Fabrication of flexible tactile force sensor using conductive ink and silicon elastomer

Fabrication of flexible tactile force sensor using conductive ink and silicon elastomer

Accepted Manuscript Title: Fabrication of flexible tactile force sensor using conductive ink and silicon elastomer Author: Chullhee Cho Youngsun Ryuh ...

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Accepted Manuscript Title: Fabrication of flexible tactile force sensor using conductive ink and silicon elastomer Author: Chullhee Cho Youngsun Ryuh PII: DOI: Reference:

S0924-4247(15)30212-0 http://dx.doi.org/doi:10.1016/j.sna.2015.10.051 SNA 9390

To appear in:

Sensors and Actuators A

Received date: Revised date: Accepted date:

31-8-2015 29-10-2015 30-10-2015

Please cite this article as: Chullhee Cho, Youngsun Ryuh, Fabrication of flexible tactile force sensor using conductive ink and silicon elastomer, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2015.10.051 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fabrication of flexible tactile force sensor using conductive ink and silicon elastomer Chullhee Cho, Youngsun Ryuh* [email protected] Robot R&BD Group, Korea Institute of Industrial Technology, Korea. *

Corresponding author.

Highlights 

We made a flexible tactile force sensor using conductive ink and silicon elastomer.



We realized good linearity, high-selectivity measurement, and good repeatability.



The CEST sensor is more competitive and extremely cheaper than the FSR sensors.



A prototype CEST sensor is now using in a real-time robotic feedback control system.

Abstract A novel design for a flexible tactile force sensor, which is called conductive elastic silicon tactile (CEST) sensor, is proposed in this paper. The flexible tactile force sensor is fabricated using a molding process of a composite material, which is a mixture of two components: conductive ink and silicon elastomer. The composite material is poured into a mold, which contains an array of cavities with desired shapes. After curing at room temperature, the solidified part is ejected out of the mold and attached to a designed flexible printed circuit board film. Five tactile sensing elements in the fabricated CEST sensor layer enable independent detection of the external normal force. The efficacy of this sensor is tested when a 0-5 N normal force is applied. Good linearity of 0.93, high selectivity in real-time measurement, and good repeatability of 98.67% are observed. By comparing the characteristics of CEST with those of one of the most used force-sensitive resistor sensor, the CEST sensor achieved competitive sensitivity with better repeatability, high selectivity, and extremely low cost. Keywords: Tactile; force sensor; flexible; conductive ink; silicon; elastomer; conductive; CEST; normal force

1. Introduction Humans explore and interact with the environment via the five main sensory receptors: vision, sound, gustation, olfaction, and touch [1,2]. To obtain relevant information from new and unstructured environment, humans utilize one or a combination of these senses [3]. In this respect, the importance of studies concerning the principle and contribution of each sensory system in acquiring and collecting information from the environment has dramatically increased, especially in improving the sensing capabilities of the forthcoming robotic system. Among these senses, many parts of the complex manipulation and exploration tasks rely on tactile perception in human skin. Tactile sensing, which corresponds to touch sensory receptors, has become an integral part in the advancement of sensors in robotics. Tactile sensing systems provide necessary information in detecting the environment through physical touch measurement, such as vibration, temperature, hardness, and applied force. Thus, tactile sensor systems are prerequisites for implementation in robots that deal with dexterous tasks [4,5]. Furthermore, developing flexible tactile sensors that emulate the natural touch of a human is also required, in which precise perception leads to sophisticated manipulation. Many studies have reported on the development of tactile sensors based on various transduction methods, including strain gauge, piezoresistive, capacitive, magnetic, inductive, and optical methods and so on [2]. Most of the pre-developed sensors were fabricated via micro electro mechanical systems (MEMS) technology because of its paramount role in making a tiny array with a fine spatial resolution. However, one of the problems suffered by these MEMS-based sensors is that they are not flexible because of their substrate, which is rigid silicon. To overcome this problem, some tactile sensors have been implemented using flexible polymer substrates such as polydimethylsiloxane [6-11]. Flexible tactile sensors that use piezoresistors, which are among the most common transduction method in the form of strain gauge, have been developed to sense the deflection of a designed membrane [12,13]. Good sensitivity and repeatability have been achieved; however, the fabrication processes were complex, and they could only detect a single contact point, which is not suitable for multiple and wide-range detection. In addition, piezoresistors are prone to temperature changes. Further, piezoresistive cantilever-type sensors have been developed, which were derived from a bio-inspired artificial hair cells. However, they still suffered from similar disadvantages mentioned earlier and were too fragile for robotic application [6, 7]. The capacitive method is also a well known and one of the most-used methods in recent

