MCMs for RF and wireless systems often use metal filled via holes to improve isolation between the stripline and microstrip interconnects. In this paper, results from a 3D-FEM electromagnetic characterization of microstrip and stripline interconnects with metal filled via fences for isolation are presented. It is shown that placement of a via hole fence closer than three times the substrate height to the transmission lines increases radiation and coupling. Radiation loss and reflections are increased when a short via fence is used in areas suspected of having high radiation. Also, via posts should not be separated by more than three times the substrate height for low radiation loss, coupling, and suppression of higher order modes in a package. Index Terms-microstrip, stripline, coupling, crosstalk, MCM, microwave transmission lines I. INTRODUCTION RF and wireless package designs must become smaller to satisfy the demands of the commercial and government markets. Simultaneously, the package must house data processing, biasing, and memory circuits in addition to the RF circuits to reduce the overall system size and complexity. Even more ambitious systems being developed by NASA include microelectromechanical (MEM) gyroscopes, active pixel sensors, and other micromachined scientific instruments with the already mentioned electronic circuits to create entire systems in a package. While the size of the package is being reduced and the complexity increased, the cost must also be reduced. To achieve these utopian goals, many MultiChip Module (MCM) technologies have been proposed [1-5], but Low Temperature Cofired Ceramic (LTCC) may be the ideal packaging technology. The material used in LTCC packages has a moderate dielectric constant, 4<_<8, which permits wider microwave transmission lines and thus lower conductor loss than circuits on Si, GaAs, or Alumina. In addition, it has a low loss tangent of 0.002 at I0 GI-/z, which results in low dielectric attenuation. LTCC packages comprise many 0.1-0. i5 mm thick ceramic layers with transmission lines on each layer [2,6]. Vertical interconnects between the layers are easily manufactured by laser or
Abstract-Coupling between microstrip lines in dense RF packages is a common problem that degrades circuit performance_ Prior 3D-FEM electromagnetic simulations have shown that metal filled via hole fences between two adjacent mlcrostrip lines actually increases coupling between the lines; however, if the top of the via posts are connected by a metal strip, coupling is reduced. In this paper, experimental verification of the 3D-FEM simulations is demonstrated for commercially fabricated LTCC packages.
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