Proc. of the 2018 International Symposium on Electromagnetic Compatibility (EMC Europe 2018), Amsterdam, The Netherlands, August 27-30, 2018.
Analysis of Transmission Characteristics of Three-Layer Flexible Printed Circuit Board Du-i Kang1,2, Hosang Lee3, Jawad Yousaf3, Wansoo Nah3 1
Department of DMC Engineering, Sungkyunkwan University, Suwon, Korea
[email protected] 2 Mobile Communications Business, Samsung Electronics. Co., Ltd., Suwon, Korea
[email protected] 3 Department of Electrical and Computer Engineering, Sungkyunkwan University, Suwon, Republic of Korea
[email protected] Abstract— A FPCB (Flexible Printed Circuit Board) has been used in various electronic products such as smart phones, wearable devices, and tablet PC’s, etc. In high performance electronic devices with multiple modules to be connected, high speed signal data usually needs to be transmitted through FPCB between the modules. Due to the laminated structure of the multilayer FPCB, however, an air gap could be introduced between the FPCB layers when it is bent, which deviates the designed characteristic impedance of the FPCB. For the prediction of the bent FPCB characteristics, a 3D electromagnetic model has been used, but the calculation time is too long, especially due to the meshed ground employed in FPCB. In this paper, we propose a circuit model which efficiently predicts the characteristic impedance of the bent FPCB with an air gap inside layers. The validity of the proposed model was verified between the EM simulation and measured results.
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Fig.1 Rigid and flex sections in FPCB: (a) a bent FPCB without buckling; (b) a bent FPCB with buckling
The number of the segments is, of course, depends on the shape of the curved part of the FPCB: the steeper the curve, the larger the number of the segments should be. The S parameters of each segment were numerically calculated using ANSYS® HFSS™ (Release 18.1.0) 3-D EM simulator and the S parameters in each segment were cascaded using DESIGNER™ circuit simulator to get the whole impedance characteristics. To check the validity of the proposed circuit model, a test sample was designed and fabricated, and the characteristic impedance was measured using reflectometry, which shows good agreement with the impedance from both the proposed model and full EM simulation.
Keywords— FPCB; air gap between layers; bent FPCB; transmission line; segmented circuit model
I. INTRODUCTION An application of the multilayer FPCB could be essential to connect the two flat PCB’s in a limited narrow space. To have the FPCB more flexible, the multilayers in a FPCB are not bonded between the two FCCL’s (Flexible Copper Clad Laminate) to keep reliability of the manufacturing process. It is not rare to see an air gap between the two FCCL’s which is bent. Fig. 1 shows a buckling in the flex region, which introduces air gaps between the layers when FPCB is bent. In this case, the mismatched characteristic impedance degrades the signal integrity of the circuit, and deteriorates the whole device performance. For the analysis of the characteristic impedance, one usually needs 3D electromagnetic analysis, however it has turned out to take a lot of time to get the results [1]. Due to these limitations, the circuit designers have been using “trial and error methods”, repeating manufacturing and measurement several times, which are time-consuming and expensive. In this paper, we propose a circuit model to efficiently analyze the signal transfer characteristics of a bent FPCB which contains an air gap. In the proposed circuit model, the curved (bent) section of the FPCB is segmented, and approximated by a short and flat transmission line, and then all the segments are cascaded in the circuit simulator.
978-1-4673-9698-1/18/$31.00 © 2018 IEEE
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II. STRUCTURE OF A MULTILAYER FPCB A. General structure of a multilayer FPCB Fig. 2 shows an example of a cross-section of a 4-layer R-F PCB (Rigid-Flexible Printed Circuit Board) and also illustrates the detailed geometrical specifications and materials information. One can see that the flex part is between the two rigid parts of the R-FPCB which is also described in the Fig. 1. As in Fig. 2, a FPCB consists of FCCL and cover-lay film (polyimide + adhesive), while a FCCL is made of a single polyimide (PI) layer with two copper layers bonded on top and bottom. The role of cover-lay film bonded on the copper foil layer is to prevent the corrosion of the copper foil and the cracks due to foreign matter or bending. Note that no adhesive is applied between the two cover-lays in the air gap.
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Proc. of the 2018 International Symposium on Electromagnetic Compatibility (EMC Europe 2018), Amsterdam, The Netherlands, August 27-30, 2018.
can see that the impedance has been reduced down to 83 due to the increase of the capacitance by the third conducting layer.