research on developing flexible tactile sensors. The developed can determine the applied force through capacitance change in terms of the change in the embedded air-gap distance and change in the overlapped areas between the top electrode and the combination of all the bottom electrodes [8-10]. Soft and sensitive force sensors have been developed, but they were susceptible to noise, especially in mesh configurations due to crosstalk noise, and they need relatively complex electronics to filter out this noise [10]. Recently, tactile sensors that use optoelectric technologies have also been studied. Optoelectronic components, which consist of LED–photoresistor couples positioned below an elastic dome-shaped layer, detect the deformation of a deformable silicon layer via the variation in the reflected light intensities and induced photocurrent flowing into a photodetector. The devised sensor provides a widerange frequency response, and it is immune to low-frequency electromagnetic interference generated by an electrical system. However, its bulky size and considerable processing power consumption are major disadvantages to apply it to robotic systems [14]. Recently, as an alternative–competitive option, the idea of developing soft tactile sensors for robotic systems has attracted the attention of researchers, and such type of sensors have already been developed. Among these sensors, pressure-sensitive rubber was used as a sensor material in many studies. A soft areal tactile sensor made of pressure-sensitive conductive rubber without wiring has been developed, and it can measure pressure distribution in real time. However, the resistivity distribution obtained from the region in the sensor must be reconstructed by a complex computation technique, called electrical impedance tomography [15]. Microstructured rubber has been used as a dielectric layer in a recent study that developed a highly sensitive flexible pressure sensor. The pyramid-structured film, which acted as a passive capacitor and integrated with an active organic field-effect transistor, demonstrated very high sensitivity. However, it is very complex, and the production cost is exorbitant [16]. As presented in the aforementioned studies, using a conductive rubber material for areal tactile sensor offers many benefits; however, we still need to resolve the accompanying complexities and expensive production cost. The current research aims at overcoming these limitations and simultaneously designing a novel simple-structured flexible tactile force sensor that is extremely cheap. New polymeric materials made of conductive ink and silicon rubber with unique electrical properties have been used as casting materials. The molding material is poured into a fabricated mold with an array of desired shapes of cavities. After

solidification, it is used as a sensing layer on a flexible printed circuit board (F-PCB) film. The conductivity of the composite material critically affects the sensitivity of the sensor. However, the conductivity of the composite significantly depends on the volume fraction of the conductive ink; hence, the effect of varying the volume fraction of the conductive ink on the characteristics of the material was investigated. The detectable force range, selectivity, sensitivity, and repeatability of the fabricated areal sensor called conductive elastic silicon tactile (CEST) force sensor were also investigated. In addition, the sensitivity and repeatability of the CEST sensor were compared with those of commercially available forcesensitive resistor (FSR) sensors.

2. Procedures of CEST sensor production The production process of the CEST sensor consists of four steps: wet mixing of the conductive ink and silicon rubber, molding, curing, and separation with the addition of a protective layer. Fig. 1 shows the schematic diagram of each step. Incorporation of conductive fillers in an insulating matrix is well known to allow generation of a new polymeric material with unique electrical properties, which can be used in electronic applications [17]. Among the conductive fillers, carbon black is one of the best candidates owing to its high electrical conductivity, low cost, innocuousness, and environmental stability [18]. Hence, carbon black-based conductive ink (Electric Paint, Bare conductive), whose the resistivity is 0.27 Ω cm was chosen to create a composite material in the water solvent. In the first step, a composite material consisting of conductive ink and liquid silicon rubber (KE45, Shin-Etsu Chemical Co.,Ltd.) was created by ultrasonic vibration using the wet mixing method. The wet mixing method enables wider dispersion of carbon particles into the silicon matrix, which leads to a lower average gap width between the conducting particles and higher probability of formation of effective conductive path, whereas inhomogeneous mixing of conductive particles are observed when the composite is fabricated by the conventional mixing method, denoted as the dry-mixing method [19-21]. An experiment that compared mixing carbon particle with the elastomer and mixing conductive ink with the elastomer using a similar process was conducted and the results of the SEM observation are shown in Fig. 2. Several agglomerates, which formed clusters and were isolated from the neighboring particles, were observed when carbon black was mixed with the elastomer, as shown in Fig.

