Oct. 2008, Volume 2, No.10 (Serial No.11)
Journal of Materials Science and Engineering, ISSN1934-8959, USA
An investigation of mechanical performance of silver
inkjet-printed structures
Umur Caglar, Kimmo Kaija, Pauliina Mansikkamäki
(Department of Electronics, Tampere University of Technology, Tampere FI-33101, Finland)
Abstract: In this paper, we investigated the mechanical
performance of inkjet-printed structures with silver nanoparticle
ink and used an adhesion pull-off test together with optimized
ISO and ASTM. Adhesive silver test patterns were
inkjet-printed on polyethylene naphthalate (PEN), polyimide
(PI), and liquid crystal polymer (LCP). During printing, the
spreading of the silver ink was controlled with an electronic
coating, and its effect on adhesion performance was evaluated.
After printing, samples were sintered in two different profiles,
and the effect of sintering on their mechanical properties was
investigated. In addition, we analyzed the dynamic mechanical
stress on the printed samples at temperatures from -60 ℃ to
100℃ and their adhesion performance after humid condition of
85℃/85% RH.
Key words: inkjet printing;
performance; adhesion
nanoparticles;
mechanical
1. Introduction
By its expedited manufacture, inkjet technology
has gained several advantages, such as flexibility, low
costs, and low environmental impacts, over the current
electronic manufacturing processes[1-3]. Unlike the
lithographic techniques, inkjet technology also boasts
fewer process steps from drawing to product[4].
Consequently, inkjet printing technology is being
implemented in electronics manufacture to produce
partly[5-6] or fully[7-9] active and passive components.
Nanoparticles have been commonly used in the
formulation of inkjet printing conductive inks because
Acknowledgment:
The authors wish to thank the VICINICS
Industrial Consortium and the Finnish Funding Agency for
Technology and Innovation (TEKES) for their support.
Corresponding author: Umur Caglar, male, researcher, M.
Sc. materials engineer; research fields: printable electronics,
inkjet printing technology. E-mail: umur.caglar@tut.fi.
of their advantages concerning to decrease sintering
temperature and to shorten the sintering process
duration. Nanoparticle silver ink has lowered the
melting temperature point compare to their bulk
form[10] and therefore less energy consumption is
possible during the sintering process. Because of these
benefits, nanoparticles have become popular in inkjet
printing technology and several silver and gold[11]
nanoparticle-based materials have been used in
researches lately. Those materials have been applied in
interconnection line production in IC packages[12-13] or
in 3-D electronics circuits[14].
Mechanical performance of the inkjet-printed
structures is important to investigate in order to
determinate the reliability of the silver structures. In
previous papers, the adhesion performance of silver
inkjet-printed structures on PI, LCP, BCB
(Benzocyclobutene), Si3N4, chip-pad metals[13], glass
slide[15], and Barium Strontium Titanate (BST) film[16]
have been investigated by a scotch tape test to define
their peel strength. Qualitative results have been gained
by empirical evaluation of inkjet-printed structures
after tests. However, electrical design and modeling
requires more quantitative results to simulate the
interfacial stress to define failure criteria. In addition,
evaluation of adhesion performance after a humid
environment or the mechanical tensile performance of
inkjet-printed structures at elevated temperatures is
also important for reliable structural design.
1.1 Usability of adhesion results in modeling
evaluation
35
An investigation of mechanical performance of silver inkjet-printed structures
Modeling can be extensively used to simulate
stresses on inkjetted circuit boards before manufacture.
Furthermore, modeling can provide valuable
information about the stresses in an electrical structure
during different phases of processing. From simulated
stress contours, it is possible to evaluate the location of
peak stresses, which may subject the structure too, e.g.,
an interfacial crack or cohesive failure. It is often faster
and cheaper to run a set of parametric simulations to
determine the effect of different factors, such as
material combinations, structural dimensions, or
component placement, e.g., stress peak values.
