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Modelling up to 45 GHz of coupling between microvias and PCB cavities considering several boundary conditions

Thierry Le Gouguec 1 Najib Mahdi 1 S. Cadiou 1 Cédric Quendo 1 E. Schlaffer 2 W Pessl 2 Alain Lefevre 3
Lab-STICC - Laboratoire des sciences et techniques de l'information, de la communication et de la connaissance, UBO - Université de Brest
Abstract : Modeling up to 45 GHz of coupling between microvias and PCB cavities considering several boundary conditions thierry le gouguec 1 , najib mahdi 1 , ste ' phane cadiou 1 , ce ' dric quendo 1 , erich schlaffer 2 , walter pessl 2 and alain lefevre 3 The recent developments in electronic cards such as the network equipment are characterized by the miniaturization of the board size and the increasing complexity of the layout. Because of these requirements, multi-layered printed circuit boards are commonly used and vias connecting signal lines on different layers, or integrated circuit devices to power and ground planes, are frequently used and often essential. However, a via is not an ideal transmission line. Besides, it creates discontinuities at high frequencies leading to high insertion loss degradation of signal which limits the performances of integrated circuit and systems. In this paper, the impacts of coupling between via and parallel-plates cavity on the response of microwave integrated devices are highlighted in the first part. Then, to describe the intrinsic interaction between the via transition and parallel-plate modes, the notion of parallel-plates matrix impedances is presented and new boundary conditions like open or plated through holes shielded boundaries of the cavities are introduced. Then, using this physics-based model, an intuitive equivalent circuit has been developed. Finally, the proposed approach and the equivalent circuits were validated by using comparisons with electromagnetic simulations and measurements in different scenarios. Three-dimensional (3D) multi-layer technologies such as low temperature co-fired ceramics (LTCC) [1] or high-density multi-layers printed circuit board (HD-PCB) [2] are currently being strongly developed because they offer considerable size reduction as well as the embedded function possibilities. For microwave applications such as filters, couplers, diplexers, etc. [3, 4], these 3D structures offer new design possibilities for frequencies up to 100 GHz. HD-PCB structures consist of several metal layers separated by dielectric substrates. The vias and microvias used in multilayer PCBs allow connecting lines of different metallic levels together or connecting devices to the power and ground plane [5]. The different metal planes can also be connected together with metallic plated through holes (PTHs). With the rise of working frequencies, the stacked multilayer PCB structures are subjected to electromagnetic phenomena like standing waves in cavities or like coupling and interaction between neighboring components. As example of HD-PCB technology, the AT&S TM (PCB manufacturer) technology used during MIDIMU-HD project funded by the Euripides council is presented in Fig. 1. This HD multilayer consists of eight metallic layers (30 mm thickness) separated by Megtron6 (Panasonic TM) sub-strate of 95 mm thickness (depending on the metal densities of each level) and with a relative permittivity 1 r ¼ 3.3 and loss tangent tan(d) ¼ 0.0065 at 40 GHz. A single microvia hole consists of a central cylinder with a diameter of 140 mm, a conductor pad with a diameter of 240 mm, and when this via passes through a metallic plane it will also have a clearance hole called anti-pad of diameter of 350 mm. AT&S is able to stack more than three microvias and to realize buried via with diameter of 200 mm. The PTHs connecting the metal level M1 to the metal level M8 are 200 mm of diameter. Obviously, these multilayer structures which involve parallel planes, dielectric layers, pads, and anti-pads are not ideal transmission components at high frequencies. The electrical behavior of a microvia can be modeled by serial inductance and resistance like is done for a metallic wire [6, 7]. The vias and microvias may cause mismatch [7], crosstalk, reflections, some additional signal delays, and consequently the degradation of signal performance. On the other hand, the coupling between vias, microvias, and parallel plates also plays an important role in the electrical performances of the via transition [8, 9]. The excitation of the parallel plate modes results in conversion of energy between propagation on line and propagation on guided plated structures which imply some transmission zeros.
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Thierry Le Gouguec, Najib Mahdi, S. Cadiou, Cédric Quendo, E. Schlaffer, et al.. Modelling up to 45 GHz of coupling between microvias and PCB cavities considering several boundary conditions. International Journal of Microwave and Wireless Technologies, Cambridge University Press/European Microwave Association 2016, 8 (3), pp.421-430. ⟨10.1017/S1759078716000192⟩. ⟨hal-01358417⟩



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