Microelectronic Engineering 193 (2018) 91–97
Contents lists available at ScienceDirect
Microelectronic Engineering
journal homepage: www.elsevier.com/locate/mee
Sintering process of inkjet-printed silver patterns using a heated inert gas
Kwon-Yong Shin a, Nam Son Park b, Jun Young Hwang a, Kyungtae Kang a, Sang-Ho Lee a,⁎
a
b
Korea Institute of Industrial Technology, 14, Hanggaul-ro, Ansan 15588, Republic of Korea
Jesagi Hankook Ltd., 17, MTV 25-ro, Siheung 15117, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 26 September 2017
Received in revised form 3 February 2018
Accepted 17 February 2018
Available online 21 February 2018
Keywords:
Inert gas
Sintering
Inkjet
Silver
Printing
a b s t r a c t
In this study, we introduce a thermal sintering process using a heated inert gas as a new sintering method. The
heated gas flow is formed by passing nitrogen (N2) gas through a heating head of 300 to 700 °C, and the desired
substrates are then exposed to the heated gas flow through a 400 mm long and 5 mm wide slit-nozzle. Sintering
performance of Ag films was characterized by analyzing the electrical resistivity and metallographic structures
according to the temperature change of the heating head. The temperature distribution of the heating region is
analyzed by infrared (IR) thermal imaging and surface temperature measured by thermo-label tape. To test
the feasibility of the proposed method for application to printed circuit board (PCB) manufacturing, we performed a reliability evaluation using the printed Ag patterns under the standard of the Institute for
Interconnecting and Packaging Electronic Circuit (IPC), TM650. Various reliability test patterns were created by
Ag inkjet-printing on both a rigid flame retardant 4 (FR4) substrate and a flexible polyimide (PI) film substrate.
The reliability evaluation includes withstanding voltage, adhesive strength, thermal shock, pressure cooker, and
bending tests. The surface wettability of the substrates was controlled to obtain high quality fine and uniform patterns by UV/O3 treatment after coating a fluoropolymer thin film.
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
Printed electronics is a technology used to create electrical devices
on various substrates using printing methods such as screen printing,
flexography, gravure, offset lithography and inkjet-printing [1]. Recently, the scientific community has shown a growing interest of developing low-cost flexible electronics by printed electronics. This interest is
driven by several factors: (1) the need for low-cost and mass-production processes; (2) numerous applications require shapeable and disposable devices; and (3) the demand for the quick realization of
electronics [2,3]. Typical printed devices include printed strain sensors,
temperature sensors, printed image sensors, printed humidity sensors,
printed biosensors and RFID tag with light detection [2–5].
In the last decade, inkjet-printing has been attracting growing interest for the production of micro-patterns as an alternative to the conventional photolithography process. The inkjet-printing method has many
advantages including low-cost manufacturing, a low-temperature process and patterning compatibility on a non-planar substrate or a flexible
substrate due to the direct-writing feature. Furthermore, it is possible to
produce micron-sized patterns in a drop-on-demand manner without
predefined masking and etching [6–10]. Inkjet-printing technology
has been steadily utilized as a fabrication process to demonstrate
field-effect transistors [6,7], conductive electrodes [8,9], memory
⁎ Corresponding author.
E-mail address: sholee7@kitech.re.kr (S.-H. Lee).
https://doi.org/10.1016/j.mee.2018.02.016
0167-9317/© 2018 Elsevier B.V. All rights reserved.
devices [11], sensors [3,5], light-emitting polymer displays [12,13] and
organic solar cells [14,15]. Recent advances in 3D printing technology
can provide electronic prototypes that can be rapidly fabricated in comparable time frames to those for traditional 2D bread-boarded prototypes by component placement and electrical interconnection on 3Dprinted thermoplastic dielectric structures [16,17]. Hybrid prototyping
with 2D printed electronics and 3D printing is referred to as 3D structural electronics. 3D printing techniques such as stereo-lithography
and fused deposition modeling have recently been explored in the context of 3D structural electronics [17]. Interest in 3D printed electronics
has motivated the development of several new types of commercially
available equipment. Nano Dimesion's DragonFly 2020 enables multilayer printing of conductive traces using an inkjet technique at a lateral
resolution of ~30 μm. The Voxel 8 combines a conventional plastic filament extrusion nozzle with a syringe-based silver ink extruder. The lateral resolution of the traces that can be printed with the Voxel 8 is 250
μm with a recommended pitch of 2 mm, and the printer allows pausing
of the printing to enable manual placement of circuit components [18].
