Optimization of Layer Thickness to Yield Predetermined Shielding Performance of Multilayer Conductor Electromagnetic Shield

Transcription

1 Optimization of Layer Thickness to Yield Predetermined Shielding Performance of Multilayer Conductor Electromagnetic Shield C Dharma Raj D Vijaya Saradhi P Hemambaradhara Rao P Chandra Sekhar GITAM University GITAM University GITAM University GITAM University Visakhapatnam Visakhapatnam Visakhapatnam Visakhapatnam dharmarajc@yahoo.com saradhi.gitam@gmail.com hem.gitam@gmail.com chandrasekhar5@yahoo.com Abstract - Electromagnetic shield made up of single conductor layer of chosen thickness and conducting material can be designed to achieve required shielding performance. This single conductor layer electromagnetic shield would be bulky and occupy more volume when the size of the electronic circuit is shrinking with the advent of microelectronic technologies. In order to optimize the thickness of the electromagnetic shield, multilayered conductor electromagnetic shield is designed and developed in this paper. Analysis was carried out of for determination of shielding effectiveness of a multilayer conductor electromagnetic shield after evolving its mathematical model. A design methodology is presented to yield predetermined shielding effectiveness for different materials in various layers of the shield. The designer can be optimizing the layers thickness in the electromagnetic shield for selected conducting materials and shielding effectiveness. Keywords - EMI, Electromagnetic shielding effectiveness, materials conductivity and permeability I. INTRODUCTION In certain engineering applications, it is often desirable to utilize a metallic thin film to attenuate optimum electromagnetic radiation at radio frequencies. Most electronic systems in use are known to generate radio frequency fields. The revolution in telecommunications continues with the fourth generation mobile phone and further the developments will become available in due course. Recent review of scientific developments concluded that biologic and epidemiologic evidence does not necessarily suggest adverse health effect from mobile phone use, but exposure to the radio frequency transmissions below the guideline levels may be of some concerns. Electromagnetic interference [] (EMI) is the performance degradation of a device caused by electromagnetic disturbances, which can be a noise, an unwanted signal, or a change in the propagation medium. The shielding effectiveness is the principal performance measure of electromagnetic compatibility [2, 3] in design problems. For various reasons, especially linked to the physical nature of materials that compose the shielding, this border is not completely impervious. Shields are used to isolate a region, either to prevent interference from outside sources (susceptibility), or to avoid the leakage of unwanted radiation due to internal sources (emission) [4]. The electromagnetic shielding [5] is based on the reflection, absorption and transmission phenomena of an electromagnetic wave. The Performance of a shield is given by its shielding effectiveness, it represents the reduction factor of the electric, magnetic or electromagnetic field and it depends on the shield material (intrinsic effectiveness) as well as the characteristics of the incident field (far or near field), which is defined by the distance between the source and victim. Single layer conductor sheet is the best example of electromagnetic shield [5]. The shielding effectiveness of such a conductor shield depends upon conducting material, shield thickness and frequency of operation. In order to achieve high values of shielding effectiveness with micro thin electromagnetic shields to combat EMC problems with microelectronic circuits, multilayer conductor [6-2] is considered as electromagnetic shield in this paper. Metallic thin film grown on plastic substrates was discussed by S.M.Yang, et.al. [6] in which the base layer is taken as plastic which will not contribute to improvement in shielding effectiveness. H.W.Deng, et.al. [7] proposed a multilayer coated conductor which can be effectively used as electromagnetic shield, but Dang only analyzed the skin depth for such a multi Proc. of the International Conference on Advanced Computing and Communication Technologies (ACCT 2) Copyright 2 RG Education Society ISBN:

