Shielding Effectiveness of Board Level Shields with Absorbing Materials

A method is proposed to extend the shielding effectiveness (SE) of board level shields (BLSs) with absorbing material. Resonances of the metallic shield reduce the electromagnetic interference (EMI) suppression and limit the working frequency range to the fundamental resonant frequency. Two strategies of the absorbing material placement are developed to improve the SE and extend the working frequency of a BLS by inserting the absorbing material over the emission source. At low frequencies (below 10 GHz), a large amount of absorber material is needed to effectively reduce the quality factor of the low-order resonances. At high frequencies (above 10 GHz), a much smaller amount of absorber is required to achieve similar effect. At least 15 dB shielding effectiveness is achieved in the frequency range up to 32 GHz, extending from the fundamental resonant frequency of the BLS cavity (about 5 GHz).