The effects of gaps introduced into a continuous EMI gasket When properly designed, a surface-mount EMI gasket can provide essentially the same shielding performance as continuous gasketing. THOMAS CLUPPER JOSEPH WHEELER W.L. Gore and Associates, Inc. Newark, DE The traditional EMI (Electromagnetic Interference) gasket comprises a continuous conductive material used to create a conformable seal between two conductive surfaces. These materials are usually made from conductive particles combined with a polymer to make a filled composite that has very low volume resistivity and good compressibility. When used to fill the gap between two metal pieces, such as a metallic enclosure and a ground trace on a printed circuit board, the gasket joint essentially prevents electromagnetic energy from getting through. New forms of EMI gaskets have been introduced in SMT (Surface Mount Technology) compatible shapes that allow for easy placement directly onto printed circuit boards (PCBs). This approach takes the place of installing the gasket onto the shield cover, where traditional gaskets have been placed. These new discrete SMT gaskets break the paradigm of the continuous EMI gasket by segmenting gasketing into separate pieces. This change allows for an overall lower cost EMI solution and greater design flexibility, because the EMI gasket can be designed in, along with the other SMT components. A question is then raised as to what effect the gaps between the SMT parts have on the gasket s overall ability to shield electromagnetic energy. This article will describe in detail the theory and application of these gaps, or apertures, and how they impact the overall shielding effectiveness of the EMI gasket. As you will see, when properly designed, an SMT EMI gasket can provide essentially the same shielding performance as the continuous EMI gasket and can also yield all of the benefits in cost and manufacturability of surface mount technology. BACKGROUND There are several references, dating back to the early 1960s, that discuss the effects of gaps, slots, or apertures in EMI enclosures. Several approaches were used to describe the effect apertures had on the shielding effectiveness (SE) of the enclosure, but none was adequate in describing the effects of periodic gaps along an EMI gasket. In order to understand the shielding effectiveness of non-continuous gaskets, a model for the continuous gasket had to be developed and then verified, using data taken from a custom test fixture. Then, an equivalent circuit was developed for the gap between each SMT component. This circuit was incorporated into the continuous gasket model to see what effects the gaps had on the overall shielding. The same fixture was then used to compare the non-continuous gasket model to actual test data. SOLID GASKET MODEL We know that, at low frequencies, EMI shielding can be roughly predicted from the DC-resistance across the gasket. 1 At ITEM 2000 www.rbitem.com 1
higher frequencies, this correlation fails because of transmission line effects, different incident wave impedances, intrinsic impedance of the materials, and skin effects. A first order model for a homogenous gasket material would be to treat it as a parallel plate transmission line filled with the gasket material, where the width of the gasket is equal to the length of the transmission line segment (Figure 1). With this model, one can include arbitrary wave impedances on either side of the gasket and can include the reflection and absorption effects in the gasket itself as well. Also, cascading S-parameters can be used 3 to derive the overall transmission coefficient (t) of the equivalent parallel plate transmission line. Then the shielding effectiveness, expressed in db, of the gasket seam can be calculated by 20 log( t ) as shown in Equation 1: S.E. =20log e w 4Z 02 Z 03 (Z + Z )(Z + Z ) - e -w (Z - Z )(Z - Z ) 01 02 03 02 01 02 03 02 Equation 1. Shielding effectiveness model for continuous EMI gasket. where Z 01 & Z 02 = wave impedance on either side of the gasket seam Z 02 = t P (1 + j) µ 0 f = impedance of the gasket seam g = t P (1 + j) µ 0 f = propagation constant within the gasket t = compression stop thickness of gasket P = perimeter, or circumference of gasket seam w = width of gasket material f = frequency r = resistivity of gasket material µ 0 = permeability of free space (4p 10-7 H/m) GASKET TEST FIXTURE A custom test fixture was developed to evaluate the shielding effectiveness of EMI gasket materials. It is based on the industry standard ARP-1705, which is a technique Figure 1. Cross section of solid gasket. Figure 2. Cross section drawing of Gore EMI gasket test fixture. that uses a coaxial type fixture to measure the transfer impedance through an EMI gasket seal. With the use of a modern network analyzer, it becomes much easier to make large dynamic range insertion measurements of devices. Therefore, a coaxial cell was developed which could be used with a network analyzer. The shielding effectiveness can then be measured directly using S-parameters obtained from the network analyzer. The cross-section drawing, shown in Figure 2, shows all parts in detail; but for descriptive purposes, it is necessary to mention only the important items. The main body of the fixture is a large cylindrical part that has four air cylinders mounted to it; these provide