Via fence

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.

If these are not properly shielded from each other, many problems can result including poor frequency response, noise performance, and distortion.

Via fences are cheap and easy to implement, but use up board space and are not as effective as solid metal walls.

Planar technologies are used at microwave frequencies and make use of printed circuit tracks as transmission lines.

That, and the drive to keep down costs, leads to a high degree of integration and circuit units in less than desirable proximity.

[1] Among the many problems that can be caused by parasitic coupling are reducing bandwidth, degrading passband flatness, reducing amplifier output power, increasing reflections, worsening noise figure, causing amplifier instability, and providing undesirable feedback paths.

[2] In stripline, via fences running parallel to the line on either side serve to tie together the groundplanes, so preventing the propagation of parallel-plate modes.

In these formats it is the usual practice to connect the top pads of the via fence with a metal track (see figure 2).

In stripline the field can only propagate between the ground planes, but in microstrip it is able to leak over the top of the via fence.

There is no electrical conductor within this space, but electromagnetic waves can exist within the enclosed dielectric material of the substrate and their direction of propagation is guided by the LWG.

[11] Via fences use up a lot of valuable substrate real estate and so will increase the overall size of the assembly.

In stripline, a rule of thumb is to place the fences at least four times the trace to groundplane distance away from the line being guarded.