EDA365. DesignCon Shielded Flex Circuitry - What Drives Performance? Jim Nadolny, Samtec

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1 DesignCon 2010 Shielded Flex Circuitry - What Drives Performance? Jim Nadolny, Samtec jim.nadolny@samtec.com Julian Ferry, Samtec julian.ferry@samtec.com

2 Abstract Flex circuitry is increasingly used in high speed digital applications and in EMI severe environments including avionics. In these environments, shielding performance is a design consideration, yet there is very little data and even less practical design guidance for EMI performance. In this paper, we will look at different flex assembly constructions and how the different constructions impact shielding performance. Solid copper ground planes, silver ink shields, and crosshatch ground planes are all possible reference plane choices, and each has their own unique SI and EMI considerations. In this paper, data and analysis is presented to derive practical design solutions. Authors Biography Jim Nadolny Samtec Senior SI/EMI Engineer - Jim has as a MSEE from the University of New Mexico and has more than 15 years experience in the connector industry. He has worked on EMI performance issues of cables and connectors in the past including the development of test chambers specifically for passive interconnects. His other major area of focus has been using S-parameters for signal integrity simulation and analysis. Currently, he is involved with developing test and analysis methods for passive interconnect. Jim is a former chair of TC-10 a technical committee on Signal Integrity within the IEEE EMC Society and is a frequent presenter within the IEEE and at DesignCon. Julian Ferry Samtec High Speed Engineering Manager - Julian earned a BSEE with an emphasis in RF and Microwave Engineering from Penn State University, University Park, PA. He has more than 20 years experience in the high speed interconnect industry focusing on test and simulation, product design and development, and team management. Julian has been granted 12 US and numerous foreign patents covering products and processes for improved signal integrity and EMC performance.

3 Introduction Flex circuitry can be used in digital microwave applications (PCIe 2.0 and 10GbE ) and is often analyzed with similar tools and techniques as traditional printed circuit boards (PCBs). One difference between flex circuitry and PCBs is that flex circuitry offers return path choices that are not typically found in PCBs. For example, cross hatch ground planes and conductive inks are much more mechanically flexible than solid copper and are design options for flex circuitry which are not typically encountered with traditional PCBs. Analyzing and understanding the SI/EMI aspects of these alternate reference structures in flex circuitry is the basis for this paper. The paper flow follows the approach a designer would take when selecting a flex circuit for an application. First, we describe a flex circuit and look at signal integrity; if the flex will not function; there is no point in being concerned about shielding performance. Next, we look to understand flex shielding and to point out some misconceptions. We follow that with analytic and computational predictions of shielding performance and compare them to measurement. Finally, we consider real world effects of shield termination and the use of high density connectors at the flex assembly level. We conclude with some application recommendations for flex circuitry. Flex Circuit Electrical Advantages Flex circuit construction is very similar to conventional PCB construction where imaging, etching and lamination are the core processes. The key difference is that a flex circuitry does not use rigid core along the entire length; rather flexible polyimide or similar materials are used. This allows flex circuits to be bent, folded, and twisted similar to a sheet of thin plastic. The degree of flexibility is determined by the thickness and materials used in the construction. Classic applications are where motion is involved such as printer heads or where a tight bend radius and high density is needed such as flat panel displays. Data rates in the 5-8 Gbps are mainstream today, and at these speeds, flex circuits can actually offer electrical advantages over traditional rigid PCB materials and designs. Common flex materials are available that demonstrate electrical stability to at least 20 GHz. And unlike typical rigid PCB materials, flex circuitry does not typically use fiber glass based reinforcement. This can be advantageous at higher speeds because material property variations caused by glass fiber bundles have been shown to introduce skew and impedance discontinuities. The mechanical advantages of flex based interconnects can make them ideal for applications involving wide physical location tolerances where typical ridged PCB based riser cards or tall stack height connectors could prove challenging. In high data rate applications, the flex circuitry must be treated as a transmission line with consideration given to characteristic impedance, loss characteristics, and crosstalk between circuits.