Fig. 1
Stresses σz [MPa] after attachment with ICA
Fig. 2
36
Electrical systems must withstand the conditions
when they are subjected to during manufacture and
operation. The chosen material combinations induce
thermo-mechanical stresses during a temperature
change. Fig. 1 shows an example of simulated stresses
after an SMD was attached with an isotropically
conductive adhesive (ICA) paste to a PI sheet. The
adhesive was cured at 120 ℃ , and when the
temperature was brought down to room temperature,
the unmatching coefficients of thermal expansion
(CTE) stressed and warped the structure.
Use conditions subject electrical structures to
mechanical stresses caused by bending, vibration,
impact, or other causes. Thin substrates with an
inkjetted circuit board are flexible and can be bent to
various shapes and purposes; e.g., an electrical system
can be wrapped around the wrist. However, because
discrete components are not bendable, their mechanics
must be experimentally tested or modeled by computer.
Fig. 2 shows the stress contours of a simple case, in
which a rigid component was attached with ICA to a
flexible polyimide foil, and the structure bent.
Experimentally determined adhesion strength values of
different material combinations can be used as failure
criteria for simulated interfacial stress values.
Stresses σz [MPa] caused by bending a PI substrate with an SMD component
An investigation of mechanical performance of silver inkjet-printed structures
In this paper, we evaluate the tensile adhesion
pull-off performance of an inkjet-printed silver layer
on various substrates and discuss the adhesion
performance of a selected inkjet-printed silver structure
after a humidity test. In addition, we investigate
separately the effect of an electronic coating and
sintering profiles on adhesion performance and of
elevated temperature on dynamic mechanical
performance. The test setup and related challenges are
described in the paper.
2. Experiment
procedures
materials
and
test
2.1 Nanoparticle-based silver ink (NPS)
The main challenge in formulating inkjet-printing
ink is nanoparticle stability, i.e., to prevent particles
from settling in the ink or to keep the formulated ink
stable at room temperature, both measures being
crucial to increasing the shelf life of the ink. Besides,
the nanoparticles of the bulk materials should be small
enough (<100nm) to minimize clogging of piezo
printhead nozzles, and the particles should disperse
well to avoid aggregation that would increase
viscosity.[17-18] The stability of the tested nanoparticle
ink (Table 1) was enhanced around the nanoparticles
with a dispersant manufactured by the gas evaporation
method. When the dispersant was removed, the
nanoparticles connected physically, enabling electrical
conductivity. Fig. 3 illustrates the sintering mechanism
of the nanoparticle-based ink in several phases.
Dispersant
Silver Nanoparticles
Heat (220-230℃)
Fig. 3
Schematic of sintering of nanoparticle-based silver ink
Table 1
Properties of tested nanoparticle-based silver ink
Particle size (nm)
Metal contents (wt%)
Viscosity (mPa.s)
Specific resistance (µΩ.cm)
Thickness (µm)
2.2 Organic substrate materials
Inkjet printing technology enables production of
electronic components on substrates which are
independent from their physical properties, e.g. rigid,
flexible, and porous. The usage of flexible organic
substrate with printing makes possible to produce
many interesting applications, e.g. e-paper,
organic/inorganic Radio Frequency Identification
(RFID) tag, Organic Light Emitting Display (OLED),
photovoltaics or as part of circuit board material. To
Before sintering
3-7 (mean diameter 5)
57-62
5-10
-
After sintering
99
3
3
enable to use of flexible organic substrates, the
electrical, chemical, and mechanical properties of the
organic substrates need to be well understood and their
engineering properties need to be selected carefully. In
this experiment, we tested several flexible substrates,
i.e. Polyethylene Naphthalate (PEN), Liquid Crystal
Polymer (LCP), and Kapton Polyimide (PI). Their
properties varied in terms of, e.g., dielectric constant,
moisture absorption, and thermal expansion. For
electrical performance with the tested NPS inkjet ink,
37
An investigation of mechanical performance of silver inkjet-printed structures
the sintering temperature was adjusted to a range of
220 ℃ -230 ℃ . At this high temperature, the above
substrates yielded certain process advantages in their
Table 2
CTE (ppm/℃)
Tensile strength (MPa)
Melting point (℃)
Dielectric constant
Moisture absorption (%)
Substrate thickness (µm)
material compatibility (selected properties of the tested
flexible substrates shown in Table 2).