Fig. 2. A cross-section of a 4-layer Rigid-Flexible PCB and materials information. The thick-dotted line represents the Rigid-Flexible PCB.
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B. Employment of a meshed ground As shown in Fig. 2, the thickness of each material is relatively thin compared with the hard PCB for the enhanced flexibility. As the number of layers increase, the total thickness becomes thicker and flexibility will be lowered. Therefore, the thickness of each material inevitably limited. Due to this thickness limitation of the dielectric material in the FPCB structure, the target impedance cannot be matched if a solid ground plane is used as the reference ground plane. Therefore, to match the characteristic impedance of the transmission line, a mesh ground is used as the reference ground of the FPCB, which also has the effect of maintaining physical flexibility. Fig. 3 shows two structures of the differential micro-strip lines of 100 impedance with a solid ground and a meshed ground plane, respectively. The two-layer FPCB designed with a solid ground has the following specification: width (W1 = W2) of the signal line is 80 ȝm, the space (S) between the two signal lines is 130 ȝm, the thickness (t) of each conductor layer is 12 ȝm, the height (h1) of the dielectric (r = 3.0) between the first and second layers is 40 ȝm, and the height (h2) of the cover-lay film (r = 3.5) is 25 ȝm. On the other hand, the FPCB with a meshed ground has a mesh pitch of 500 ȝm, the mesh width of 100 ȝm, and a dielectric thickness (h1) of 11 ȝm.
(b) Fig. 3. Comparison of the FPCB structure of differential micro-strip lines: (a) with a solid ground plane; (b) with a meshed ground
With dielectric of 40 ȝm in thickness as in Fig. 3(a), the characteristic impedance becomes of 100 as in Fig. 4(a), but the flexibility of FPCB is greatly reduced, which cannot be used as an FPCB. To make it flexible, the width of dielectric has been reduced to 11 ȝm, then in this case the characteristic impedance is down to 40 as described (a) in Fig. 4. With the same dielectric thickness of 11 ȝm, however, the characteristic impedance becomes of around 100 if the meshed ground is employed, again in (a) of Fig. 4. Therefore, a meshed ground design has been generally used as a reference ground plane in FPCB, which satisfies both the flexibility requirement and the target impedance of 100 [2]. However, since the electromagnetic fields passing through the aperture in the meshes also induce electromagnetic interference adjacent to the meshed ground, it could deteriorate the signal transfer characteristics. The adjacent conductors may be other signal lines or conducting plane beneath the meshes. In this case, the signal transfer characteristics in the FPCB should be analyzed including the conductors beneath the meshed ground. Fig. 4(b) compares the characteristic impedance of the differential micro-strip lines with and without the third conducting layer beneath the meshed ground in Fig. 3(b). One
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(b) Fig. 4. Comparison the characteristic impedances of differential mode microstrip lines: (a) with a solid ground plane when h1 = 40 ȝm, and h1 = 11 ȝm, and with a meshed ground plane when h1 = 11 ȝm; (b) with a meshed ground plane h1 = 11 ȝm, with and without the 3rd conducting plane under the meshed ground.
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Proc. of the 2018 International Symposium on Electromagnetic Compatibility (EMC Europe 2018), Amsterdam, The Netherlands, August 27-30, 2018.
III. CHARACTERISTIC IMPEDANCE OF A BENT FPCB
2π r ×
A. Shape of a bent FPCB Fig. 5 shows two pictures of FPCB, the shape of which could depend on the assembly environment. The characteristic impedance could be varying as the FPCB is arbitrarily bent, and one cannot analyze all the possible bent shapes of FPCB, of course. That is, we need a bent FPCB model, to systematically handle a bent FPCB. In the next section, a simple model to describe a bent FPCB is presented.
angle in degree = arc 360o
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The arc is the length of the transmission line of the FPCB. The angle (ș) and r are the angle and radius corresponding to the specific arc as described in Fig. 7. z
y x
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Fig. 7. Simplified model of a bent FPCB: (a) trimetric view of the bent FPCB model; (b) side view of the bent FPCB
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Fig. 5. Two examples of FPCBs: (a) flat FPCB, (b) bent FPCB
B. EM model of a bent FPCB Fig. 6 shows the design parameters for a bent FPCB. The third conducting layer was added to the two-layer FPCB of Fig. 3(b), and the first dielectric height (h1) between the first layer and the second layer is changed from 11 ȝm to 20 ȝm so that the differential mode micro-strip transmission line is matched to 100 .