2 layer conductor. M.Milutinov [8], Craig [9], A.Massarini [], et.al. discussed about multilayer electromagnetic shields to analyze various shielding parameters using various techniques. A model, based on the transmission line and plane wave theory, is developed to analyze the shielding effectiveness of multilayer conductor electromagnetic shields. Analyses show that among the absorption, reflection is dominant, where as absorption is negligible because of small film thickness. Better shielding can be achieved by having the impedance ratio of the adjacent layers higher than, i.e. by placing the thin film of higher impedance as the inner layer. Without the correct sequence of placement, more layers do not necessarily lead to better shielding. The typical shielding materials used are the copper, aluminum, silver, steel and nickel. Shielding effectiveness for different materials as a function of frequency is analyzed. The frequency verses shielding effectiveness characteristics of multilayer conductor electromagnetic shield sequences are also analyzed, the results obtained are presented. II. SHIELDING EFFECTIVENESS A conductor of high conductivity and low permeability has low intrinsic impedance. When a radio wave propagates from a medium of high intrinsic impedance into a medium of low intrinsic impedance, the reflection coefficient is high. From the plane-wave theory of shielding in the far field, high shielding effectiveness occurs in a shield material of high conductivity and low permeability. The shielding effectiveness SE [5] in decibels of a metallic thin film is expressed as The absorption loss A is directly related as follows to the thickness t of the film and the attenuation constant α: A=3.4t Where σ the relative conductivity of material, µ the relative permeability of material s. A =A + A + A + A, A =3.4 t t t 3 3 (3) Where the relative permeability of base conductor, is the relative permeability of micro thin film layer, µ is the relative permeability of micro thin film layer2, is the relative conductivity of base conductor, is the relative conductivity of micro thin film layer, is the relative conductivity of micro thin film layer 2 The characteristic impedance [6] of a media is defined by the ratio of the electric field E to the magnetic field H. It is given by the relation Z = µ (4) SE = A + R + M () A is absorption or penetration loss in decibels inside the shield, R is reflection loss in decibels from the multiple boundaries of the shield, and M is multiple internal reflections when the absorption loss. From electromagnetic wave theory, the propagation constant Γ, for a uniform plane wave in a good conducting material, is expressed by Γ= (+j) µ /, for >>. (2) Where: α the attenuation constant in nepers, β the phase constant in radians per meter, ω expressed the frequency in hertz, the conductivity in., μ the permeability and ε the permittivity. For an insulating media, the conductivity is very weak and thus, the free space impedance is: Z = 377 Ω For a conducting media, the impedance is given by Z = µ For shields, it is preferable to express their properties comparatively to those of the copper. The conductivity of the copper is taken equal to 5.82xI7S.m. The characteristic impedance of the shield then becomes 353

3 Proc. of the International Conference on Advanced Computing and Communication Technologies (ACCT 2) Z = (5) The reflection losses are due to a missed adaptation between the incident wave impedance [6] and the intrinsic impedance of the shield z, and are given in db by R = 2log R = R + R + R..+ R Table.. Relative Conductivity and Permeability of various materials Material Relative conductivity ( ). Copper uminum.6 Silver.5 Gold.7 Nickel.23 Relative permeability ) H. R = 2 log + 2 log log (6) Where z the impedance of the base conductor, z is the impedance of micro thin film layer, is the impedance of micro thin film layer2. The losses by multiples reflection are given by M=2log M = M + M + M + M III. RESULTS AND CONCLUSION: The typical materials used for shielding were copper, aluminum, silver, nickel and gold. In order to do a comparative study between the different sequences of materials, the equation () was considered and the results obtained are presented on the figures to 3. It is observed that thickness is decreasing exponentially with increasing frequency. Figure is a plot between thicknesses versus frequency at S-band frequency range for a predetermined value of 6dB shielding effectiveness. From the figure it is evident that thickness is exponentially decreasing with increase in frequency. Among the considered conducting materials, Nickel yields least thickness for single layer design. M = 2log + 2log + 2log (7).4 x -4.2 S band(2-4ghz)frequency Vs Thickness Cu Ni Where t thickness of base conductor, thickness of micro thin film layer, thickness of micro thin film layer2, δ skin depth of base conductor, δ skin depth of micro thin film layer, δ skin depth of micro thin film layer 2. thickness in mills Skin depth is the traveling distance of the wave as it decreases in magnitude /e of its original value where e is exponential term, and it is given by δ = / μ / frequency in hz Figure.: Frequency vs. thickness for different conducting materials to yield 6 db Shielding Effectiveness. 354