2(a). The measured resistivity was approximately 34.47 Ω cm. However, when the conductive ink was mixed with the elastomer, wider distribution of carbon black particles were observed similar to that in Fig. 2(b), and the measured resistivity was approximately 26.65 Ω cm. This wider dispersion leads to significant improvement for practical applications, such as gradual fall in resistivity with pressure and very good repeatability in successive measurements [19]. However, the resistivity of the composite material, which is related to conductivity, significantly drops by orders of magnitude at a critical volume fraction of conductive ink without applying any external pressure. Therefore, investigating the critical volume faction of the conductive ink, which allows strong pressure-dependent resistivity, is important so that the composite can be used as a force sensor. This critical volume fraction, also called percolation threshold, can be determined by the percolation and effective medium theories. The result of this value is discussed later in Section 4. The composite material was then poured into a mold, which contains an array of cavities with desired shapes, as shown in Fig. 1(b). In this study, the mold had an array of nine identical cone-shaped cavities. This mold was fabricated by conventional micromachining. After pouring, the composite material was cured at room temperature for approximately 8 h under uniform pressure at the top surface, as shown in Fig. 1(c). When the composite material completely solidified, it was ejected out of the mold, as shown in Fig. 1(d). This solidified layer had an array of nine sensing elements, which are called taxels in the studies of tactile sensors. To prevent any noise during sensing, such as the noise that occurs from a touch by a human finger, and to protect the sensor-array layer, the molding process of a silicon rubber layer on top of the sensor array layer was implemented, as shown in Fig. 1(e). In the final step, the fabricated sensor layer was assembled on a detect layer of the designed F-PCB, as shown in Fig. 1(f).

3. Experimental setup and details The experimental setup for investigating the characteristics of the fabricated CEST sensor is shown in Fig. 3. The sensor array layer contains nine identical-shaped taxels, and the layer is assembled on the designed F-PCB. Each taxel is connected to its corresponding copper pad on the F-PCB, which is also connected to the electrical components such as GND, VCC, and signal

acquisition ports. An external normal force is applied on the CEST sensor by a feeding system constructed using an FT sensor (SI-125-03 Nano25). The feeding system can be moved in the XYZ plane. The raw signal data generated by the applied external normal force were acquired through a data acquisition system, and the signal data were filtered and analyzed by the designed software. The characteristics of each taxel were independently investigated by applying a normal force using a 3-mm-diameter rod attached to the FT sensor. Table 1 lists the details of the physical properties of the CEST sensor.

4. Characteristics of the CEST sensor 4.1 Percolation threshold of the conductive ink The mixture of conductive ink and liquid silicon rubber created a new polymeric composite with unique electrical properties. The conductivity of the composite clearly increases as the amount of conductive ink in the composite increases, whereas the resistivity of the composite increases as the amount of silicon rubber in the composite increases. However, the resistivity significantly drops by many orders of magnitude after a critical volume fraction of the conductive ink has been reached even without any external force [19,20]. Over many years, various researchers have investigated the resistivity of conductive composites as a function of pressure, but the use of materials as a pressure sensor could not be supported due to its inability to control the sudden resistivity drop in successive resistivity measurement. This result is important in that the fabricated composite must have a strong pressure-dependent resistivity so that it can be used as pressure-sensing elements. According to the percolation and effective medium theories, a critical volume fraction exists when a sudden drop in resistivity occurs. To determine the percolation-threshold volume fraction of the conductive ink, measurement of the resistivity of 10 different volume fraction groups using five different samples with the same ratio were conducted for five repetitive measurements. The resistivity values of the samples were measured using the four-point probe resistivity measurement system CMT series (Advanced Instrument Technology), and the maximum and minimum resistivity values in each sample were excluded in calculating the overall mean and standard deviation. Fig. 4 shows the resistivity curve of the composite without any external pressure, and the corresponding data are also presented. The resistivity was up to 3.8 kΩ cm when the volume fraction of the conductive ink was approximately 38%. This extremely high resistivity value

