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MYU Tokyo
S & M 1178

Simple Fabrication Method to Produce
Flexible Electrode Capable of Soldering
Shigeru Toyama,* Yasuhiro Tanaka, Yurie Ishikawa,1 and Kazuhiro Hara1
Research Institute, National Rehabilitation Center for Persons with Disabilities,
4-1 Namiki, Tokorozawa-shi, Saitama 359-8555, Japan
1School of Engineering, Tokyo Denki University, 5 Senju Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan(Received November 9, 2015; accepted Decemeber 16, 2015)

Keywords: flexible sensor, laser printer, toner, electron beam evaporation, solder, thermally resistant film,
wearable sensor

We have developed a simple method of producing a customized flexible electrode that is capableof soldering.  This method consists of a few fabrication steps.  The first step is the laser printing ofa reversal pattern of the electrode on a thermally resistant plastic film.  The second step is electron
beam evaporation to fabricate multilayered metals, such as Ni-Ag, on the film.  The third step isa kind of liftoff process, which is the removal of unnecessary metals together with the underlying
toner by sonication after immersing the film in an organic solvent.  After a thorough washing, we
can obtain the desired patterned-electrode film.   Several film electrodes were fabricated by thismethod.  Consequently, some electrodes tightly adhered to the basal film and endured soldering bycareful choice of the combination of metal layers. Although the conductance of these electrodes
was less than the expected value estimated from the bulk metal conductance, the method can beused for the development of small electrodes. This time, we fabricated electrodes with several
metals such as Ag, Au, and Pt at the surface, which seem not only useful as sensor electrodes but
also as electrode pads. The method also may be useful for developing sensors on the laboratory
scale.

1.

Introduction

Today, flexible electrode substrates are widely used in various electronic devices.   They areoften used at the joints of moving parts, such as the hinge of a foldable laptop computer and mobile
devices. Flexible substrates are also used as connecting cables inside many electronic devices tosimplify the assembly of multiple electronic components in a limited space. For example, they
connect the main circuit board and liquid crystal display (LCD) panel in liquid crystal televisions.
Moreover, flexible substrates have become important components for wearable devices in order to
fit the surface shape of the human body.(1–4)
To develop new wearable devices, we have to prepare customized flexible electrode substrates.  
Although it is customary to outsource the preparation of substrates, which have most appropriate
features and accurate size, it is sometimes more convenient if they are produced in our own
laboratory. In the latter case, we have to carry out two types of fabrication. One is the deposition ofa conductive material on the substrate, and the other is patterning. We can use several conventional*

Corresponding author: e-mail: toyama-shigeru@rehab.go.jp
ISSN 0914-4935 © MYU K.K.

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methods, such as electroplating, vacuum evaporation, or sputtering, as the former procedure,and photolithography or screen printing as the latter. However, conventional patterning methods
often require much time and effort. Recently, inkjet printing using a conductive ink has attracted
considerable attention as a new and simple method of preparing electrode substrates.(5) This method
enables the deposition of a conductive material and patterning simultaneously. Another simple but
uncommon method is the line patterning method (LPM).(6) This method provides electrodes madeof organic conductive material on a polyethylene terephthalate (PET) film.  The method is a kind of
liftoff process using a laser printer as a patterning device. The substantive point is that the toner issoluble in organic solvents such as toluene. The treatment of the organic solvent after the printing
and deposition of metal layers removes the metal pattern with the underlying toner. The advantageof this method is that the entire fabrication, including designing the electrode pattern, is possible tocarry out in a day. Therefore, it may promote development cycle of sensors.
In the past, using PET films, we extended the LPM to prepare various metal electrodes forhomemade sensors such as glucose sensors(7,8) and shear force sensors.(9) This time, we further
explored the method to prepare electrode substrates that are capable of being soldered.

2.