4 Proc. of the International Conference on Advanced Computing and Communication Technologies (ACCT 2) Figure 2 is a plot between thickness and frequency in S-band frequency range for a selected value of 6dB shielding effectiveness for multilayer electromagnetic conductor shield of copper as base conductor and nickel, silver and aluminum, as micro thin films. It shows that variation of thickness in base conductor copper and micro thin films aluminum and silver cause a drastic change in thickness versus frequency characteristic. Whereas variation of thickness in nickel micro thin film does not show much affect. So, for wide band frequency applications to achieve minimum overall thickness of multi layer electromagnetic conductor shield, nickel thickness can be chosen to minimum. Thickness in M ills 2.5 x -5 C band(4-8ghz)frequency Vs Thickness of each layer of /cu/au/ cu Au 3.5 x S band(2-4ghz)frequency Vs Thickness of each layer of Cu///Ni Cu Ni Frequency in Hz Figure3: Frequency vs thickness for different multilayer conducting materials to yield 6 db Shielding Effectiveness for c-band frequency ranges. T h ic k n e s s in m ills ACKNOWLEDGMENT We thank the GITAM University and its management for the encouragement extended to us in carrying out this work. We also grateful to the faculty and staff of department of electronics and communications engineering GITAM University, for providing all the infrastructure and support required in making this work successful Frequency in Hz Figure2: Frequency vs thickness for different multilayer conducting materials to yield 6 db Shielding Effectiveness for S-band frequency range. From figure 3 is a plot between thickness versus frequency in C-band frequency range for a selected value of 6dB shielding effectiveness for multi layer electromagnetic conductor shield of which silver is base conductor and copper, gold and aluminum are micro thin films. It can be observed that this combination to yield 6 db of shielding effectiveness results in very low thickness values of microfilm materials and thus, such combination is not suggested in practical applications. REFERENCES []. D.R.J.White Electro Magnetic Interference and Compatibility, EMI Control Methods and Techniques. MD:Don White Consultants,Inc.,vol.3,973,pp.-. [2]. C.R. Paul, "Introduction to Electromagnetic Compatibility", John Wiley & Sons, 26. [3]. Wilhelm Rotkiewics, "Electromagnetic Compatibility in Radio Engineering". Elsevier, ISBN: [4]. L. Klinkenbusch, "On the Shielding Effectiveness of Enclosures", IEEE Trans. on Electromagnetic Compatibility, vol. 47, no. 3, August 25, pp [5]. Richard.B.Schulz, et.al. Shielding Theory and Practice, IEEE Transaction on Electromagnetic Compatibility, vol.3, No.3, pp.87-2, August.988. [6]. S.M.Yang, et.al., Electromagnetic Shielding Effectiveness of Multilayer Metallic Thin Film on Plastic Substrates, Journal of Applied Polymer Science, vol., 28, pp [7]. H.W.Deng et.al Effective Skin Depth For Multilayer Coated Conductor Progress in Electro Magnetic Reasearch M, vol.3,973,pp.-. [8]. Miodarag Milutinov et.al Shielding Effect of Non- Ferrous Metallic Plates in Vicinity of Three Phase Conductors, Serbian Journal of Electrical Engineering, vol. no 2, Nov 25, pp

5 Proc. of the International Conference on Advanced Computing and Communication Technologies (ACCT 2) [9]. Craig et.al., EMI Shielding Characteristics of Perm alloy Multilayer Thin Films IEEE Aerospace applications conference, Feb 994,, pp []. A.Massarini et.al., Shielding Effectiveness of Multilayered Shielded for Magnetic Fields No sinusoidal Sources. IEEE conference Electromagnetic Compatibility, Feb 27 pp []. Hoang Ngoc Nhan et.al., Modeling of Electromagnetic Shielding Effectiveness of Multilayer Conducting Composites in the Micro Wave Band IEEE Symposium Communication and Electronics, Oct 26 pp [2]. Shidan et.al., Determination of Shielding Effectiveness of Multilayer Shield by Making of Transmission Line Theory IEEE Electro Magnetic Compatibility And Electromagnetic Ecology, International Symposium, June 27 pp