is not appropriate in using the composite as sensing materials. In addition, when the volume fraction of the conductive ink was less than 38%, the resistivity of the samples without an external force was not measured. When an external force was applied, the resistivity of these samples was detected to be in the 104 scale. However, the resistivity of the composite significantly dropped when the volume fraction of the conductive ink was approximately 45%. The mean resistivity of the composite was approximately 48.75 Ω cm. Hence, we note that the percolation threshold value of the volume fraction of the conductive ink is approximately 45%. In addition, we observed that in the case where the amount of liquid silicon rubber mixture was too low, the composite became too brittle, resulting in breakage after the curing step. When the volume fraction of the conductive ink exceeded 57%, the mixed material could not be ejected from the mold without cracks. Because the composite requires high conductivity with appropriate elastic property to be used as a sensing element, a 54% volume fraction conductive ink was selected to create composites as sensing elements.

4.2 Purpose of dummy elements The fabricated CEST sensor contains an array of nine identical taxels in the sensor array layer. Among these taxels shown in Fig. 5(a), five taxels, including one in the center, are actual sensing elements, and the other four taxels are dummy elements, as shown in Fig. 5(b). The dummy elements are connected to the GND pad on the detect layer while voltage is applied through the taxels. The sensor array acts as a variable resistor, as shown in Fig. 5(c), which makes the CEST sensor detects applied force by analyzing the change in the voltage using a pull-up circuit. When pressure is applied to the taxels, the volume of the taxels changes, which leads to changes in the number of particles per unit area and resulting in resistivity [19]. The purpose of the constructed dummy elements is to provide stability with strong adhesion and resilience between the sensor and detect layers and to reduce any disturbance occurring from adjacent elements when an external force is applied on the CEST sensor. These characteristics were examined through finite element method (FEM) analysis before the actual experiments. The initial design concepts of the CEST sensor were of three types: one with four-sided walls around the sensor layer to provide strong adhesion and stability to the detect layer [Fig. 6(a)], another within a solid body [Fig. 6(b)], and another one with a sensor array with a dummy array [Fig. 6(c)]. First, to compare the sensitivity of a single taxel when an external force of 1 N is applied on it, FEM analysis of the three different

types of models was conducted, and the results of the stresses applied on the taxels are shown in Fig. 6. We must note that when the applied stress on the taxel is higher, the volume change in the taxel is larger. Fig. 6(a) shows that the average stress examined on the taxel was approximately 4.03 kg/cm2 for Type A, Fig. 6(b) shows 1.07 kg/cm2 for Type B, and Fig. 6(c) shows the expected value of 3.46 kg/cm2 for Type C. These results indicate that relatively much more amount of applied force was absorbed by the solid part of the sensor layer, especially for Type B, which is inappropriate as a sensor property. The most sensitive reaction was observed in the Type-A model. However, another analysis was conducted to determine the feasibility of detecting the area where the sensor layer can practically detect the location of the applied force. The results are shown in Fig. 7 when the force was applied near the edge of the sensor layer. As predicted, a large amount of force was absorbed in the solid part in Type B; thus, the stress applied on the taxel near the edge of the sensor layer was marginal, which was approximately 0.59 kg/cm2. Similarly, because Type A has walls around the sensor layer, only 2.92 kg/cm2 of stress was applied on the taxel because the concomitant reaction force on the wall was substantial. In addition, if the external force is applied on any location where no sensing elements are installed below, the net reaction force that occurs from the side walls becomes too complex to estimate the location and magnitude of the actual force applied on the sensing elements.