Recently, demonstrated electronic devices include a 3D-printed spherical dipole antenna [19], a surface acoustic wave transponder [20], a 3Dprinted conditioning circuit [16], and a three-axis magnetic flux sensor
system [17].
In inkjet printing of a conductive nanoparticles pattern, selection of
the sintering method is a critical factor to obtain stable conductivity of
the printed patterns because the electrical performance depends on
the inter-connection of individual nanoparticles onto a substrate after
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K.-Y. Shin et al. / Microelectronic Engineering 193 (2018) 91–97
deposition by inkjet printing [21]. Generally, Ag nanoparticle ink consists of nanoparticles and a solvent. Before sintering, there are no significant changes in the particle size and neck growth. However, when the
ink is sintered, neck growth gradually takes place at the grain boundary
surface and the pores are gradually reduced between the particles. A
continuous conductive printed pattern is then created [22]. In recent
years, many research results related to sintering technology such as
laser sintering [23], thermal sintering [24,25], electrical sintering [26],
microwave sintering [27], plasma sintering [22], and photonic sintering
[28], for printed electronics applications have been reported. Also, several theoretical studies were introduced to understand the nanoparticle
sintering process at the level of the nanoscale process [29–32]. In particular, a theoretical approach to understanding the electrical and thermal
properties is extremely important for applications of nanostructured
materials in microelectronics, thermoelectric, and thermal barrier coatings. Ertekin et al. proposed a generalized model to describe the thermal
conductivity for nanostructured materials [30,31] and Kang et al. proposed a charge-transport model for a fundamental understanding of
conducting polymers [32].
In this study, we introduce a thermal sintering process using a
heated gas flow as a new sintering method and evaluate its feasibility
for PCB manufacturing through a reliability characterization of inkjetprinted Ag patterns. The gas flow is heated by passing N2 gas through
a heating head of 300 to 700 °C, and then the heated gas flow is jetted
from a nozzle with a length of 400 mm and, a width of 5 mm onto the
substrates. In order to control the sintering characteristics, we controlled the gas flow rate, heating head temperature and exposure time
of the heated gas flow. Sintering performance on printed Ag patterns
was characterized by analyzing the electrical resistivity and metallographic structures according to the temperature change of the heating
head. A reliability evaluation was performed according to the standard
of the IPC, TM650 commonly used in the PCB industry. The test patterns
were fabricated by inkjet printing of an Ag nanoparticle ink on a FR4
rigid substrate and a PI flexible film, respectively.
surface. We determined the surface energy through Wu's harmonic
mean model using the average contact angles of DI water and formamide as input parameters.
Rigid FR4 and flexible PI substrates were used to fabricate the specimens for the withstanding voltage, adhesive strength, thermal shock
and pressure cooker tests. The dielectric withstanding voltage, thermal
shock, and pressure cooker tests were conducted with a high-speed
electric load (6330, CHROMA, Taiwan), high temperature tester (TSA41L, ECPEC, Japan), and, a pressure cooker tester (PCT-183CP, Lab Companion, Korea), respectively, and the American Society for Testing and
Materials (ASTM) D3359 adhesive strength test rating system was
used along with Scotch tape (610, 3M, Korea) to measure the adhesion
of the Ag patterns. Flexible PI (DSF-600, Doosan Electronic Corp., Korea)
films were used to fabricate the specimens for the bending test, which
was carried out using a bending tester (MIT-SA, Toyoseiki, Japan).