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Fig. 6. The 3-layer FPCB design parameters
In addition, each copper layer was modeled as a sheet conductor plane in 3D EM simulation, which was necessary for the code to be running without breakdown. With the thickness of the conductor plane introduced, the EM simulation was just forced to be down due to the heavy computation load. For the bent 3-layer FPCB, the first and second layers are bonded in a raw material called FCCL, so they are bent in the +z direction as shown in Fig 7. On the other hand, the third layer, which was not bonded to the second layer, are bent in the -z direction to approximate the buckling shape. In this case, the gap between the two layers is referred to an air gap, and the height of the air gap affects the impedance of the transmission line of the first layer. The angle (degree, ș) of the bent FPCB is calculated as follows [3].
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(c) Fig. 8 EM-simulated signal transfer characteristics of the bent FPCB of 30 mm arc in length for 10 bent angles: (a) characteristics impedance; (b) reflection characteristics; (c) transmission characteristics
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Proc. of the 2018 International Symposium on Electromagnetic Compatibility (EMC Europe 2018), Amsterdam, The Netherlands, August 27-30, 2018.
Figure 10 (a) shows a schematic of circuit simulator which uses the extracted S-parameters of the whole bent FPCB for the TDR simulation. Figure 10 (b) illustrates a schematic of circuit simulator for the proposed segmented model. In each segmented model, S-parameters of each segmented unit are cascaded in series. The 18 cascaded two-port elements ensure the total transmission line length to be of 30 mm long [4].
Fig. 8 shows the EM-simulated characteristic impedances, reflection coefficients (S11) and transmission coefficients (S21) of the differential transmission line for 30 mm arc in length. Fig. 8(a) compares the characteristic impedances between the flat FPCB (ș = 0°) as the reference and the bent FPCBs with the bent angle from 0.2° to 30°. As the gap between the third conducting layer and the second meshed ground plane, i.e., the height of the air gap, is increased, the characteristic impedance is also increased mainly due to the decrease in capacitance. The increase of the characteristic impedance by the bent angle is gradually saturated and does not increase anymore, and the value of the saturated maximum impedance approaches the impedance of the 2-layer FPCB without the third conducting layer. One can also note that even with very small angle introduced (~0.2°) the change of characteristic impedance, and S-parameters is quite abrupt. IV. SEGMENTED EM AND CIRCUIT MODEL FOR A BENT FPCB A. Proposed circuit model In spite of the relatively simple bent model of FPCB (DUT) shown in Section III, it takes a long time to get the results of full 3D EM simulation, as described in Table I. In this paper, we propose a segmented model to divide a whole bent FPCB into smaller segments to reduce the simulation time, which will prove to be valid. Figure 9 illustrates the proposed segmentation procedure for the designed bent FPCB. Fig. 9(a) shows a side view of the bent FPCB. As shown in Fig. 9(b), the transmission line of the first layer and the meshed ground of the second layer are electrically connected by the lumped port, and the third conducting layer is floated. Each segmented unit model, in Fig. 9(a), has an air gap between the second and third layers. In addition, the segmented layers are assumed to be flat which are actually in the shape of arcs. The approximation of flat layers could be possible due to the small angle of bend. Since the segmented units are to be cascaded, the length of a segmented unit (unit length) was determined to be the multiple of the mesh pitch in x-axis to decrease the discontinuity at each unit as shown in Fig. 9(c). In Fig.9(c), one unit length equals two mesh pitches. For further simplification in the model, the segmented units can be combined depending on the shape of the FPCB. In Fig. 9(a), for example, two segments were combined to be one bigger block. For each segmented unit, S-parameters are extracted in EM simulation, and then the signal transfer characteristics can be computed using a circuit simulator. To check the validity of the proposed model, the signal transfer characteristics of the bent FPCB with the bent angle of 0.2° were obtained from both the whole FPCB model and the segmented model as well. In the full model, the Sparameters were calculated for the full bent FPCB, and in the segmented model the S-parameters in each flat segment were calculated and then they are cascaded in series in a circuit model as described in Fig. 10. In each segment, the unit length was chosen to be of 1.7 mm so the two mesh pitches in x-axis can fit the unit length.
