluid
volume in the capillary of the print head, there exists
a pulse duration time where the droplet volume
reaches a maximum. This can be seen for example in
the curve with a value of tP of 28 µs and a pulse
voltage of 180 V. For the two green curves, corresponding to voltage pulses of 240 and 250 V, the
energy provided to the droplet between 14 µs and
40 µs is so high that the droplet, including its filament, does not fit on the screen after detaching from
the nozzle. A measurement can not be taken in this
case. Although the filament of the droplet is too long
and exeeds the observation area, the printing process
does not stop.
Another interruption of the curves can be seen in
figure 4, where the droplet velocity is depicted. If the
voltage and thus the provided energy is too high, the
generated volume is divided into two droplets in spite
of a single one, therefore no single droplet volume is
calculated. When building up conductive lines by the
DoD method, a splitting of the generated droplet is
not desired. Satellite droplets can reach the substrate
in unwanted positions and can cause short circuits.
Consequently only single droplets containing the
whole generated volume are taken into account in this
study, the cases of satellite droplets are not
considered. Below 140 V pulse voltage, the energy
provided to the droplet formation process is not
sufficient, so the filament does not detach from the
100 µm nozzle.
140 V
200 V
Voltage Range
94 - 103
0.3 - 1.0
48 - 217
0.9 - 4.9
60 - 214
0.2 - 4.3
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
128 - 166
0.1 - 0.5
72 - 218
0.2 - 3.5
95 - 315
2.8 - 4.2
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
V min V max
122 - 179
0.6 - 2.5
103 - 174
1.0 - 2.8
156 - 295
1.5 - 6.8
v min v max
90 - 200 V
150 V
100 µm
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
60 - 130 V
90 V
3.2. Interruption of the curves
Range with
t P-variation
Voltage
160 V
230 V
Voltage Range 140 - 250 V
A shorter pulse duration allows for smaller droplets
and there are overlapping ranges, which can be of
special interest. In defined (U, tP) combinations it is
possible to produce smaller droplets with the 100 µm
nozzle compared to the 50 µm nozzle. In the case of
printing conductive inks, that contain metal particles,
the advantages are obvious: The risk of nozzle
clogging is minimized when using the 100 µm nozzle
for printing conductive lines. Another advantage
arises: By having the possibility to control droplet
velocity and volume, the line width for an inksubstrate combination can be varied between 120 and
250 µm, dependend on the interfacial tension, and,
especially, the velocity and volume of the droplet. For
a good resolution and to facilitate most-densely
placed conductor boards, small widths of the
conductive paths and low droplet velocities and
volumes are desired to minimize the spreading effect
at impact on the substrate.
3.3. Comparison of different nozzle diameters
4. Influence of the medium’s viscosity and surface
tension
Figures 3 and 4 show, that the droplet volume and
velocity can be controlled significantly by changing
the pulse duration and the pulse voltage. The same
can be observed with the 50 µm nozzle and the 70 µm
nozzle print heads. The measured volumes and
velocities are listed in table 1, voltages are given
where maximum and minimum values of V and v
In the first chapters of this article, the solvent
ethylene glycole is used for printing tests to create the
print head’s characteristic diagrams. Ethylene glycole
is printable at room temperature with a dynamic
viscosity of 17.4 mPas at 25 °C. Having a surface
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CICMT 2008
tension of 46.8 mN/m, this solvent shows a very good
droplet formation behaviour. That means, there exists
a wide range of pulse voltages and pulse durations
where droplets can be generated in a stable way, and
the surface tension allows for a fast formation of a
spheric droplet.
When formulating a drop-on-demand-ink, an adjustment of the ink’s surface tension might be difficult.
The addition of new components often changes the
ink’s surface tension, but the viscosity as well. E.g.
water with a viscosity/surface tension combination of
0.9 mPas/72.1 mN/m at 25 °C cannot be printed very
well. Many satellite droplets occur and the printing
process is not reproducible, often the droplet generation just stops.
This part of the article shows the influence of the
medium’s viscosity and surface tension on the
printing process, especially on the droplet parameters
velocity and volume. Three solvents with different
surface tensions are used in this study: Ethylene
glycole, terpineol and a glycerin derivate.
The solvents are newtonian fluids and thus the
viscosity does not depend on the shear rate. For
particle loaded inks, the newtonian property is desired
as well, but it is not realizable at all times. During
droplet generation, the shear rate in the nozzle
exceeds 100000 s-1. Practical experience has shown
that 1000 s-1 is a suitable shear rate for judgement of
the ink printability.
