PCB ground fill design guidelines for radiated EMI

This paper is to alert PCB designers about potential EMI problems caused by ground fills in outer layers of PCB. Ground fills are frequently used on top and bottom layers of PCBs for manufacturing or perceived shielding reasons. However, if not well applied they may cause EMI problem, especially if they form resonators. Several types of situations need to be distinguished, by (1) the resonating structure and (2) the excitation structure. Examples of problematic structures are shown by simulation and measurement. Possible guidelines for mitigating such problems are discussed.