Properties of tested organic substrates
Kapton Polyimide (PI)
20
139-231
3.50
1.80
125
Polyethylene Naphthalate (PEN)
18-20
250
270
2.90
0.40
100
Liquid Crystal Polymer (LCP)
17
294
310
2.85
0.04
100
Chemical bonding structure
2.3 Adhesion test setup
Samples were prepared for an adhesion test by
inkjet-printing NPS ink on the substrates. Piezo
printhead type inkjet printing poses several challenges,
and that is why important parameters such as droplet
firing voltage, jetting waveform, printhead nozzle size,
piezo-head pulse shape, and shooting repeatability
during printing must be carefully defined. In addition,
piezo printhead temperature is material-dependent;
thus substrate temperature will affect the quality of the
printed structures. Another way to increase the quality
of the printed structures is to modify the surface of the
substrate to control droplet formation and the contact
angle between ink droplet and substrate. Oxygen
plasma or UV/Ozone surface treatment cleans the
substrate and minimizes dust and other contamination.
Furthermore, coating the substrate electronically helps
adjust the droplet size on the plastic to print highly
aligned and narrow interconnection lines or wide and
thin patterns[12,19-20]. In this experiment, the effect of an
electronic coating on adhesion performance was
examined for selected substrate.
Sample preparation continued then by attaching
the inkjet-printed structures on flexible substrates to a
rigid board. A 1.5-mm thick, one-sided copper FR4
board was selected to lend rigidity to the organic
substrates. The copper side of the FR4 board had the
added advantage of its surface, which was smooth
38
enough to attach organic substrates with an
epoxy-based adhesive. The adhesive was also used to
attach an aluminum pull stud, 7 mm in diameter, to the
NPS inkjet-printed structure. The most important
criterion for selecting the adhesive was its curing
temperature, which was to be low enough to prevent
undesired chemical reactions in the inkjet-printed NPS
layer. The adhesive’s datasheet gave a curing
temperature and duration of 90℃ and 30 minutes,
respectively.
Fig. 4 illustrates the final adhesion test setup. It
represents a new mechanical test method for printed
structures
compared
to
those
previously
[13,16,21-22]
reported
and
those
developed
in
[23-24]
industry
.
Fig. 4
Schematic description of adhesion test
setup with material interfaces
2.4 Adhesion test
The organic substrates were cleaned with
isopropanol to wipe dust away and were subjected
again to UV/Ozone surface treatment for 5 minutes.
An investigation of mechanical performance of silver inkjet-printed structures
Afterwards, adhesion test sample files were
inkjet-printed with high resolution image on the
organic substrates. The adhesion strength of NPS
inkjet-printed structures on PI, PEN, and LCP were
Table 3
Surface treatment:
Isopropanol + UV/Ozone
Sintering temperature (℃)
Electronic surface coating
Sintering duration (min.)
examined and the effect of electronic coating material
on PI substrates were also investigated. Table 3
describes the material combinations and the selected
parameters for the analysis of adhesion strength.
Adhesion test samples
Kapton Polyimide (PI)
Polyethylene Naphthalate (PEN)
Liquid Crystal Polymer (LCP)
Yes
Yes
Yes
220
No
60
220
No
60
220
Yes
60
220
No
60
250
No
30
The sintering temperature and the duration is one
of the most important parameters that can affect the
adhesion test results. Various sintering temperature and
durations were studied on silver nanoparticles and 230
℃ for an hour was given as the recommended
sintering profile[13]. In this experiment, 220℃ for an
hour was recommended from the ink supplier and in
addition, 250℃ for 30 minutes were also studied to
define the effect of sintering profile on adhesion
performance. After sintering process, the samples were
cooled down to room temperature to attach to FR4 rigid
board. Before the aluminum adhesion pull stud were
attached to the NPS inkjet-printed structure, the upper
and the lower surface parts of the test samples were
cleaned with isopropanol to remove the impurities,
since that can affect the adherence quality of the
epoxy-based adhesive to the FR4 rigid board and to the
adhesion pull stud.