the force needed to apply pressure to the top contact plate. This force compresses the gasket under test between the top contact plate and the bottom contact plate. The bottom contact plate is supported by the dielectric support disc. This arrangement provides a completely shielded enclosure that allows for accurate and repeatable SE measurements. With the fixture loaded, the signal is injected from the network analyzer into the top coaxial port. If any signal leaks through the gasket, it will continue through the coaxial test fixture and out the bottom coaxial port. The ratio of these two signals will yield the transmission coefficient, that when converted to db, yields the shielding effectiveness of the gasket under test. A great deal of modeling, using W.L. Gore & Associates, Inc. proprietary field solving software, was performed to ensure that the impedance mismatch effects of the fixture would be minimized. Also, other features were designed into the fixture to facilitate the testing of gaskets using various compression stops, compression forces, or environmental conditions, through the use of a removable contact plate/gasket assembly. COMPARISON OF CONTINUOUS GASKET MODEL TO TEST DATA A gasket material was chosen that would yield results in the middle of the measurable range i.e., above the noise 2 www.rbitem.com ITEM 2000
DC (m Ohms) Figure 3. Predicted low frequency SE vs. actual data. floor. First, the DC resistance across the gasket interface was measured and used, as described in Reference 1, to predict the low frequency SE. Then, this result was compared to the actual SE data produced from the test fixture at 10 MHz. As Figure 3 illustrates, the results correlate very well. Next, the SE was measured for the gasket material at two different compression levels and was compared to the model described in Equation 1. The results are shown in Figure 4. A MODEL FOR APERTURES The basic theory for apertures was developed around wire mesh screen used as an EMI shield and as a seethrough view-port. 4 The best example of this is the door to a microwave oven. The basic theory treats one aperture as a slot antenna that will radiate completely as l/2 approaches the dimension of the largest width of the slot. As long as the periodic array of apertures are in close proximity to one another, the shielding effectiveness can be represented as shown in Equation 2. S.E. =20log( /2 d (2) where l = wavelength d = aperture width ( Figure 5. Example of a single slot type aperture in a metal plate. When a situation arises in which the gaps are far apart from one another, Equation 2 does not hold; and another analysis must be used. Figure 5 shows an example of a single aperture within a shield. References 5 and 6 analyze this problem by using Babinet s principle to transform the slot antenna to a simple dipole. Then, the input impedance to the dipole can be used to calculate the amount of incident signal that is reflected back from the aperture. Then, the slot is treated as a waveguide beyond cutoff, and a simple calculation is used to determine how much of the remaining signal actually gets through. 7 The main problem with this approach is that it does not yield realistic SE numbers for small gaps along an EMI gasket seam. Therefore, another approach must be used. Both References 6 and 8 describe the equivalent circuit of a slot as a shunt inductance, with the lower frequency effects dominated by the surrounding shield material. Also, Reference 5 indicates that the cumulative effect of multiple independent apertures will be a decrease in SE as 20 log (n); where n is the number of apertures. This observation would suggest that the combined equivalent circuit of n apertures could be treated as n-inductors in series. The final step would be to treat the width of the gasket as the depth of the waveguide beyond the cutoff section and to include this effect in the calculations. Figure 6 shows the equivalent circuit of the gasket with gap effects included. To determine the effective inductance of various slot widths, a test fixture similar to the one described in ASTM- D-4935 was used. This approach allows for measurement of just the inductance of the gap, ignoring any ef- Z g = effective impedance of gasket L*n = effective inductance of gap times the number of gaps Figure 4. Actual data vs. modeled data for continuous gasket model. Figure 6. Equivalent circuit of gasket with periodic gaps. ITEM 2000 www.rbitem.com 3
Figure 7. SE data and equivalent model for various gap sizes. Figure 9. Screen shot of EMI gasket modeling program. fects caused by gasket impedance or a waveguide-beyond-cutoff. Figure 7 shows a series of four experiments of eight gaps each; each trial uses a different gap size. The modeled and actual data are shown to be in very good agreement. Next, to prove that the number of gaps can be effectively used as a multiplier to the gap inductance, data were collected holding the gap width constant while doubling the number of gaps for each series of data. Theoretically, this step should manifest itself as a 6-dB loss each time. Figure 8 shows the data and corresponding model for each case. Indeed, there is a 6-dB offset for each doubling of the number of gaps. The last step is to introduce the effects of the gasket width as an attenuation created by the waveguide-beyond-cutoff phenomenon. The