Fig. 1 show schematic images of the heated N2 gas flow-based sintering
process and a picture of the sintering system (HBS-I, Jesagi Hankook Co.,
Korea) used in this study. The N2 gas flow was heated by passing N2 gas
through a heating head of 300 to 700 °C. The heated gas flow was jetted
through a nozzle with a length of 400 mm and a width of 5 mm. The gas
feeding flow rate was 40 L/min and the gap distance between the nozzle
and the substrate was 3 mm. The substrate was moved to the heated gas
exposure region after loading the substrates onto the stage.
3. Results and discussion
Fig. 2 shows the horizontal temperature distribution of the heated
gas flow around the slit nozzle at various measuring positions with
the head heating time. We measured the temperature at the gap area
Slit nozzle
Heated gas flow
(Inert gas)
2. Experimental methods
We used Ag ink (DGP 40LT-15C, Advanced Nano Products Co.,
Korea) in which ~50 nm Ag nanoparticles were dispersed in a
triethylene glycol monomethyl ether solution. The ink contained Ag
nanoparticles in an amount of 30.1 wt.%. A piezoelectric print-head system with a 19 μm nozzle diameter (DMP-2800, Dimatix, Fujifilm, USA)
was used for printing the Ag ink. The printing system was composed of a
print-head, a motorized X-Y stage, a heatable working table and an
alignment system. Before printing the patterns, we first optimized the
droplets ejected from the nozzle by controlling the voltage and the
waveform of the piezo actuator drive to ensure stable single droplet deposition during all the experiments.
FR4 and PI were used as the substrate material. The substrates were
ultra-sonicated in acetone and 2-propanol alcohol for 10 min to remove
surface contamination, rinsed with de-ionized water, and subsequently
dried in a convection oven at 110 °C for 10 min. The cleaned substrate
was coated with fluorocarbon (FC) solution to produce a hydrophobic
surface. A hydrophobic FC thin film was spun onto the substrate at
2000 rpm for 1 min using a fluoropolymer solution: a mixture of FC
722 and FC 40 (Fluorad™, 3 M, Korea). The FC-coated substrate was
then immediately loaded into a convection oven and baked at 110 °C
for 10 min. In order to increase the hydrophilicity of the hydrophobic
surface, UV/O3 treatment was performed using an UV/O3 cleaner (AH1700, Vision Semicon Co., Korea). The UV/O3 treatment was conducted
for 6 min to control the surface wettability of the substrate. We measured the contact angles of the FC-coated and UV/O3 treated substrates
using a contact angle measuring instrument (DSA 100, Krüss, Germany). Three test liquids were used for the contact angle measurement:
DI water, diiodomethane and formamide. Contact angles were measured using the sessile drop method at five different locations on the
Thin film
Substrate
Stage
(a)
Gas heating head
Gas flow rate
controller
Stage speed controller
Temperature controller
(b)
Fig. 1. Heated N2 gas flow-based sintering system: (a) a schematic image of the sintering
process; (c) a picture of the sintering system.
K.-Y. Shin et al. / Microelectronic Engineering 193 (2018) 91–97
Nozzle
Measurement
position
5 cm
Front
93
Nozzle
5 cm
Left 5cm
Left 2.5cm
Substrate
Right 5cm
Right 2.5cm
Center
(a)
A
a
B
b
C
c
D
d
E
(b)
Fig. 2. Horizontal temperature distribution of the heated gas flow around the slit nozzle: (a) IR camera image of nozzle front side; (b) temperature change at various measuring positions
with head heating time.
between the nozzle and the substrate by an IR thermal image camera
system (A40, FLIR, USA). The heated gas flow was generated through a
slit nozzle of 400 mm length and 5 mm width. Fig. 2(a) shows an IR
thermal image of the nozzle front side at a gap distance = 3 mm
between the nozzle and the substrate. We measured the temperature
at the center, 2.5 cm and 5 cm away from the center, respectively.
