A via fence, also called a picket fence, is a structure used in planar electronic circuit technologies to improve isolation between components which would otherwise be coupled by electromagnetic fields. It consists of a row of via holes which, if spaced close enough together, form a barrier to electromagnetic wave propagation of slab modes in the substrate. Additionally if radiation in the air above the board is also to be suppressed, then a strip pad with via fence allows a shielding can to be electrically attached to the top side, but electrically behave as if it continued through the PCB.
Modern electronics have components and sub-units at high densities to achieve small size. Typically, many functions are integrated on to the same board or die. If these are not properly shielded from each other, many problems can result including poor frequency response, noise performance, and distortion.
Via fences are used to shield microstrip and stripline transmission lines, guard edges of printed circuit boards, shield functional circuit units from each other, and to form the walls of waveguides integrated into a planar format. Via fences are cheap and easy to implement, but use up board space and are not as effective as solid metal walls.