theory described above is used in a proprietary program, developed at Gore, to predict the SE of the GORE-SHIELD SMT EMI Gaskets. Although it is understood that the program can provide only a first order approximation, it is very useful in the design and application of EMI gaskets of this type. Figure 9 shows a screen shot of the program. COMPARISON OF COMPLETE MODEL TO TEST DATA Using the software to provide the model, and the test SE (db) (16) 2MM HOLES Model 16x2mm gaps (8) 2MM HOLES Model 8x2mm gaps (4) 2MM HOLES Model 4x2mm gaps 0 0.5 1 1.5 2 2.5 3 Figure 8. SE with different number of 2-mm gaps. fixture (described earlier) to provide the data, an experiment was performed to compare the theory in this article to actual EMI test data. Using 1-mm and 2-mm wide gaskets formed into 50.8-mm diameter rings, a number of gaps of various widths were cut into each gasket, along the perimeter (pd = 159.6 mm). Figure 10 shows plots of both modeled and actual test data for the 1-mm wide gasket, where 16 gaps of both 1- mm and 2-mm widths were cut along its perimeter Figure 11 shows plots of both modeled and actual test data for the 2-mm wide gasket where the gap width was held at 4 mm; but the number of gaps was varied from 4 to 8 to 16. Notice that the only effect of doubling the number of gaps was to decrease the SE by 6 db each time. The last plot, Figure 12, shows the 2-mm wide gasket with the gap width held at 6 mm and with the number of gaps varied from four to eight. Again, notice the 6-dB difference between four and eight gaps. CONCLUSIONS The information presented in this article gives both a theoretical and a practical look at the effect of gaps in an EMI gasket. These insights can be used to design effective shielding solutions using the proprietary gasket tech- 1mm wide gasket with various gaps 9B-1 (16 1mm slots) Model 16x1mm gaps 9B-1 (16 2mm slots) Model 16x2mm gaps Figure 10. SE data and model for 1-mm wide gasket with (16) 1- and 2-mm gaps. 4 www.rbitem.com ITEM 2000
Gasket A (16) 4mm gap Model 16x4mm gap Gasket A (8) 4mm gap Model 8x4mm gap Gasket A (4) 4mm gap Model 4x4mm gap Figure 11. SE data and model for 2-mm wide gasket with (4), (8), and (16) 4-mm gaps. nology. With proper placement of the SMT parts onto the PCB, the right amount of shielding for sensitive components can be achieved while the overall cost of the shielding solution is held to a minimum. Figure 13 shows a screen plot of the gasket modeling program, where a typical example of an EMI gasket design using the SMT parts is shown. Clearly, using these gaskets achieves essentially the same shielding performance as a continuous EMI gasket, while retaining the surface-mount technology benefits of lower cost and higher manufacturablity. The research covered in this article describes just a part of W.L. Gore s ongoing effort to further our understanding of EMI shielding at the printed circuit board level. Gasket A (8) 6mm gap Model 8x6mm gap Gasket A (4) 6mm gap Model 4x6mm gap Figure 12. SE data and model for 2-mm wide gasket with (4) and (8) 6-mm gaps. Figure 13. An EMI gasket designed using 25 SMT gasket pieces around a 200-mm perimeter. REFERENCES 1) Correlating DC resistance to the shielding effectiveness of an EMI gasket, Thomas Clupper, ITEM 1999, Robar Industries, West Conshohocken, PA, 1999 2) Basic Theory of Waveguide Junctions and Introductory Microwave Analysis, by Kerns and Beatty, Pergamon Press, 1967 3) A Handbook on Electromagnetic Shielding Materials and Performance, Donald White, Interference Control Technologies, Gainesville, VA, 1980 4) A Handbook on Shielding Design Methodology and Procedures, Donald White, Interference Control Technologies, Gainesville, VA, 1986 5) Electromagnetic Shielding: Vol 3, Donald White and Michel Mardiguian, Interference Control Technologies, Gainesville, VA, 1988 6) Waveguide Handbook, N. Marcuvitz, Peter Peregrinus, Ltd., London, 1986 7) Principles of Microwave Circuits, Ed.by C.G. Montgomery, R.H. Dicke, E.M. Purcell, Peter Peregrinus, Ltd., London, 1987 BIBLIOGRAPHY Fields and Waves in Communication Electronics, by Ramo, Whinnery, and Van Duzer, 2nd ed., Wiley, 1984 Controlling Radiated Emissions by Design, Michel Mardiguian, Chapman and Hall, NY, 1992 EMC and the Printed Circuit Board, Mark Montrose, The IEEE, NY, 1999 GORE-SHIELD is a registered trademark of W.L. Gore & Associates, Inc. TOM CLUPPER received his BSEE from Penn State, and an MSEE from the University of Delaware. Tom has been employed with W.L. Gore & Assoc., Inc. for 15 years. During his tenure with Gore, he has worked with the Gore Microwave Cable Assembly business developing the 50- GHz and 65-GHz flexible VNA test cables, the R&D group developing new microwave materials, and with the EMI group working on product development, product characterization, and application support. Currently Tom works as a New Product Development Engineer in the Gore Wireless Products group. Phone: (410) 506-3876; email: tclupper @wlgore.com. JOSEPH WHEELER has a BA in Economics from the University of Delaware. He has been employed with W.L. Gore & Assoc., Inc. developing manufacturing control software for high volume production and on integrated testing software applications. Currently he heads up the manufacturing Information Technology effort for the Wireless Products group. ITEM 2000 www.rbitem.com 5