Materials and Methods

2.1

Fabrication of electrode pattern

Electrode fabrication consisted mainly of three steps (Fig. 1).  The first step was the printing ofa reversal pattern of electrodes on a substrate film.  A laser printer (Seiko Epson Co., LP-S5000)
with its standard toner cartridge (black toner) was used for printing. In most experiments, athermally resistive polyimide film (Du Pont-Toray Co., Ltd., Kapton®, thickness: 50 μm) was used

Fig. 1. Fabrication of electrodes on flexible film.  There are three main steps.  The first step is laser printing ofreversal electrode patterns.  The next step is electron beam evaporation of multilayered metals.  The final step is a
lift-off process. Unnecessary metal is removed with underlying toner by dissolving it in organic solvent.

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Sensors and Materials, Vol. 28, No. 4 (2016)as the substrate for electrodes.  In some experiments, we used a PET film for comparison.  After
printing, metal layers were fabricated using an electron beam evaporator (ULVAC, Inc., EBX-8C).
Evaporation was carried out at a pressure below 5 × 10−5 Torr. Multi-layered metals were fabricatedby exchanging the source metals without opening the evaporation chamber. The thickness of eachmetal was controlled automatically using a quartz crystal microbalance (QCM) thickness monitor(ULVAC, Inc., CRTM-5A), which was installed in the evaporator.  After evaporation, the film wascut into small pieces and put into a small bottle filled with acetone.  Then, the bottle was soaked
in an ultrasonic bath and sonicated for several minutes. Finally, the sheet was gently washed withpure water and then gently wiped with an ethanol-containing tissue. We fabricated various samples
on which line patterns with different widths and multi-layered metals were deposited.
2.2

Measurement of width of line patterns

The line pattern of the sample was photographed with a scale, and the width was estimated fromthe photograph using measurement software (freeware of NIH, ImageJ 1.47v).
2.3

Adhesion test method

The method of adhesion test is explained as shown in Fig. 2. The preparation of the test sample
is shown in Fig 2(a).  A metal wire was jointed to the surface of a strip of flexible film (2 × 5 cm2),
which was coated with multilayered metals. In some cases, the solder was used as a joint material,

(a)

(b)

(c)

(d)

Fig. 2. General view of adhesion test: (a) preparation method of test sample, (b) photo of a test sample, (c)experimental setup for adhesion test, and (d) microscopy image of the attached site with a scale.

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and in other cases, epoxide adhesive was used.  Then, the opposite side of the film was bonded ona thick metal plate with double-side tape.  Both ends of the film were folded back under the metal
plate, and another metal plate was placed under the first plate and fixed with bolts.  The entire test
sample was pinched in a vice [Fig. 2(b)] and placed on a tension tester (Aikoh Engineering Co,
Ltd., Model-1308) where a force gage (Nidec-Shimpo Corp., FGP-5) was installed [Fig. 2(c)]. The
end of the metal wire was hung on a hook of the force gage. Then, the force gage was verticallypulled up at a constant speed (5 mm/min), and the maximum tensile force (i.e., tensile force at the
break point of adhesion) was recorded. After the tensile test, the wire above the adhesion point
was cut off, the adhesion part of the wrecked sample was photographed with a scale [Fig. 2(d)],and the adhesion area was estimated from the photograph with measurement software. Finally,
the maximum tensile force was divided by the adhesion area to calculate the minimum adhesion
strength.
2.4

Measurement of conductance of line patterns

The conductance of line patterns was measured with a digital multimeter (CD771, Sanwa
Electric Instrument Co., Ltd. in some cases and 7351E, ADC Co. in other cases). The probes wereplaced on each line pattern by maintaining a distance of 10 mm, and the measured ohmic value wasused to calculate conductance.
2.5

Soldering test

Soldering capability was judged using a commercial solder (H-716, Hozan Tool Industrial Co.,
Ltd.), which was composed of 60% Sn and 40% Pb.  In some experiments, we used flux (HB-20F,
Sanhayato Corp.) before soldering.