However, when no walls but only dummy elements are present, the sensitivity of detecting the location of the applied force was most significant among the design models, which was approximately 5.33 kg/cm2. As a result, Type C was selected as the CEST sensor layer owing to its better availability in terms of detection area with good sensitivity of the taxel, even though it is slightly lower than Type A. After Type C was chosen, another FEM analysis was conducted to confirm the stress distribution on the taxels and the displacement of each taxel. Fig. 8 shows the results of the FEM analysis under two different force conditions: uniform load distribution and point load between taxels. Fig. 8(a) and (b) show that when an external force uniformly distributed over the CEST sensor area is applied, the displacement of all taxels and the stresses on each taxel were equally changed. In addition, in the case where a point load was applied among the four taxels with the same distance from one another, the same amount of displacement and stresses were obtained for each adjacent taxel. Consequently, we confirm through the FEM analysis results that the designed sensor can

detect any location by calculating the proportional changes in the deformation of each taxel, and it can detect the amount of force applied within the range of the measureable capacity of the fabricated sensor. Fig. 9 shows the fabricated prototype of a CEST sensor based on the aforementioned design concept.

4.3 Sensitivity, selectivity, and repeatability The characteristics, namely, sensitivity, selectivity, and repeatability, of the CEST sensor when a normal force was applied on it were investigated. Fig. 10 shows that the output of the sensor varies linearly with the force applied to the right above a taxel. Linear regression analysis was performed to determine the linearity of the sensor, and the result was approximately 0.93. The measureable force range of an individual taxel was approximately from 0 to 5 N. Further, all taxels in the sensor array can endure up to approximately 30 N at once. In addition, the CEST sensor can measure the external normal force in real time. Fig. 11 shows the results of real-time measurement of the experiment where an applied normal force on the sensor decreased by three discrete steps. The output values of the CEST sensor were compared with the result of a reference force sensor (SI-125-03 Nano 25). The results show that a sensing taxel in the CEST sensor can measure the applied normal force in real time in the case where the applied force changed in the range of approximately from -4 to -2 N (the negative sign means the applied force is pressing downward). At the same time, the sensor output, which is the voltage, changed from approximately 3 to 5 V. Furthermore, Fig. 12 shows the noise data of the CEST sensor without the application of a force. The noise level is approximately ±2.44mV, and according to this noise value, the minimal detection force derived was approximately 0.011N. Selectivity, i.e., how much can an individual taxel separately detect the multi-applied force, was also investigated. Fig. 13(a) shows the result when a randomly frequent force was applied only on sensing taxel 1 among the five discrete sensing taxels. Fig. 13(a) shows that when a force was applied only on taxel 1, the output (voltage) of taxel 1 significantly changed according to the applied force, whereas the output of the other taxels remained at the same value as that before the excitation. In addition, the CEST sensor also sh

owed very

good selectivity when multiple forces were applied on different taxels at the same time, as shown in Fig. 13(b). In this experiment, two discrete forces with the same magnitude and

frequency were initially applied at once on taxels 2 and 3. After 5 s, the force was sequentially applied only on taxels 2 and 3. As a result, good selectivity was achieved. Finally, an experiment to evaluate the repeatability of the CEST sensor was conducted. The force in all the repetitions was equally applied 10 times under the same condition, and the sensor output was observed. Fig. 14 shows the results of the experiment: standard deviation of approximately 0.0332 V among the 10 repetition times, approximately 15.2% hysteresis, and 98.67% repeatability were observed. By comparing these values with the characteristics of the FSR 400 series, which is one of the most widely used and commercially available force sensors, the CEST sensor achieved competitive or even better characteristics in terms of repeatability because the FSR series has only ± 2% repeatability and ± 10% hysteresis.

5. Conclusion In this paper, a novel flexible tactile force sensor was proposed and fabricated. A new flexible tactile force sensor fabricated via the molding process using a composite of conductive ink and silicon elastomer was successfully demonstrated. This paper described the detailed design and experimental characterization of the fabricated sensor. The composite material created by wet mixing method was poured into a mold containing an array of cavities with desired shapes. Through curing at room temperature and ejection steps, the solidified layer was combined with the detect layer of an F-PCB film. The cost of this entire process, which represents the production of the CEST sensor, is very cheap. The sensor layer of the fabricated sensor has nine discrete taxels. Among these, five taxels were used as sensing elements, whereas the other four taxels were connected to GND and acted as dummy taxels. These dummy taxels provided strong adhesion to the detect layer and eliminated much noise due to any reaction force that could be brought about by adjacent taxels when an external force is applied nearby. To investigate the appropriateness of the design of the proposed sensor, FEM analysis was conducted. The results of the FEM analysis confirmed that the identical dummy taxels could reduce the complexity of estimating the applied force value better than the initial design concept of sensors surrounded by four sidewalls. The fabricated flexible sensor was shown to be capable of independently measuring force components of up to 5 N using a single taxel with a good linearity of 0.93, capability of realtime measurement with high selectivity, and good repeatability of 98.67%. These

characteristics are competitive with or even better than the commercial FSR sensor. Currently, the prototype of the proposed sensor is being integrated into robotic applications for use in a real-time feedback control system and fingertip of a robotic hand. Further developments are expected in applying the tactile skin to human–object interface research.

