Electrical signals that propagate through vias between layers of metal planes in circuit boards will generate radial waves that are guided by the planes. Multiple reflections of these parallel plate waves from the edges of the circuit board will cause resonances that greatly increase the effective impedance between the two planes at the resonant frequencies. Such resonances are highly undesirable for operation of high performance electronic packaging systems, because they degrade signal qualities, increase crosstalk level, and enhance simultaneous switching noise. In this paper, it is shown that the magnitude of the resonances can be greatly reduced by incorporating an absorbing material between the metal planes at the perimeter of the circuit. We describe for the first time experimental results that make use of separate input and output vias to transmit and receive signals. As a result, the signal integrity of the system can be greatly improved. By using absorbing materials whose loss depends upon magnetic, rather than electrical effects means that it is possible to choose materials whose resistivity is of the order of 10{sup}11 Ω-cm, making it possible to place the materials directly between power and ground planes without introducing any DC current leakage. These materials are available commercially in flex form and can be chosen to enhance losses at either a low frequency band below 1 GHz or at frequencies above 5 GHz to accommodate different needs. The paper also describes methods of using composite materials to increase loss at both low and high frequencies.
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