IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 44, NO. 1, FEBRUARY 2002 243 Short Papers_______________________________________________________________________________ ESD Field Penetration Into a Populated Metallic Enclosure: A Hybrid Time-Domain Approach G. Caccavo, G. Cerri, V. Mariani Primiani, L. Pierantoni, and P. Russo Abstract—This paper presents a method for the analysis of the elec- tromagnetic field inside a metallic enclosure populated by printed circuit boards (PCBs) and a comparison with an analogous field into an empty box is also reported. The field source is represented by an electrostatic dis- charge (ESD) current flowing along a monopole and producing an elec- tric field coupling into a shielded structure through a slot. The model is based on a time domain approach and the solution of the electromagnetic problem is achieved by a hybrid method, characterized by the combination of the Method of Moments in Time Domain (MoMTD) and Finite Differ- ence in Time Domain (FDTD); this technique allows to investigate the com- plex geometry of the problem and to evaluate the strong coupling between the source and the victim structure. All simulated results are validated by measurements. Index Terms—Electrical equipment enclosures, electrostatic discharge, hybrid MoMTD-FDTD, transient response. I. INTRODUCTION The evaluation of the shielding effectiveness (SE) of a metallic en- closure is a classical topic in the EMC literature, because of its funda- mental importance to design and to manufacture external cases of elec- tronic equipment. In fact, a well designed shield assures both a good immunity of the internal circuitry against external fields and a low radi- ated emission from internal currents, to mitigate electromagnetic pol- lution. Many aspects concerning the electromagnetic characteristics of metallic shields were widely investigated, using the most suitable ana- lytical and numerical techniques, adopting time- or frequency-domain methods, applying theoretical and experimental approaches. In the case of simple geometry, analytical or semi-analytical models can be used. This approach is useful to outline explicitly the effect of physical pa- rameters involved in the shielding mechanism [1]–[3], and sometimes this also allows to extract an equivalent network [4], providing the eval- uation of the SE in a closed form, very useful in the design stage. Another basic aspect concerns the shielding performance degrada- tion due to the presence of slots in metal enclosures; this problem was also intensively investigated and a lot of contributions were published. These papers analyzed slots and apertures having different positions on the enclosure walls, various transversal dimensions, and particular shapes, such as circular, rectangular, and cross [5]–[12]. In some cases, array of slots were also considered, because, from a practical point of view, they are widely applied in cooling systems of electronic devices [13]–[15]. Moreover, an analogous and important issue is represented by cable penetration inside the enclosures through apertures; this is a very topical problem, because this structure allows the penetration of common mode currents down to the lowest frequencies [16]–[18]. Very critical also is the evaluation of the SE at very low frequencies espe- cially for magnetic fields, when the most effective shielding mecha- nisms (reflection and absorption) fail; in this case, active shields were recently proposed and the nonlinear behavior of the material was prop- erly taken into account [19]–[25]. Manuscript received January 15, 2001; revised November 5, 2001. The authors are with the Dipartimento di Elettronica ed Automatica, Univer- sità di Ancona, 60131 Ancona, Italy. Publisher Item Identifier S 0018-9375(02)01433-3. To improve the performance of shielding walls, composite materials (chiral and anisotropic layers), wire mesh structures, and conductive plastic enclosures were also investigated [26]–[39]. Another aspect is constituted by the analysis of gaskets, slits, cracks, and joints: because of their irregular shape and geometry, they produce an unwanted elec- tromagnetic leakage which is not easy to control [40]–[43]. The above review of published contributions is not exhaustive, is limited to recent literature, and is reported to highlight the progressive research effort developed regarding some of the most important aspects involved in the application of shielding enclosures. However, they deal with empty boxes or boxes filled with little perturbing objects, such as straight wires or small loops. On the contrary, a more realistic model should include the presence of subsystems or mother board supporting several plugged-in printed circuit boards (PCBs). A pioneering work about this topic is reported in [44], where the effect of the presence of metal plates was investigated in the frequency domain and for a box excited by an internal small loop. The goal of the paper is the development of a method for the mod- eling of the electromagnetic coupling between an antenna, excited by an ESD current, and a victim circuit located inside an enclosure popu- lated by PCBs. The antenna is placed in front of the slotted wall of the metallic enclosure. If the ESD excitation of shielded PCBs is consid- ered, three coupling mechanisms can be highlighted: 1) the injection of a direct ESD conduction current; 2) the coupling through the mag- netic; or 3) electric field resulting from the ESD current. In particular, this paper addresses coupling of energy from an ESD current through the electric field that may impact the functioning electronics, though the conduction current itself may not actually have a path through the functioning circuit. The geometry of the problem is complicated by internal metallic plates that simulate PCB ground planes. They greatly perturb the in- ternal field of the box, which cannot be considered an empty resonant cavity; therefore, a modal analysis is very difficult to apply, whereas an FDTD approach seems to be the most suitable to evaluate the electro- magnetic field inside the box. On the other hand, extending the FDTD space to the external region should require the availability of a very large computer memory and a lot of computation time. So, the ex- ternal region was characterized by the proper free-space Green’s func- tion, and the current flowing along the antenna was evaluated by a marching-on-in-time procedure [method of moments in time doman (MoMTD)], that requires the discretization of the antenna only. This hybrid technique allows us to solve the problem directly in the time domain, which is very convenient for treating fast transient fields, be- cause the time history of the interfering pulse is achieved without pro- cessing the source and the induced disturbance with direct and inverse fast Fourier transforms (FFTs), respectively. The paper also provides a comparison between the internal field in- duced inside an empty box and one induced inside a populated box, showing surprisingly that, in the second case, fields reach peak values higher than the ones evaluated in the first case. The analyzed structures were manufactured and measurements were carried out to validate the model; the comparisons between experimental data and numerical re- sults show a very satisfactory agreement. To strongly stress the ability of the method to deal with transient fields, the structure was also excited by a pulse generated by the time-domain facility of a network analyzer (NWA), which allows us to perform more accurate measurements in a more controlled setup. 0018–9375/02$17.00 © 2002 IEEE