The adhesive was used in controlled amounts,
occurrence of air bubbles was minimized, and the
adhesive was deposited evenly along the substrate’s
lower surface. The samples were placed in a thermal
oven to pre-cure the adhesive. Afterwards, the
aluminum pull stud was attached to the NPS layer with
the same adhesive, and the whole system was placed in
the oven. Curing was finalized at 90℃ for 30 minutes,
and the samples were cooled down to room
temperature.
2.5 Dynamic mechanical analysis (DMA)
The dynamic mechanical analysis provides
important knowledge about tested materials, e.g.
molecular structure, product properties, and processing
conditions. From the standardized dynamic mechanical
test results, it is possible to determine storage modulus,
loss modulus of the tested material at elevated
temperature as a function of time and frequency.[25-26]
The NPS ink material was inkjet-printed on PI
substrate and the samples with several geometry, i.e.
semicircular lines, square, and rectangular areas were
sintered at 220℃ for 60 minutes in this experiment.
The PI substrate dimensions were selected to be 10mm
(width) × 15mm (length) × 0.055mm (thickness).
The test aim was to determine the mechanical
characteristic, i.e. tension at elevated temperature and
was to investigate the possible mechanical degradation
of the inkjet-printed NPS structures. The optimized
industrial test standard[26] was used and temperature
range from -60℃ to 100℃ was selected. A frequency
of DMA test was 1Hz to 10Hz with 40µm amplitude.
2.6 Humidity test
The reliability of an NPS inkjet-printed structure
must be evaluated in varying temperature and
environmental conditions to determine any changes in
the structure’s adhesion performance. Accelerated life
tests in harsh conditions such as high humidity and
high temperature usually accelerate the failure
mechanism, which facilitates detection of possible
failures much sooner than in the product’s regular
service[27]. In this study, the NPS inkjet-printed
39
An investigation of mechanical performance of silver inkjet-printed structures
structure on the PI substrate was tested according to a
common industrial standard[28] at 85 ℃ and 85%
relative humidity (RH) for 1,000 hours. Because of its
high moisture absorption rate, the PI substrate was
tested to determine its humidity-related adhesion
pull-off reliability. Samples were prepared as described
above (printing, sintering), and the PI substrate was
cleaned with isopropanol and UV/Ozone. After surface
preparation, an electronic coating was applied and
printing finalized. The structures were sintered at 220
℃ for 60 minutes, and then tested for humidity, and
after that for adhesion pull-off.
3. Results and discussion
3.1 Adhesion test results
Adhesion tests were run at room temperature at
50% RH. In Fig. 5, “Ink” represents inkjet-printed NPS
structure, “S1” sintering profile at 220 ℃ for 60
minutes, “S2” a sintering profile of 250 ℃ for 30
minutes, and “EC” stands for electronically coated
substrate. The results did not fluctuate more than
expected, and they show the reliability of the test setup
and the procedure. The NPS inkjet-printed structure on
the PEN substrate shows the highest adhesion
compared to the PI and LCP substrates.
Mean
Ink_LCP_S1
Ink_PEN_S1
Max
Ink_PI_S2
Ink_PI_S1_EC
Min
0.0
50.0
100.0
150.0
200.0
250.0
Pull-Off Breaking Force (N)
Fig. 5
40
Adhesion pull-off breaking force results of
NPS inkjet-printed structures
Fig. 6 shows the measured adhesion breaking
strength of the NPS inkjet-printed structures according
to the following formula:
σ = F/ A
(1)
where σ is breaking strength (megapascals), F is
breaking force (N), and A is area of the dolly (square
millimeters). The microscopic visual inspection was
carried out and the fracture surfaces were determined
according to the ISO standard.[23] The interface fracture
between the substrates and inkjet-printed layer was
described as C/D and the fracture between
inkjet-printed layer and adhesive materials was
described as D/Y. The adhesive fracture mechanism
was considered a partial cohesive failure.
