Analysis of Signal Integrity in a Microstrip Transmission Line Passing through a PCB Shield

The paper presents the results of signal integrity analysis for a microstrip line that passes through an aperture in a typical board level shield. The effect of the shield on the microstrip line per-unit-length parameters (resistance, inductance, capacitance, conductance, and characteristic impedance) was evaluated using quasi-static simulation. Then, the scattering matrices (S-parameters) were compared for microstrip lines without and with the shield. The time responses of these microstrip lines to the influence of the pulse signal were obtained in the final stage. As a result, it was shown that shielding a printed circuit board can significantly affect the integrity of signals travelling through it. For example, placing a shield over a microstrip line can cause a local change in characteristic impedance of almost $20 \%$. This, in turn, results in reflected pulses at the microstrip line input. Moreover, the amplitude of these pulses can be up to $10 \%$ of the original input signal.