Improving signal integrity in circuit boards by incorporating embedded edge terminations

Much attention has been paid toward signal and power integrity in devices, circuit boards as well as entire systems. Resonances set up between the power and ground planes due to multiple reflections from the edges of the circuit board will affect signal integrity. The impedance seen by a via passing between the power and ground planes can be very high at the resonant frequencies. This gives rise to the effects of crosstalk and simultaneous switching noise (SSN) which would adversely effect the operation of the device. An attempt has been made in this paper to cover all the topics in [1] and [2], which describe a method to incorporate lossy (absorbing) material at the edges of a circuit board to reduce the wave reflections. The "lossy material" is usually a material of very high resistivity but which shows large magnetic losses at UHF and microwave frequencies [3]. Thus this material is suitable to be placed directly between the power and ground planes without introducing any dc leakage currents. Experiments were carried out on a bare copper circuit board with a FR4 dielectric. The absorber used in the experiments is available commercially in flexible, castable and a hard dense form. It is shown that it is possible to reduce the impedances at the resonant frequencies to quite an extent over a broad frequency band by applying the lossy material at the edges of the board. Various configurations of applying the material are also described.