3.

Results

We deposited Cr on a polyimide film in the first trial because Cr is often used as an adhesive
layer between Au (or Ag) and other materials.  However, the Cr thin film was easily peeled off fromthe film with an adhesive tape, and we stopped using it in further experiments.  Therefore, we cameup with a way of using Ni as an adhesive layer instead of Cr.  In fact, the Ni thin film could not bepeeled off the polyimide film with an adhesive tape.
Figure 3 shows a sample of Ni (thickness: 170 nm) deposited on a polyimide film.  At firstglance, the patterning resolution seems simply dependent on that of the laser printer. However,
the edge of the line patterns seems coarse in the microscopy images [Fig. 3(b)], suggesting that theresolution of the line patterns was dependent on the size of the toner particles. Moreover, we canobserve many cracks on the surface of Ni in the microscopy image. Figure 4 shows the measured
widths of the line patterns of Ni as a function of their designed widths. Because the edge of thepattern was shaped concavo-convex, we tried to measure the width between the centerlines of bothsides. Although the actual width was about 3.5% narrower than the designed width, an almost
linear correlation was obtained. To seek the reason for the decrease in width, we checked the sizeof the film at each step of the fabrication.  However, no shrinkage of the film was observed.
The Ni pattern was not peeled off with adhesive tape. However, no line exhibited electric
conductivity. Therefore, the Ni layer alone cannot be a candidate electrode. Moreover, it turned out

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(a)

(b)
Fig. 3. A sample of fabricated electrode patternof Ni on polyimide film: (a) entire view and (b)microscopic view of the line pattern with a thicknessof 1.5 pt (0.53 mm).

Fig. 4. Correlation between the actual width of theline pattern versus the designed width. The line is
obtained by linear curve fitting (y = −0.0246 + 0.965 x,
R = 0.9996).

that the solder alone was not mounted on the Ni surface, but it was mounted when the Ni surface
was pretreated with flux.
When Ag was deposited after Ni on a polyimide film, it was observed that Ni-Ag was not peeled
off with adhesive tape, the line patterns were conductive, and soldering was possible. Figures 5(a)and 5(b) show the soldering results of the front and back sides of Ni-Ag deposited on the polyimide
film.   The photo in Fig. 5(b) clearly shows that the solder did not penetrate the metal layer.  
Moreover, the pattern was still conductive even after soldering.
When Ni was deposited after Ag, the Ag-Ni could not be peeled off with adhesive tape, and theline patterns were conductive. However, when soldering was carried out on Ag-Ni, the thin metal
layer was absorbed into the solder [Fig. 5(c)]. On the basis of these results, we realized that thedeposition sequence of metals was important for soldering capability. Thus, we carried out further
testing experiments with various combinations of multilayered metal-deposited films.
Multilayered metal patterns such as Ag-Ni (100-200 nm), Ni-Ag (100-100 nm), Ni-Ag-Ni (100100-100 nm), Ni-Au (100-100 nm), Ag-Pt (100-100 nm), and Pt-Ag-Pt (100-100-100 nm) were
fabricated on polyimide films.   Figure 6 shows the conductance of multilayered metal-depositedfilms as a function of the designed width of line patterns.   All of them exhibited almost linear
correlation.

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(a)

(b)

(c)

Fig. 5. Photographs after soldering: (a) front side of a Ni-Ag-deposited film, (b) back side of the same film, and (c)front side of a Ag-Ni-deposited film.

Fig. 6. Conductance of line pattern as a function of width.  The lines are obtained by linear curve fitting (Ag-Ni:
y = −0.0346 + 0.399 x, R = 0.989; Ni-Ag: y = −0.00571 + 0.268 x, R = 0.978; Ni-Ag-Ni: y = −0.0144 + 0.356 x, R = 0.972;
Ni-Au: y = −0.017 + 0.169 x, R = 0.973; Ag-Pt: y = −0.0123 + 0.229 x, R = 0.999; Pt-Ag-Pt: y = −0.00984 + 0.263 x,
R = 0.996).