The head temperature was held for 20 min at 300 °C, 400 °C, 500 °C,
600 °C, and 700 °C, respectively, at sections A, B, C, D, and E in Fig. 2
(b), and ramped up for 5 min from 300 °C to 400 °C, 400 °C to 500 °C,
500 °C to 600 °C, and 600 °C to 700 °C, respectively, at sections a, b, c, d
Table 1
Evaluation results of colorimetric thermo-label sensors by hot plate heating and thermocouple sensor monitoring.
Hot plate set temperature (°C)
Measured temperature (°C)
Thermo-label color change
Measurement setup
200
202
210
210
230
234
250
254
94
K.-Y. Shin et al. / Microelectronic Engineering 193 (2018) 91–97
Nozzle
(a)
(b)
(c)
Thermo-label
Stage
Stage
5 cm
5 cm
Fig. 3. Surface temperature analysis of a substrate during exposure of heated N2 gas flow using colorimetric thermo-label sensors: (a) scheme of surface temperature measurement; (b)
before exposure; (c) after exposure.
in Fig. 2(b). After the head temperature reached 600 °C, the temperature at the center region was above 250 °C. The temperature at the
5 cm-away left/right position was 40–42% lower than the center temperature (Fig. 2(b)).
Before sintering the Ag patterns, we analyzed the surface temperature change of the substrate during exposure of the heated gas flow
using colorimetric thermo-label sensors (5E, Nichiyu Giken Kogyo Co.,
Japan). The actual surface temperature can be measured by monitoring
the color change of thermo-label tapes. First, we evaluated the accuracy
of the thermo-label sensor using a hot plate and a thermocouple sensor
monitor (GL240, Basecamp C&M Co., Ltd., Korea). After attaching the
thermo-label sensor and the thermocouple on the hot plate, we monitored color changes by elevating the temperature as shown in the measurement set-up presented in Table 1. As soon as the temperature
increased from 200 °C to 250 °C, the white dots of the thermo-label
tapes started to change to black. In the results, five dots changed to
black at 250 °C (Table 1). The target surface temperature is 250 °C,
which is recommended for sintering of the Ag nanoparticle ink used in
this study [24]. Next, we loaded quartz plate (40 cm × 40 cm) with
ten attached thermos-label sensors into the heated gas flow-based
sintering system (Fig. 3(a)). The quartz substrate was moved toward
the heated gas exposure region at a stage velocity = 3.5 mm/s. The
heating head temperature was 600 °C and the N2 flow rate was
40 L/min. The color of all five dots changed to black (Fig. 3(b)). This indicates that the heated N2 gas flow induced a surface temperature above
250 °C at a head temperature of 600 °C.
The electrical resistivity of the Ag thin films was measured to investigate the thermal sintering performance of the heated N2 gas flowbased sintering system. The electrical resistivity is a key variable to characterize the sintered Ag patterns as a metal electrode after the sintering
process. The electrical resistivity (ρ) of the fully sintered film was
calculated by multiplying the sheet resistance by the film thickness
[21]. We prepared an Ag ink-coated glass substrate and the thickness
of the Ag film was about 330 nm. The sheet resistance decreased gradually from 1.6 Ω/□ to 0.12 Ω/□ when the head temperature increased
from 300 °C to 600 °C. The Ag-coated glass substrate was exposed to
the heated N2 gas flow for 10 min. The electrical resistivity of the Ag
film reaches about 3.96 × 10−8 Ω·m after being sintered by the heated
N2 gas flow at 600 °C (Fig. 4). The calculated electrical resistivity of the
Ag film sintered by the heated N2 gas flow was approximately 2.5
times that of bulk Ag metal (1.6 × 10−8 Ω·m), but ~30% lower than
that of the Ag film sintered in a convection oven at 250 °C for 60 min
(5.8 × 10−8 Ω·m) [24]. Fig. 4 shows the change of the sheet resistance
and electrical resistivity with the head temperature.
