Improving signal integrity in circuit boards by incorporating absorbing materials

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 since they degrade signal qualities, increase crosstalk level and enhance simultaneous switching noise. In this paper we show 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 board. As a result the signal integrity of the system is improved. By using absorbing materials whose loss depends upon magnetic rather than electric effects, it is possible to choose materials whose resistivity is of the order of 10/sup 12/ ohm-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 flexible and hard, dense forms and can be chosen to enhance losses at either the UHF band or various microwave frequency bands to accommodate different needs. Results of theoretical computation are compared with experiments performed on test boards characterized using a vector network analyzer between 50 MHz and 6 GHz. Significant reduction in input impedance of the test structure at resonance frequencies is obtained, which shows the effectiveness of the proposed method and the accuracy of the calculation method. The paper also evaluates several ways of applying the lossy material.