The adherence of the deposited metals on basal films was also evaluated.  Because the peelingtest of metal thin layers with adhesive tape was merely a qualitative method, a force gage was used
for the evaluation of adhesion strength. Table 1 shows the results of the minimum adhesion strength
test of multilayered metals on films.  Here, the term “minimum” is used because the metals were
not peeled off at these values. During the test, events such as the rupture of the basal sheet occurred
instead of the peeling off of metals. All of the multilayered metals in Table 1 exhibited particularly285

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Table 1Results of adhesion tests of samples with various multilayered metals.
Deposited layers[thickness (nm)]Sheet

Adhesiveagent

Ni (170)
Ag-Ni (100-200)

Polyimide
Polyimide

Epoxy
Epoxy

Minimumadhesionstrength (MPa)
0.28
0.13

Ni-Ag (100-100)

Polyimide

Solder

1.26

Ni-Ag-Ni (100-100-100)

Polyimide
Polyethylene
terephthalate
Polyimide
Polyimide

Epoxy

0.34

Epoxy pulled off
Epoxy pulled off
Further test
stopped to avoid
damage to gage
Wire dropped

Epoxy

0.33

Sheet ruptured

Solder
Solder0.88
1.03

Sheet ruptured
Sheet ruptured

Soldering testing
○*
×
○
○*
○
×
○

Conductivity**
×
○
○
○
○
○
○

Ni-Ag-Ni (100-100-100)
Ni-Au (100-100)
Pt-Ag-Pt (100-100-100)

Table 2
Qualitative summary of experimental results.
Deposited layers (nm)AdhesivenessNi (170)○
Ag-Ni (100-200)○
Ni-Ag (100-100)○
Ni-Ag-Ni (100-100-100)○
Ni-Au (100-100)○
Ag-Pt (100-100)○
Pt-Ag-Pt (100-100-100)○

Remark*Adhered only when flux was used.
**Conductivity is indicated as ○ when the measured value at the line width of 0.7 mm was larger than 0.1 S.

strong adhesion (note that the atmospheric pressure is about 0.1 MPa). Ni was a superior adhesive
between the polyimide film and Ag/Au in our experiment.  Moreover, Pt adhered to the polyimide
film and Ag.  Because we used PET film instead of polyimide film previously, adhesion tests witha PET film as a comparison sample were carried out.  In the case of Ni-Ag-Ni deposited PET film,
the multilayered metal tightly adhered to the film.
In addition, a soldering test was carried out and the results are shown in Table 2. In this table,
conductivity was judged as acceptable when the value at a line width of 0.7 mm was larger than 0.1S.
According to this table, polyimide films deposited with Ni-Ag, Ni-Ag-Ni, Ni-Au, and Pt-Ag-Pt canbe used as solderable electrodes.
Because all the experiments explained here were done with lead-containing solder, we carriedout further soldering tests using a lead-free solder (H-716, Hozan Tool Industrial Co., Ltd.), which
was composed of ca. 99.3% Sn and 0.7% Cu.  Polyimide films deposited with Ni-Ag (100-100 nm)
and Ni-Au (100-100 nm) were solderable with the lead-free solder.

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4.

Sensors and Materials, Vol. 28, No. 4 (2016)