We compared metallographic structures of the Ag films after
sintering in a convection oven and a heated gas flow sintering system,
respectively. Fig. 5 shows the sintering conditions and scanning electron
microscope (SEM) images of the Ag film before and after sintering. For
oven sintering, we observed the overall intra-microstructure of neck
growth, grain growth, and pore growth (Figs. 5(c) and (d)). However,
heated-gas sintering show a closely-packed microstructure with large
grains of about 300 nm and very few pores were found (Figs. 5(e) and
(f)). The SEM analytical results reveal why the electrical resistivity of
the Ag film sintered by the heated N2 gas flow is ~30% lower than that
of the Ag film sintered in a convection oven. It is presumed that the additional connections among the nanoparticles without pore growth led
to a decrease in the electrical resistivity.
To assess the feasibility of applying the proposed method to PCB
manufacturing, we performed a reliability evaluation with the inkjetprinted Ag patterns under the standard of the IPC, TM650. Various test
patterns were created by Ag inkjet-printing on both a rigid FR4 substrate and a flexible PI film substrate for the reliability analysis. In the
inkjet-printing process, the ink wettability of the substrate is a critical
factor to make fine patterns [8,33,34]. In this study, wettability is controlled by a FC thin film coating followed by UV/O3 treatment to print
fine and uniform Ag patterns with the desired dimensions. UV/O3 treatment can increase the surface energy of a non-wettable FC thin film. To
examine the influence of the UV/O3 treatment on the surface wettability
of the FC thin film, the surface energy of the substrates was evaluated by
measuring the contact angles of two polar liquids (D.I. water and formamide) and one nonpolar liquid (diiodomethane). Table 2 shows the
contact angles of the three test liquids, the surface energies, and microscopic images of printed single dots and lines for each surface treatment
condition. Straight lines could be ink-jet printed with a 50% overlap rate
without a merging or bulging phenomenon after UV/O3 treatment for
4 min. The treatment time was increased from 4 min to 10 min at an interval of 2 min and the surface energy was increased from 15.4 mJ/m2 to
43.8 mJ/m2. This means that the UV/O3 treatment contributes to creating high wettability. Regarding the surface energy of the FC thin film,
Fig. 4. Change of sheet resistance and electrical resistivity versus head temperature.
K.-Y. Shin et al. / Microelectronic Engineering 193 (2018) 91–97
Sintering
conditions
95
Oven sintering
Heated gas flow
sintering
250 C, 1 hr
Head Temp. = 600 C,
10 min
(a)
(c)
(e)
(b)
(d)
(f)
Before sintering
Observation view
Surface view
( 100,000)
Cross sectional view
( 100,000)
Fig. 5. Scanning electron microscope images of Ag film before and after sintering and sintering conditions.
the diameter of the printed Ag dots increased gradually from ~40 μm to
~155 μm and the line width increased from ~ 50 μm to ~200 μm.
In this study, we selected five different test items and conditions
used in the conventional PCB industry according to the standard of the
IPC, TM650. Table 3 presents a summary of the test conditions, substrate
type, and evaluation results for the reliability test items including the
withstanding voltage, adhesive strength, thermal shock, pressure
cooker, and bending tests. Fig. 6 shows a photographic image of
inkjet-printed test patterns for the reliability analysis items (Fig. 6(a))
and the schematic design and dimensions of each test pattern (Fig. 6
(b)). The withstanding voltage test was conducted by transmitting
5000 V DC, 1 mA for 60 s through both ends of a 5 cm line pattern and
examining it for any evidence of spark discharge, short circuiting, or dielectric breakdown. The test patterns were printed with a line width
and spacing of 100 μm and a thickness of ~1 μm, as shown in Fig. 6(b).
The test patterns were printed with a line width and spacing of 100
μm and a thickness of ~1 μm, as shown in Fig. 6(b). Figs. 7(a) and (b)
shows photographic images of the withstanding voltage test pattern
with the inset showing magnified images. The Ag patterns on the FR4
substrate maintained its initial shape in the withstanding voltage test
without damage due to spark discharge, short circuiting, or dielectric
breakdown phenomena (Fig. 7(a)), but the Ag patterns on the PI substrate were seriously damaged by the withstanding voltage test and
showed failure phenomena of peel-off and disconnection (Fig. 7(b)).