3.50
3.00
2.50
Pull-Off Breaking
Strength (MPa) 2.00
and Fracture
Surfaces
1.50
1.00
0.50
0.00
Ink_PI_S1_EC
Fig. 6
Ink_PI_S1
Ink_PI_S2
Ink_PEN_S1
Ink_LCP_S1
Adhesion pull-off breaking strength results
and interface fracture surfaces
Electronic coating controlled the droplet
spreading in order to create more precise structures. On
the other hand, surface fracture mechanism and the
breaking strength of the electronic-coated PI substrate
were indicated that the adhesion performance was
weakened when the electronic-coating was applied.
The NPS droplets have smaller area to create
mechanical contact with the substrate. The contact
angle of the NPS droplets were smaller when
electronic-coating material was not applied. The good
adhesion usually requires good wetting which means
the contact angle of each droplet should be as close as
possible to 0º.
In terms of adhesion, the differences in the
sintering profiles of the NPS inkjet-printed structures in
An investigation of mechanical performance of silver inkjet-printed structures
the selected conditions were similar. When the adhesion
breaking mechanism and the amount of pulled-off NPS
inkjet-printed structures were carefully examined, the
sintering profiles of 220℃ for 60 minutes and 250℃
for 30 minutes yielded similar results.
3.2 Adhesion test results after humidity test
The NPS inkjet-printed structures were tested
with a Steady-State Temperature Humidity Bias Life
Test to assess any variation in their breaking force and
breaking strength in a humid environment. PI material
was selected as a test substrate because of its high
moisture absorption rate over the other substrates. The
results show that humidity lowered the adhesion
breaking force, yielding a mean of 60-65N to 20-25N.
The results also underline the importance of the
substrate’s moisture uptake to minimize variation in
the adhesion breaking force compared to adhesion
results in dry conditions.
3.3 DMA test results
Several geometrical NPS inkjet-printed structures
on the PI substrate were tested with results shown in
Fig. 7. The semicircular line structure is described as
“Ag multi circles,” the square structure as “Ag layer”
(3 mm×3 mm), and the rectangular area as “Capacitor
Ag layer.” In addition, bare PI film without print is
described as “Polyimide film.” The complex Young’s
modulus of the structures was calculated for each
sample based on the formula:
E* = E’ + iE”
(2)
where E* is the complex Young’s modulus, E’ is the
storage modulus, i is the square root of minus one, and
E” the loss modulus of the samples[29]. NPS
inkjet-printed structures have shown a good degree of
tensile endurance at elevated temperature. After the
test, the samples were visually inspected, but no
degradation was observed. The measurements of the
largest NPS inkjet-printed area, the capacitor Ag layer,
were close to those of the polyimide film. The
previously presented[30] experimental results were
showed that the test results varied based on the amount
of the NPS layer on the PI.
Complex modulus︱E︱[Pa]
Capacitor Ag layer
Ag layer (3mm×3mm)
Ag multi circles
Polyimide film
9,00E+009
8,00E+009
7,00E+009
6,00E+009
5,00E+009
4,00E+009
3,00E+009
2,00E+009
-60 -50 -40 -30 -20 -10 0
10 20 30 40 50 60
70 80 901 00 110 120
Temperature[℃]
Fig. 7 DMA results of several geometric NPS
inkjet-printed structures and bare PI film
4. Conclusions
Inkjet-printed NPS layers were subjected to
several mechanical tests in dry and humid conditions
and separately to elevated temperatures. NPS ink and
substrate properties were described and the test setup
and procedures explained. NPS inkjet-printed layers
were tested according to modified industrial adhesion
pull-off standards and their interfacial breaking
mechanisms and test results reported. Our results show
that an NPS inkjet-printed structure on a PEN substrate
has the highest adhesion performance. The effect of an
electronic coating on adhesion performance was
studied, and the PI substrate showed lower
performance when a surface coating was applied. The
selected sintering temperatures gave similar
performance results for adhesion. Moisture affected
adhesion greatly, especially when a substrate with high
moisture absorption was selected. NPS inkjet-printed
structures were also evaluated for their tension
performance at elevated temperature, and all
mechanical test results were considered suitable for
producing electronic components.
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An investigation of mechanical performance of silver inkjet-printed structures
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(Edited by Tsyung and Edward)