4 Flex circuitry is an ideal transmission line media for high data rate applications for lengths up to several feet. At longer lengths, cables with low loss dielectrics and larger conductor cross section have an inherent performance advantage. Ground Planes, Shields, and Return Paths It is important to differentiate the terms shield, return path, and ground plane. In this paper, the term ground plane is used as a mechanical, or physical term, to describe a uniform conducting layer in the flex stack-up. Ground planes are wide, while traces are much narrower. Return path is an electrical term used to describe a conductor which caries a portion of the signal current. The return path can be a ground plane, a nearby trace, or possibly even an ill defined group of conductors in a poorly designed system. A shield is a conductive layer which may or may not have a DC connection to the signal. A shield s main purpose is to reduce radiated emissions. A ground plane can function as a shield, but as we shall see later, some ground plane materials work well as a shield, but not as a return path. We will focus on the most common types of flex circuits which can also be considered the building block for more complicated flex designs. A two layer flex circuit for high speed applications will have a signal layer and a ground plane return path; that is, an embedded microstrip cross section. A three layer flex circuit will have a signal layer between two ground planes; a stripline cross section. There are several options for constructing a ground plane in a flex circuit. Conductive inks can be applied, typically to outer surfaces. Inks are attractive from mechanical and cost perspectives, but they often exhibit higher resistance and less consistency than solid copper foils. Cross hatch copper ground planes are more flexible than solid copper, and they are common in flex circuitry. Finally, traditional solid copper ground planes can be used where the highest electrical performance is required. Signal Integrity Aspects of Return Path Options The first electrical consideration of a high speed flex circuit design needs to be Signal Integrity (SI). If there is so much loss or crosstalk in the design that the data is corrupted, there is not much point in going further. To investigate this aspect, an array of samples were designed and manufactured for a set of experiments. The sample matrix is shown below in Figure 1.

5 Microstrip solid copper ground plane cross hatch ground plane Stripline solid copper ground planes cross hatch ground planes conductive ink ground plane 1 solid copper ground plane, 1 conductive ink ground plane Figure 1. Flex Circuit Sample Matrix The samples were 20 inches long and are designed for 50 ohm characteristic impedance. The flex circuits are commercially offered by Samtec as an HFEM Series (stripline) and a HFEM2 Series (microstrip). These assemblies mate with Final Inch Test and Evaluation Boards available from Samtec which permit the connection of test equipment to the samples. For this test, an Agilent 4-port network analyzer was used to acquire Touchstone files which were post processed in Agilent Advanced Design System for display. Signal density is a driving factor in many commercial applications and contrasts sharply with SI design guidelines. A SI standard design guideline is that the edge-to-edge spacing between traces needs to be at least three times the trace width for reasonably low crosstalk. To contrast that, designs that are much more aggressive than that are common to achieve the required connectivity. The samples in this study reflect the density requirements customers demand and have an edge-to-edge spacing 1.5 times the trace width. This is the case for both the microstrip and stripline designs. Figure 2 shows that for the microstrip flex circuits, there is an appreciable difference in insertion loss, particularly for the sample with a conductive ink return path. This is not the case for the stripline flex circuits. For the stripline flex circuit with a conductive ink ground plane, there is also a copper ground plane which is why we don t see the elevated loss in this case. Figure 2. Flex Circuit Insertion Loss Figure 3 shows that the far end crosstalk (FEXT) is higher in the microstrip flex circuits compared to the stripline flex circuit. Note that the connector and flex circuit crosstalk is included in these graphs, and it is difficult to discern which is dominating the crosstalk

6 profile. It is obvious that the crosshatch ground plane case has much higher FEXT in the microstrip flex circuit. The effective pulse risetime for the time domain portion of Figure 3 is 20 ps. Figure 3. Flex Circuit FEXT Figure 4 shows the near end crosstalk (NEXT) for the microstrip and stripline flex circuits. The time domain representation is informative as one can separate the connector crosstalk at the beginning of the trace from the flex circuitry crosstalk. The flex circuit samples which used a conductive ink had elevated near end crosstalk. The effective pulse risetime for the time domain portion of Figure 4 is 20 ps.