The adhesive strength test was performed using the ASTM D3359 rating
method for the cross-cut tape test. Square 5 mm × 5 mm patterns were
printed with a thickness of ~1 μm for ASTM D3359. The ASTM D3359
rating method employs a scale from 0B to 5B and the rating is calculated
by observing the ratio of the pattern test area removed by a strip of 3M
610 Scotch tape [35]. The ASTM D3359 adhesive strength rating
achieved by the test patterns was 5B for FR4 and 3B for PI, as shown
in Figs. 7(c) and (d). The thermal shock test was conducted by observing
the change in the insulation resistance after 100 cycles of temperature
variation from −55 °C to 125 °C for 30 min. The test pattern was a single
line of 4 cm length, 100 μm width and ~1 μm thickness. The variation in
the resistance after the thermal shock tests was found to be ±4.5% for
FR4 and ±4.3% for PI, respectively, with no signs of spark discharge or
short circuiting. The pressure cooker test was conducted by measuring
the insulation resistance through both ends between two independent
Ag lines at 121 °C, 2 atm, and 97% RH after 48 h. The pressure cooker
Table 2
Contact angles of the three test liquids, surface energies, and printed Ag pattern images for UV/O3 treatment time of FC films.
UV/O3 treatment time of FC film
Surface energy (mJ/m2)
Contact angle of probe liquids (degree)
D.I. water
Diiodomethane
Formamide
4 min
87.7
84.2
25.4
15.4
6 min
85.1
72.0
15.9
21.8
8 min
55.3
53.9
60.1
30.4
10 min
68.5
34.9
50.4
43.8
Single dot
(ϕ: diameter)
Line
(W: line width)
96
K.-Y. Shin et al. / Microelectronic Engineering 193 (2018) 91–97
Table 3
Reliability test items, conditions, substrates, and evaluation results (RH: Relative Humidity).
Test items
Conditions
Substrates
Evaluation results
Withstanding voltage
No spark discharge, short circuiting, or dielectric breakdown at 5000 V DC, 1 mA, 60 s
Adhesive strength
No peel off in 1 mm cross-cut tape test
Thermal shock
Variation of resistance within ±10% at −55 °C, 15 min and 125 °C, 15 min, 100 cycles
Pressure cooker
Insulation resistance above 100 MΩ at 121 °C, 2 atm, 97% RH; 48 h
Bending
Angle of rotation: 135°, velocity of rotation: 175 times/min above 10,000 times
FR4
PI
FR4
PI
FR4
PI
FR4
PI
PI
Pass
Fail
Pass
Fail
Pass
Pass
Pass
Pass
Pass
test is a standard for determining the occurrence of leakage current. The
pressure cooker test was conducted by measuring the insulation resistance through both ends between two independent Ag lines at 121 °C,
2 atm, and 97% RH after 48 h. Two lines of 4 cm length, 1 mm width,
and ~1 μm thickness were printed with a spacing of 100 μm. In the pressure cooker test, the average insulation resistance values and standard
deviation after 48 h were 7.3 × 106 ± 2.2 MΩ for FR4 and 7.9 × 106
± 2.1 MΩ for PI, respectively. A remarkable difference in the resistance
was not observed. The serpentine-shape patterns for the bending test
were printed with a line-width and spacing of 1 mm and length of
130 mm and thickness of ~1 μm on the PI substrate. The bending test
was conducted by measuring the rotation number of the printed Ag patterns that could be bent or folded over just before failure. The samples
were folded through an angle of 135° from their initial position at a rotation velocity of 175 cycles/min. The inkjet-printed Ag patterns passed
the test criterion of more than 10,000 times, showing a value of 11,878
± 1888 times for three samples.
