How to Use a Guard Ring in a PCB

Guard rings in a PCB are used in some RF designs as a way to confine the electromagnetic field in the interior region of a PCB. The confinement is aided by a set of vias, which provide a structure to reflect incoming or outgoing radiation from a PCB, which might lead to an EMI problem or failure. 

Sometimes, a designer will use a guard ring as a standard practice, such as in instances where it is not needed. The reality is that guard rings do not solve radiated EMI problems, they only suppress radiation to the point where it might not be noticed in a radiated emissions test. If you find that you need to include a guard ring, then pay attention to these guidelines.

How Guard Rings Are Placed in PCBs

Guard ring is a very simple structure: it is a ground region that spans around the edge of the PCB and it contains an array of stitching vias. It resembles a via fence along the edge of the PCB, which is designed to block radiation from entering or exiting the edge of the PCB. It is often used as a measure to suppress problems with radiated EMI and EMI susceptibility.

The use of a guard ring follows some simple guidelines:

• The ring is placed on all layers where possible

• Through-hole vias are used for the via fence

• The ring should connect back to earth or system ground

• The ground connection should be at one point (by the power input)

• If the ring is exposed through solder mask, it can be used to connect to chassis

The blue region in the layout below is a guard ring that is connected to a different net than the main system ground (VSSD). This guard ring region connects back to earth via an AC power input, which ties the entire system to a single reference voltage. While not visible in the view below, the ring region contains stitching vias along the edge; the ring also connects to the mounting holes in the corners of the PCB

Sometimes, the guard ring is not actually a separate ring of copper that acts like a chassis ground or shield. The “guard ring” can just be a line of stitching vias around the edge of the PCB, which then passes through the ground plane. In this case there is probably no chassis ground or earth ground, the ground is floating with respect to all other grounds in other devices. An example is the common RF mixer board shown below.

In this case, any protective mechanism that might have been available by having a separate ground ring around the PCB is gone. Anytime there is ESD it will enter the main ground and can damage circuits in the PCB. However, the board still has the capability to suppress noise because of the via fencing; it still acts as a shield up to some maximum frequency.

Regardless of the use of pour as a guard ring, the important design parameter required to ensure the via fencing works is the hole-to-hole spacing between vias. The vias that board the PCB edge will provide high shielding effectiveness up to the following wavelength limit:

Wavelength > (S/4)

Where S is the spacing between via walls. All wavelengths longer than the above limit can be effectively shielded with high effectiveness. Remember to convert the wavelength back to frequency using the Dk value!

Not All PCBs Need a Guard Ring

In the examples above, the guard ring is being used to contain radiation from the edge of the PCB, or as a convenient ground connection to a metal chassis element in the enclosure. While these are the common reasons to use a guard ring in a PCB layout, this does not mean that every device needs to have a guard ring.

There are some good reasons to not use a guard ring around the edge of the PCB. For example, the PCB might be very dense and there is not enough room to place the guard ring around the design. As another example, the use of a guard ring is sometimes based on the presence of shrouded connectors, but if there are no shrouded connectors or metal chassis then the PCB guard ring may not be needed.

If the intention of adding a guard ring to a PCB layout is to suppress radiated emissions from a PCB, the source of the radiation should be investigated first. This is often done in simulation with an electromagnetic field solver, or compliance testing of prototypes. Focus on finding the root cause of the problem before using a measure like stitching vias in a guard ring to suppress radiated emissions.

Whenever you need to investigate and diagnose tough EMI problems, use the complete set of system analysis tools from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity. Cadence PCB design products also integrate with a multiphysics field solver for thermal analysis, including verification of thermally sensitive chip and package designs.

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