Discussion

Several solderable electrodes were fabricated. The capability of soldering was dependent on thedeposition sequence of metals.  Soldering was possible for a Ni-Ag-deposited film, whereas it was
not possible for a Ag-Ni-deposited film.  Although our aim is to simply provide useful electrodes,
we would like to hypothesize a mechanism of soldering to explain our results using Fig. 7. In thecase of the Ag-deposited film, Ag is easily absorbed into the solder because both form an amalgam.  
On the other hand, in the case of the Ni-deposited film, Ni does not form an amalgam with thesolder, leaving the solder on Ni.  However, we cannot use the Ni-deposited film because it has poor
conductivity. Moreover, it is well known that the Ni layer alone has a poor solderability surface.(10)In the case of the Ni-Ag-deposited film, it is presumed that the Ag layer forms an amalgam with thesolder, but amalgamation does not proceed further with the Ni layer, and the solder remains on theNi.  In the case of the Ag-Ni-deposited film, it is presumed that the solder absorbs the entire thinmetal layer.
The relative decrease in the width of line patterns is attributed to the shrinkage of the film
due to several reasons.   The first is heating during laser printing.   The second is heating during
the electron-beam (EB) evaporation.  And the final one is the treatment with acetone.   However,as already explained in the results, we did not observe any shrinkages of the film throughout theelectrode fabrication.
We can estimate the theoretical conductivity of each line pattern in Fig. 6 from the specificresistance of bulk metals. For example, the theoretical conductance of Ni-Ag (100-100 nm), witha line width of 0.7 mm and a length of 10 mm, was calculated to be 0.54 S from the bulk specificresistances of Ni (6.84 × 10−8 Ωm) and Ag (1.59 × 10−8 Ωm).(11) The measured conductivity of
Ni-Ag (0.19 S) was about a third of the calculated value. Likewise, the calculated conductanceof Ni-Au (100-100 nm) from the bulk specific resistances of Ni and Au (2.35 × 10−8 Ωm)(11) was
calculated to be 0.40 S. In this case, the measured conductivity of Ni-Au (0.11 S) was also lessthan a third of the calculated value.

Fig. 7.

Tentative hypothesis for the capability of soldering.

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In general, the decrease in conductivity of the thin metal layer is mainly attributed to the size
effect and structural defects.(10) When the thickness of the metal layer is less than that of the mean
free path of electrons in the bulk, the size effect is not negligible. According to the reference, themean free paths of electrons in Ni, Ag, and Au at room temperature are 420–580, 520–570, and
360–400 Å, respectively.(10) Therefore, the size effect was not dominant in our case because thethicknesses of these metal layers were more than 100 nm in all cases. Therefore, the decrease in
conductance could possibly be attributed to a structural defect, such as atomic vacancy, lattice
dislocation, and/or generation of grain boundary.
Despite the decrease in conductance, our fabrication method would be useful to develop
sensor devices because sensors are usually not large, and thus the loss of conductance of basal
metal patterns is negligible in most cases. Particularly in the case of chemical sensors, often
noble metals are used as electrodes because they have special features. Our method is suitable fordeveloping chemical sensors because it provides electrodes with a Ag, Au, or Pt surface. In ourprevious papers,(7,8) we already reported glucose sensors fabricated on a PET film using similar
patterning methods with additional processing. In that case, a Pt-based glucose sensitive electrode
where glucose oxidase was fixed, a Ag/AgCl reference electrode, and a Pt counter electrode were
fabricated. Therefore, it is possible to fabricate such multielectrode-based chemical sensors, which
are solderable at the end terminal, if the electrode fabrication method shown here and the postprocessing written in our previous papers are used in combination.
According to our experience in fabricating sensors,(7–9) it requires only one day to perform theentire process to obtain basal electrode films for sensors including the design of electrodes patterns.  
The preparation method for electrode films proposed here may enhance the development cycle ofsensors in laboratories.

5.

Conclusions

We have developed a simple method of preparing metal electrodes on a flexible substrate.  Thismethod uses a thermally stable film, i.e., a polyimide film, and presents depositing sequences ofmetals, thus providing soldering-capable electrode substrates. We can install circuit components
and/or connect cables directly on electrode surfaces.   Moreover, we can stack multiple flexible
electrode films by jointing their edges by soldering.  Therefore, this method would be convenient
for making prototypes of sensors in the laboratory.

Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research (A24240083 & C15K01496)
from JSPS.

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