I suppose that I am like many people in that I am nostalgic about the old west and cowboys; of course, my “experience” has come entirely through movies, books or emotional recants from others. Many of those sources blame the railroad for the abrupt end to the long interstate cattle drives that were the primary reason for the profession. However, others claim that barbed wire should be given the credit for bringing order to the plains. Specifically, the fencing produced made it more difficult for rustlers and easier to keep would-be strays in.
Fence to keep the strays in
Similarly, when designing circuit boards there are times when you a “fence” is just what you need. On boards, these are not called fences; instead, they are referred to as PCB guard rings. And these rings are typically copper traces that enclose nodes, components or circuits on your board. Adding these simple traces can have a great benefit for your design. For example, they can supply isolation, improve signal integrity and add operational stability if used properly. Let’s take a look at when PCB guard rings should be included as part of your board layout and then see how to effectively design them.
When Should Guard Rings be Used?
Just as the name implies, guard rings are shields. And these rings are somewhat similar to other types of shielding, such as Faraday shields that mitigate the effects of stray capacitance, in that their primary purpose is to provide isolation. However, guard rings are actual copper traces, just as other signal paths on the board; therefore, they are not intended to block radiation, but carry current. And the currents carried are surface currents, which would otherwise flow or stray to other components or conductors on your board.
In the figure below, an evaluation board for the Analog Devices ADA4530 operational amplifier is shown. As illustrated, this board employs two PCB guard rings (Shield 1 and Shield 2) to mitigate surface stray or leakage currents. As this op-amp has a high-input impedance and low-output impedance, it is susceptible to feedback from parasitic capacitance or current leakage, which the rings prevent. Additionally, this board employs a via fence to provide protection on internal layers.
ADA4530 Evaluation Board with PCB Guard Rings (Image Source)
The circuit board above gives a good example of when a guard or shield may be beneficial for your design. In particular, when voltage differences exist that could provide unwanted paths for current. In these cases, the objective is to provide an alternative path for the stray current, which can be to a lower impedance node in the circuit, the component power (VSS) or ground (VDD), by matching the voltage level of the node to which current flow is undesired. Now that we know when to use PCB guard rings, let’s see why we use them.
Why Use Guard Rings?
PCBAs today can be simple and they can be complex. Complex boards tend to have multiple signal types, including AC, propagating along surface traces and through vias. Many of these boards perform functions that require a high degree of precision. For example, biomedical device PCBs, such as those used in electrocardiograms, are complex and must generate highly accurate readings to prevent misdiagnosis. Input signals are typically very low voltage and high gain amplifiers, like op-amps where the gain may be up 100,000, are used. These higher-level voltages can then be converted to digital for processing.
For low voltage components, common board EMI like radiated emission is not a significant issue; therefore, common shielding that encloses the component is not required. However, small variations in current due to leakage can significantly impact the output and thus the measurements and readings. And rings (or driven guards as they are sometimes referred to) are an inexpensive easy to design protection element. However, the following considerations should be addressed when incorporating PCB guard rings.
How to Design Effective PCBA Guard Rings
⬜ Ensure inputs are encircled
As the input node(s) are typically the high impedance node being protected, it should be enclosed by the ring.
⬜ Connect ring to appropriate nodes
The ring should be connected according to the correct potential polarity choice. The ring should be connected to the low impedance node or to either the positive or negative input source.
⬜ Minimize thermocouple effect
The thermocouple effect is the conversion of a temperature difference between two dissimilar metals that connect at two junctions into a voltage potential. This is different from the Seebeck effect, where a single connection of two different conductors converts directly into electricity. In either case, a significant current may be generated that can affect precision in low voltage circuitry. To minimize these effects avoid crossing other metal types with your ring.
⬜ Observe clearance and creepage guidelines
Guard rings are conductors and are susceptible to EMI from nearby traces and conductive elements. Therefore, it is critical that adequate clearance and creepage rules be observed.
|Tempo‘s Custom PCB Manufacturing Service
A PCB guard ring can be a critical element depending upon your board design. If needed, these protection traces can be a simple addition to your layout. However, you should follow good guidelines and enlist your contract manufacturer (CM) for assistance to ensure DFM rules, such as appropriate clearances, are incorporated. Tempo Automation, the industry leader in smart PCB manufacturing, has the experience and expertise to convert your complex, precision design into a high-quality, reliable PCBA fast.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.