No damage
Spark discharge and short circuiting
0% peel off (5B)
~13% peel off (3B)
±4.5%
±4.3%
7.3 × 106 ± 2.2 MΩ
7.9 × 106 ± 2.2 MΩ
11,878 ± 1888 times/min
conventional oven sintering, the observations of lower resistivity and
a closely-packed metallographic structure supported that the heated
inert gas sintering process is applicable to printing-based PCB production. However, for PI film-based flexible PCB production, the proposed
sintering method is limited by poor adhesion of the printed Ag patterns
to PI films. Further optimization of the proposed sintering method
should be accompanied with a specific approach to find suitable substrate surface conditions and Ag nanoparticle-based ink formulation.
Also, future studies will focus on investigating the compatibility of the
16 cm
16 cm
4. Conclusions
Heated inert gas sintering was studied as a thermal sintering method
of printed metal conductive patterns and the feasibility of its application
to PCB manufacturing was evaluated by a reliability characterization of
inkjet-printed Ag patterns sintered by the proposed approach. The inert
gas flow was heated by passing N2 gas through a heating head of 300 to
700 °C and the heated N2 gas flow was jetted from a nozzle with a length
of 400 mm, and a width of 5 mm onto the substrates. The IR thermal
image analysis showed that the temperature at the center region of
the heated N2 gas is above 250 °C and at a point 5 cm-away left/right
is 40–42% lower after the head temperature reached 600 °C. It was further confirmed that the surface temperature is above 250 °C by temperature measurement using colorimetric thermo-label sensors. In
particular, heated inert gas sintering showed better performance than
the oven sintering method in the analysis of electrical resistivity and
metallographic structures according to the temperature change of the
heating head. The electrical resistivity of the heated N2 gas-sintered
Ag film is ~30% lower than that of the Ag film sintered in a convection
oven although its electrical resistivity is approximately 2.5 times greater
than that of bulk Ag. Heated N2 gas sintering yield a closely-packed microstructure with a large grain growth of about 300 nm and few or no
pores within the intra-microstructure overall. The surface wettability
of the substrate was controlled by FC thin film coating followed by
UV/O3 treatment to print fine and uniform Ag patterns. A reliability
evaluation of the inkjet-printed Ag patterns for PCB application was performed under the standard of the IPC, TM650. Various test patterns
were fabricated by Ag inkjet-printing on both a FR4 substrate and a PI
film substrate. Ag patterns printed on the FR4 substrate passed all test
items: withstanding voltage test, adhesive strength test, thermal
shock test, and pressure cooker test, but Ag patterns printed on the PI
substrate failed the withstanding voltage test and showed poor adhesion in the adhesive strength test. In conclusion, compared with
(a)
5 mm
4 cm
5 mm
Adhesion strength test pattern
Thermal shock test pattern
4 cm
13 cm
1 mm
100
Pressure cooker test pattern
Bending test pattern
5 cm
L/S=100
/100
Withstanding voltage test pattern
(b)
Fig. 6. Standard test patterns for reliability analysis items: (a) a photograph of inkjetprinted test patterns on PI films; (b) schematic design and dimension of each test pattern.
K.-Y. Shin et al. / Microelectronic Engineering 193 (2018) 91–97
97
100 m
100 m
(a)
(b)
1 mm
1 mm
(c)
(d)
Fig. 7. Photographic and microscopic images: (a) FR4 substrate after withstanding voltage test; (b) PI substrate after withstanding voltage test; (c) FR4 substrate after adhesive strength
test; and (d) PI substrate after adhesive strength test (the insets show magnified pattern images in Figs. 7(a) and (b)).
heated gas sintering process with a roll-to-roll process for flexible electronic device production.
[16]
Acknowledgment
[17]
We would like to acknowledge the financial support from the Industrial Technology Innovation Program (Project grant No. 10063277) of
the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of
Korea and the R&D Convergence Program (Project grant No. CAP-1504-KITECH) of the National Research Council of Science & Technology
(NST) of the Republic of Korea.
[18]
[19]
[20]
[21]
[22]
[23]
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