In table 2, the marked data indicate the temperature
where the solvent’s viscosity is nearly 14 mPas. By
keeping the viscosity constant, the influence of the
different surface tensions can be shown. The nozzle is
heated up to this indicated temperature for each print
medium. Higher nozzle temperatures are also
permitted and enable printing, but viscosities are not
comparable then. The surface tensions given in
table 2 are valid for 25 °C. They only change very
slightly with temperature and can be assumed to be
constant.
4.2. Influence of the viscosity on V and v
The volume and velocity of DoD-generated droplets
can be varied by changing the voltage pulse height U
and thus the provided energy or by changing the pulse
duration tP (cf. figures 3 and 4 and table 1). Another
parameter which can be used to increase V and v is
the nozzle temperature and hence the viscosity of the
printed medium. This can be seen in figures 5 and 6
for the medium glycerin. No droplets are generated
below a nozzle temperature of 45 °C, the viscosity is
too high and out of the range where the print head
allows for a detachment of the fluid from the nozzle.
The duration of the piezo pulse is kept constant at
tP = 25 µs for this study, only voltage and temperature
are modified.
4.1. Viscosity of the print medium
To allow for stable printing, the solvent or ink should
not exceed a print head specific viscosity. In the case
of the print head that is used in these experiments, the
viscosity where droplet formation starts is about 40
mPas in the nozzle for media with a high surface
tension like the glycerin derivate. For media with a
lower surface tension, like most non-aqueos liquids,
the viscosity must be lower. Table 2 shows the
temperature dependency of the dynamic viscosity of
ethylene glycole, terpineol and a glycerin derivate.
Table 2: Temperature dependency of viscosities
Viscosity in mPas at 1000 s-1
Nozzle
Temperature
TER
EG
a) Isolines of constant voltage
GLY
30 °C
-
14.2
-
35 °C
18.9
11.7
-
40 °C
13.6
9.7
-
45 °C
10.0
8.2
38.4
50 °C
7.7
7.0
31.8
55 °C
6.1
6.0
26.8
60 °C
4.9
5.3
23.2
65 °C
4.1
4.7
20.3
70 °C
3.4
4.2
18.1
75 °C
2.9
3.7
16.3
80 °C
2.5
-
14.9
85 °C
2.2
-
13.6
Surface
Tension
31.4 mN/m
46.8 mN/m
62.5 mN/m
If the provided energy is low e.g. in case of 125 V, a
very high temperature and low viscosity of the
medium is needed to run the printing process. In case
of higher voltages, e.g. 195 V, the energy is sufficient
to allow for printing with a higher viscosity of the
medium. The droplet formation starts at a lower
temperature (45 °C) meaning the ranges of volume
and velocity are wider, using glycerin they span from
180 pL to 290 pL and from 0.8 m/s to 6.2 m/s (with
U = 195 V). Consequently, higher piezo activating
voltages help to increase the ranges of V and v when
increasing the nozzle temperature in parallel to lower
the ink viscosity (45 °C – 85 °C).
b) Isolines of constant nozzle temperature
For the next study, a constant nozzle temperature is
considered. It can be said that the voltage ranges
where a stable droplet formation process can be
observed is increasing with decreasing viscosity.
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CICMT 2008
320
300
280
260
V droplet in pL
240
195V
190V
185V
180V
175V
170V
165V
160V
155V
150V
145V
140V
135V
130V
125V
220
200
180
160
140
120
100
40
45
50
55
60
65
70
75
80
85
90
Nozzle Temperature in °C
Figure 5: Temperature dependency of measured droplet volume with medium glycerin (100 µm nozzle)
7
6
v droplet in m/s
5
195V
190V
185V
180V
175V
170V
165V
160V
155V
150V
145V
140V
135V
130V
125V
4
3
2
1
0
40
45
50
55
60
65
70
75
80
85
Nozzle Temperature in °C
Figure 6: Temperature dependency of measured droplet velocity with medium glycerin (100 µm nozzle)
000422
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CICMT 2008
Using the glycerin derivate at a temperature of 45 °C,
a voltage of 195 V is needed to produce droplets and
it is the only value that works. For 85 °C, droplets are
generated within a voltage range from 125 to 195 V
(15 different voltages tested). The lower viscosity
reduces the minimum voltage to start droplet generation. Table 3 lists the intervals of achievable
droplet volume and velocity ranges at a certain nozzle
temperature. The change in V and v is realized by
increasing the piezo activating voltage from a
minimum value, where droplet generation begins, up
to a maximum value, where the droplet splits into
many smaller droplets due to the high amount of
supplied energy. The column for glycerin corresponds
to figures 5 and 6 and table 3 is supplemented by the
solvents ethylene glycol and terpineol. The areas
marked red indicate the temperatures where the
medium’s viscosity is about 14 mPas.
tension of 46.8 mN/m, with the advantage of being
printable even at room temperature. When formulating a colloidal DoD-ink, components with a low
boiling point in the liquid phase can cause problems
at higher nozzle temperatures. As a result, nozzle
clogging effects can occur and stop the printing
process.