Acknowledgements This work was supported in part the by Korea Institute of Industrial Technology (KITECH), research grant EO15022, and in the part by the Development of Human Coexistence Robot Platform and Smart Process Technology for Manufacturing Process Innovation, funded by the Ministry of Knowledge Economy (MKE, Korea).

Biographies Chullhee Cho received the B.S. and M.S. degrees in Mechanical & Aerospace Engineering from Seoul National University, Korea, in 2011 and 2013, respectively. Since 2013 he has been a Research Engineer with the Robot Group in Korea Institute of Industrial Technology (KITECH). His research interests include biomimetic robotics, sensors and micro/nano manufacturing.

Young-sun Ryuh received the B.S., M.S., and Ph.D. degrees in Biosystem Engineering (Robotics) from Seoul National University, Korea, in 1984, 1986 and 1997, respectively. From 1991 to 2000, he was with the SJMC Research Center as a director, and from 2000 to 2003, he was a CEO with Roboland Co., Ltd. Since 2003, he has been a Director with the Biomimetic Robot Research Group and Marine Robot Center of Korea Institute of Industrial Technology (KITECH). His research interests include Biomimetic Robotics, sensors and rehabilitation.

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Figure Captions

Fig. 1 Schematic diagrams of the production procedure of the CEST sensor

Fig. 2 SEM observation of (a) mixing carbon particle with elastomer, the resistivity is 34.47 ohm c m; and (b) mixing conductive ink with elastomer using the similar mixing process, the resistivity is 26.65 ohm cm.

Fig. 3 Schematics of the experimental setup

Fig. 4 Resistivity curve of the composite without any external pressure

Fig. 5 Disposition of taxels and circuit diagram: (a) arrangement of actual sensing elements, (b) arr angement of dummy elements, and (c) pull-up circuit diagram of the sensor.

Fig. 6 FEM analysis of a single taxel from three different design models when the force is applied right on the taxel. (a) Type A. (b) Type B. (c) Type C.

Fig. 7 FEM analysis of a single taxel from three different design models when the force is applied near the edge of the sensor layer: (a) Type A, (b) Type B, and (c) Type C.

Fig. 8 FEM analysis of two different conditions of applied force: (a) change in displacement of taxe ls and (b) change in stress distributions when a uniformly distributed load is applied on the CEST sensor; (c) change in the displacement of taxels and (d) change in stress distribution when a point load between four taxels is applied on the CEST sensor.

Fig. 9 Prototype of the CEST sensor

Fig. 10 Linear regression analysis of the sensor output and sensitivity

Fig. 11 Real-time measurement experiments comparing the results of the CEST sensor output with those of the reference force sensor (SI-125-03 Nano25) at the same time base.

Fig. 12 Noise level of the CEST sensor without appyling force.

Fig. 13 Selectivity of the CEST sensor taxels. (a) Sensor output when a random force was applied only on taxel 1. (b) Sensor output when random forces were separately applied on taxels 2 and 3 one at a time.

Parameters

Value (unit)

Repetition rate

10 times

Standard deviation

0.0332 V

Repeatability

98.67 % (±1.33%)

Hysteresis

± 15.2% ((RF+- RF-)/ RF+)

Fig. 14 Repeatability of the CEST sensor for 10 repetition times

Tables Table 1. Physical properties of the CEST sensor

Physical properties

Value (unit)

Dimension of sensor layer

20 × 20 × 5 (mm)

Taxel diameter

3 (mm)

Number of taxels

3 × 3 array, nine taxels

Taxel interval

5 (mm)

Dimension of F-PCB

30 × 80 × 0.15 (mm)