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(c) Fig. 9. Segmented unit of the bent FPCB: (a) Side view and segmented model; (b) Enlarged view; (c) Top view
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Proc. of the 2018 International Symposium on Electromagnetic Compatibility (EMC Europe 2018), Amsterdam, The Netherlands, August 27-30, 2018.
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(b) (b) Fig. 10. Time domain reflectometry circuit simulation using the extracted S parameters for: (a) the whole FPCB model; (b) the proposed segmented model
TABEL I.Comparison of simulation time Average simulation time in 3D EM simulation for30 mm long bent FPCB Cases
(Intel Xeon(R) E5-2687W v2 @ 3.40GHz with 128GB RAM)
Whole FPCB model
~ 8 hours
Proposed segmented model
~ 2 hours
(c) Fig. 11. Comparison of the simulation in the signal transfer characteristics between the whole FPCB model and the proposed segmented model: (a) characteristic impedance; (b) magnitude of S12 [dB]; (c) phase of S12 [rad]
Figure 11 shows the characteristic impedance and transfer characteristics for the whole FPCB model and the segmented model in Fig. 10. As shown in Fig. 11, the results of the characteristic impedance for the whole FPCB model and the proposed segmented model are well matched. Table I shows the comparison of the simulation time in HFSS to extract the Sparameters from the whole FPCB model and segmented models with the specified computer hardware specifications. It can be noted that on average it took more than 8 hours to analyze the 3D full wave EM simulation of the whole FPCB model, however, it took within 2 hours for the proposed segmentation method.
V. MEASURED AND SIMULATED CHARACTERISTIC IMPEDANCE A. Prototype FPCB design and experiment A prototype FPCB for experiment was designed and fabricated as in Fig. 12 and Fig. 13(a) shows the fabricated FPCB which is bent. The rise time of the input signal was set to 100 ps for the TDR measurement. Fig. 13(b) illustrates the captured TDR test screen for the flat and 30o bent FPCB’s. Note that the white dotted box in the captured screen is due to the test pad (See Fig. 12), and are not used for the data comparison in Fig. 14. Only the signals in the yellow dotted box were used for the comparison, and one can see that the flat FPCB is matched to the impedance of 100 , and the impedance of 30o bent FPCB gradually saturates to an impedance of 130 , which is equivalent to the peak impedance in the simulation.
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Proc. of the 2018 International Symposium on Electromagnetic Compatibility (EMC Europe 2018), Amsterdam, The Netherlands, August 27-30, 2018.
circuit model) is shown in Fig. 14. Good agreement between the simulated and measured results confirms that the proposed segmented model is quite accurate for the predication of the characteristic impedance of a bent FPCB with lesser computational resources requirements. We tried to analyze a complicated bent FPCB model having complex shape, and found that the 3D analysis tool (HFSS) was forcibly terminated without completing the analysis due to the heavy computational load. This suggests that the proposed segmentation strategy could be helpful for the prediction of signal transfer characteristics in a complex FPCB structure at design stage.
Fig. 12. A schematic for the prototype FPCB
VI. CONCLUSION In this work, an efficient segmented model is proposed for the prediction of the transfer characteristics for a bent FPCB. Full wave 3D EM simulation models for the whole FPCB and for the segmented FPCB were developed, and a prototype FPCB was designed and fabricated. The measured and simulated results show that the signal transfer characteristics of the bent FPCB are well matched with the measurement in the developed models. Also it was found that the proposed segmented unit model reduces the simulation time by the factor of 75% compared with the whole FPCB EM simulation, which verifies the validity of the proposed model. The proposed segmented model approach could be useful for analysis of signal transfer characteristics of the bent FPCB with arbitrary bent shape and length.
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ACKNOWLEDGMENTS
Fig. 13. (a) Fabricated prototype FPCB; (b) TDR test results in time domain. The white dotted box is due to the test pad, and the yellow dotted box corresponds to the characteristic impedance to be compared with the simulated waveform.
This study was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2016R1A2B4015020) and by DMC (Digital Media and Communication) academic training system between Sungkyunkwan University and Samsung Electronics. Co., Ltd. REFERENCES [1]
[2]
[3]
Fig. 14 Characteristic impedances for the measured data (solid line), data from the whole FPCB model (dashed green line), and the data from the proposed segmented model (dotted line) with the angles of 0o and 30o
[4]
The comparison of the measured and simulated results (for the whole FPCB EM model and the proposed segmented
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