7 Figure 4. Flex Circuit NEXT From a flex circuit design perspective, electrical and mechanical tradeoffs exist. Solid copper ground planes are superior to the cross hatch ground plane or the conductive ink ground plane; however, they are mechanically more stiff. Care must be taken with crosstalk, particularly with a microstrip cross hatch ground plane construction. Shielding and EMI Reduction The concept of shielding can be straightforward or utterly baffling for many designers. A radiating noise source inside a sealed rectangular box which has several holes is conceptually simple. The leaky bucket analogy is used, and we can visualize more radiation leaking out of the bucket as the holes get larger. Going one step further, it is not difficult to follow that that the maximum aperture dimension is more critical than the cross sectional area of the aperture. So slots and seams are generally a bigger leakage source than holes in shielded enclosures. But this concept of shielding falls apart completely when discussing flex circuits and printed circuit boards. German, Ott and Paul [1] showed that radiated emissions from a PCB were reduced by db when an image plane was introduced. The image plane was a sheet of copper placed in close proximity to the reference circuit. Is the circuit shielded? Not really because the top half of the circuit is completely open. Our leaky bucket analogy is completely inappropriate for understanding the mechanisms for emissions reduction in

8 this case. This study was done in 1990, and it expanded the importance in understanding ground noise. A ground plane has finite inductance, and longitudinal voltages will be developed across the ground when RF currents flow across the plane. Longitudinal means the voltage is developed along the ground plane; the RF potential at one end of the ground plane is different than the RF potential at the other end of the ground plane. Formalisms exist to describe the finite inductance of the ground plane. Holloway and Kuester [2] describe it as the net ground plane inductance. The net ground plane inductance is the self partial inductance of the ground plane minus the mutual partial inductance of the ground plane. Koledintseva, Drewniak, Van Doran and Hockanson [3], [4] describe a parasitic ground plane inductance and consider the ground plane flux passing through a common mode ground plane loop. This approach can be extended from PCB s to connectors as well [5], [6]. Another approach common in coaxial cable shielding considers the transfer impedance of the shield [7]. The transfer impedance is the transfer resistance plus the transfer inductive reactance. All of these methods describe ways to calculate and/or measure ground noise voltage developed by virtue of the ground plane inductance. It is helpful to think of ground noise, or transfer impedance, when trying to understand PCB, flex circuit, or connector emissions. Prediction of Flex Circuit Shielding Performance Commercial microwave analysis tools [8] can be used to analyze the transfer impedance of flex circuitry. A relationship between transfer impedance and shielding performance can be developed provided the shielding performance is uniquely defined. We will make use of the earlier work by Hoeft [7] which shows a simple relationship between transfer impedance and shielding performance using the reverberation chamber method. SE = log Z Total In the above equation, SE is the shielding effectiveness and Z Total is the total transfer impedance due to electric and magnetic field coupling. The above relationship was verified in [9] and [7] for a small aperture in very well shielded coaxial cable. A 2D quasi-static analysis of a flex circuit cross section can be performed which results in per unit length transmission line parameters. Once this analysis is performed, dynamic tuning of length is possible. The cross section can be parameterized and tuned real-time making transfer impedance analysis possible in several minutes. This approach means that modern microwave analysis tools can be used as a design tool as opposed to an analysis tools which requires more than several hours of computation time. Analysis tools, which use full wave methods, can be used for transfer impedance calculations of more complicated geometries that are not well represented by a simple 2D cross section. To verify the approach, we can compare the results from a 2D analysis with previous work on ground plane inductance [3]. Figure 5 shows the result of that comparison, and there is good correlation between the earlier work and the results from commercial tools.

