Suppression of GHz Noise Emitted From a Four-Layered PWB With a Ferrite-Plated Inner Ground Layer

Ferrite films with a chemical composition Ni_0.2Zn_0.3Fe₂O₄ of 3 mum thickness were deposited at 90^degC from an aqueous solution directly on the inner ground and/or the power source layers of the following two different types of four-layered printed wiring boards (PWBs), in order to suppress the electromagnetic noise emission due to high frequency currents generated in the inner layers. PWB (a), which is the most commonly used structure, is composed of two outer signal layers, an inner ground layer, and an inner power layer. PWB (b) is composed of two outer signal layers, which contain the power supply lines as well, and two inner ground layers. Radiated emission from the ferrite film-embedded PWB (a) was successfully suppressed to be 2 to 5 dB lower than that without the ferrite film. This tendency became more prominent with increasing frequency. A similar noise suppression effect was also observed for the ferrite film-embedded PWB (b), up to 4 dB. This suggests that we can suppress radiated emission also in the case where the ground and the power planes are not placed symmetrically in the inner layers. The frequency dependence of radiated noise suppression effect by the ferrite film corresponded to the frequency dependence of transmission attenuation power ratio (P _loss/P _in) , i.e. the suppression effect upon conducted noise due to magnetic loss characteristics of the film. This indicates that the suppression of radiated emission from the four-layered PWBs was due to the attenuation of GHz-range noise current in the ground and/or the power layers.