5. Conclusions
Conductive lines of different widths and thicknesses
can be printed at the IfA using a DoD printer and
colloidal inks. The print head is based on a piezo
actuator and can handle a couple of inks with
different viscosities and surface tensions. With a
Matlab based bitmap evaluation program, the DoD
generated droplets’ velocities and volumes are
measured. In a first step, a characteristic diagram
shows the dependency of velocity and volume on the
pulse duration and on the piezo pulse voltage. The
volume of a droplet can be tripled by adjusting the
pulse duration and it is even possible to produce
smaller droplets with a 100 µm nozzle than with a
50 µm nozzle, dependend on the print parameters. In
a second step, three different media with different
surface tensions are analyzed. Varying the viscosity
by heating the print head nozzle, the influence of the
solvent’s viscosity on the volume and velocity ranges
of the generated droplets are pointed out. Having the
same viscosity when using an adjusted nozzle
temperature, the influence of the inks’ surface
tensions on the printing process can be shown.
In a next step, the shape of the piezo signal will be
varied to be able to optimize the print parameters and
signal shape to enlarge the amount of printable inks.
Table 3: Volume and velocity ranges at different
nozzle temperatures
Nozzle
Temperature
30 °C
35 °C
40 °C
45 °C
50 °C
55 °C
60 °C
65 °C
70 °C
75 °C
80 °C
85 °C
TER
EG
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
V in pL
v in m/s
no droplet
formation
169 - 181
0.9 - 1.7
150 - 192
0.8 - 3.9
143 - 248
1.0 - 6.4
146 - 298
1.1 - 7.3
143 - 301
0.9 - 7.8
131 - 314
0.9 - 8.2
121 - 315
0.9 - 8.2
114 - 321
1.0 - 8.4
104 - 301
1.1 - 7.6
138 - 380
1.5 - 6.2
134 - 340
1.0 - 8.1
162 - 280
1.0 - 5.0
156 - 305
1.0 - 5.5
155 - 344
1.1 - 6.2
131 - 351
0.6 - 6.3
146 - 356
0.9 - 6.4
153 - 362
1.2 - 6.4
176 - 363
1.3 - 6.7
178 - 367
1.4 - 6.5
174 - 351
1.3 - 6.7
169 - 346
1.5 - 6.6
Surface
Tension
31.4 mN/m
46.8 mN/m
no single
droplet
GLY
no droplet
formation
179
0.8
169 - 220
1.1 - 2.5
151 - 225
0.8 - 3.8
144 - 241
0.9 - 5.3
151 - 267
1.2 - 5.7
135 - 281
1.1 - 6.0
129 - 285
1.0 - 6.2
125 - 295
0.9 - 6.2
122 - 294
1.0 - 6.3
References
[1] J. Perelaer, “Ink-jet Printing and Microwave
Sintering of Conductive Silver Tracks”, Journal of
Advanced Materials, Vol. 18, pp. 2101-2104, 2006
[2] U. Currle, “Observations on Particle Loaded
Silver Inks”, 16th European Microelectronics and
Packaging Conference, Oulu, Finland, June 17.-20.,
2007
[3] Microdrop Technologies GmbH, Micro Dispensing Systems of liquids in the nano- to picoliter range,
Norderstedt, Germany, http://www.microdrop.de/
[4] S. S. Sadhal, “Transport Phenomena with Drops
and Bubbles”, Springer-Verlag, New York, Chapter 2, p. 17, 1997
[5] D. Cibis, “System Analysis of a DoD Print Head
for Direct Writing of Conductive Circuits”,
International Journal of Applied Ceramic Technology, Vol. 4 [5], pp. 428-435, 2007
62.5 mN/m
As the viscosities in the areas marked red are almost
the same, the influence of the solvents’ surface
tensions on the volume and velocity ranges can be
shown. Terpineol with the lowest surface tension
allows for the slowest droplet speed at maximum
voltage and a small volume range at low values.
Glycerin with the highest surface tension allows for
droplets with the fastest speed and a much bigger
volume range. However, the best droplet formation
behaviour and the most stable printing process
without interruptions due to nozzle plate cleaning is
realized with ethylene glycole, having a surface
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