9 It is useful to analyze Figure 5 to gain an understanding of emission reduction from a planar transmission structure. Recall that transfer impedance is proportional to the net ground plane inductance, and that is plotted on the vertical axis of Figure 5. Note that the ratio of ground plane width to trace height above the ground plane is the critical parameter. This implies that a narrow trace close to the ground plane has lower emissions than a wide trace further from the ground plane, even if both geometries result in a 50 ohm characteristic impedance. ADS 2009 Multilayer T-Line Model trace width = 6.5 mils Figure 5. Parasitic Mutual Ground Plane Inductance for microstrip geometry versus the ratio of ground plane width (w) and distance from the ground plane to the trace (h) [3] A second method to verify that a 2D quasi static tool is giving reasonable estimates of the transfer impedance is to recall the definition of transfer impedance at low frequency. At low frequencies, the inductive reactance of the ground plane is small, and the transfer impedance simplifies to the DC resistance of the ground plane. Given the thickness, width, and length of the ground structure, the DC resistance can be calculated by hand and compared to what the tool provides. Correlation between the hand calculation of DC resistance and the analysis tool agreed within 5%. We now have a method to simulate the shielding performance based on a flex circuit construction. Applying this technique to the samples described in Figure 1, we can generate the results in Figure 6.

10 Figure 6. Transfer Impedance of Flex Circuitry Figure 6 shows that the transfer impedance of the stripline flex circuit is lower than that of the microstrip flex circuit. This is expected as the stripline circuit has two ground planes inductances essentially in parallel. The construction of the stripline circuit also has the trace closer to the ground plane which decreases the net inductance of the ground plane. This transmission line is 20 inches long and begins to show resonance effects above 200 MHz. Measurement Setup The Mode Stirred Chamber Method is documented in IEC and was used in this testing. The method relies on exposing a device to electromagnetic energy in a large resonant cavity (shielded room). An electrically large tuner perturbs the boundary conditions of the cavity resulting in different standing wave patterns and a randomized excitation of the device. Multiple device measurements are made at different tuner positions, and the results are averaged. Shielding effectiveness is defined to be relative to an in-band reference antenna for IEC If the shielding effectiveness is 20 db, it means that the received power with the sample in place is 20 db lower than the received power when a reference antenna is in place. A log periodic antenna serves as the reference from 200 MHz to 2 GHz, and a double-ridge guide horn antenna is the reference from 2 GHz to 20 GHz. This method has a practical high frequency limit determined by the instrumentation used, in this case 20 GHz. The low frequency limit is determined by the size of the chamber, which in this case is 200 MHz. The system used for this testing is a SMART 200 system by ETS-Lindgren and is shown in Figure 7.

11 Control PC Multidevice Controller P2 Receive line VNA SW AMP AMP P1 Detection shielded box DUT 20 flex circuit Termination shielded box Figure 7. Mode Stirred Chamber Method The sample construction was optimized for shield termination. The outer insulating film was removed exposing the ground planes. These surfaces could them be clamped between brass blocks. Figure 8 shows the shield termination approach. Tuner Transmit Antennas Figure 8. Flex Circuitry Shield Termination A second phase of the test program measured the shielding performance when the flex circuitry was terminated using a high density multipin connector. The connector and test board is documented in [11]. A photograph of the flex circuitry terminated with a test board is shown in Figure 9.

12 Figure 9. Flex Circuitry Terminated to a QTE/QSE Final Inch Test Board Measurement Results The results of the shielding tests are shown in Figure 10. Figure 10 shows the stripline flex circuit has roughly db of higher shielding effectiveness than a microstrip flex circuit. This compares fairly well with the simulated results that used a quasi static 2D field solver to compute the net ground plane inductance. The net ground plane inductance could then be transformed to a shielding effectiveness value. At 200 MHz, the analysis predicted a SE of 50 db for the solid copper microstrip case and 70 db for the solid copper stripline case. The analysis showed a generally flat response over several decades of frequency, and this also compares well with the measured data. Figure 10. Shielding Effectiveness of Microstrip (left) and Stripline (right) Flex Circuitry For the microstrip case, all three different ground plane constructions had similar performances. The conductive ink sample had roughly 10 db reduced performance in the 800 MHz 1 GHz range, but generally performed as well as a much stiffer solid copper

13 ground plane. For the stripline flex case, the cross hatch ground plane had roughly 15 db lower performance than either the conductive ink or solid copper ground plane constructions. The second part of the testing considered the case where the flex circuitry is terminated with a high density, multipin connector and test board. Shield termination and connector effects can also be analyzed via a return path inductance/transfer impedance method [5], [6], but is not the focus of this effort. Figure 11 shows the effect of real world termination. With the connectors and test boards, we see similar shielding performance for the microstrip and stripline flex circuits. We also see a decrease in shielding effectiveness with frequency for both stripline and microstrip samples. One conclusion that can be drawn is that at the assembly level the shielding benefits of a stripline flex circuit are compromised by the high density connector. Figure 11. Shielding Effectiveness of Microstrip (left) and Stripline (right) Flex Circuitry Terminated with a Connector and Test Board Conclusion and Application Recommendations In this paper, we have tried to show the value in thinking of flex circuitry shielding performance in terms of transfer impedance and the return path net inductance. We have shown that commercial microwave analysis tools can be used to compute the transfer impedance of a flex circuit, and that the tuning features allow for real time design decisions. The use of alternate ground plane materials has more of an effect on signal integrity than it does on shielding effectiveness for the samples studied in this effort. A microstrip flex circuit with conductive ink has significantly more conductor loss compared to a conventional copper ground plane. The choice of ground plane material can also have a significant impact on crosstalk. The samples in this study were designed for very high density and relatively moderate performance, so the crosstalk in the flex circuit is

14 significant. In the case of a microstrip flex circuit, the conductive ink ground plane had nearly twice the crosstalk compared to the copper ground plane. Figure 12 provides a design checklist for flex circuits based on the results presented here. Ground plane Insertion Loss Crosstalk Shielding Flexability Microstrip solid copper crosshatch copper conductive ink Stripline solid copper crosshatch coper conductive ink Figure 12. Qualitative Comparison of Flex Circuit Ground Plane Options

15 References [1] R. German, H. Ott, C. Paul, Effect of Image Plane on Printed Circuit Board Radiation, 1990 International Symposium on Electromagnetic Compatibility, Washington DC, [2] C. Holloway, E. Kuester, Net and Partial Inductance of a Microstrip Ground Plane, IEEE Transactions on Electromagnet Compatibility, February 1998 [3] M. Koledinsteva, J. Drewniak, T. Van Doren, D. Hokanson, External Parasitic Inductance in Microstrip and Stripline Geometries of Finite Size, IEEE International Symposium on Electromagnet Compatibility, 2002 [4] M. Koledinsteva, J. Drewniak, T. Van Doren, D. Hokanson, Method of Edge Currents for Calculating Mutual External Inductance in a Microstrip Structure, PIER 80, 2008 [5] D. Hockanson, X. Ye, et al, FDTD and Experimental Investigations of EMI from Stacked-Card PCB Interconnections, IEEE Transactions on EMC, February 2001 [6] X. Ye, J. Drewniak, J. Nadolny, D. Hockanson, High Performance Inter-PCB Connectors: Analysis of EMI Characteristics [7] L. Hoeft, A Simplified Relationship Between Surface Transfer Impedance and Mode Stirred Chamber Shielding Effectiveness of Cables and Connectors, IEEE International Symposium on Electromagnetic Compatibility, Sorrento, Italy, September [8] Advanced Design System 2009, Update 1, Agilent Technologies [9] J. Nadolny, J. Ferry, C. Arroyo Mode Conversion and EMI Performance of Shielded Cable Assemblies for 10 Gbps Data Transmission, DesignCon 2008 [10] IEC , Testing and Measurement Techniques - Reverberation Chamber Test Methods [11] Samtec, Final Inch Test and Evaluation Kit User s Guide FI-QTE/QSE-01, s%20guide.pdf