When designing a PCB, I’ve learned that it’s often best to design the ground signals to:
Follow the signal in reverse, creating a return path that mirrors the outgoing path.
Avoid daisy chaining and instead connect back in a “star” configuration.
Minimize the return path length.
Maintain the same trace width as the signal.
However, here’s where I’m confused: I’ve also seen recommendations to do a polygon pour for the GND net on both sides of the PCB. I primarily work with through-hole components on two-sided boards operating at <= 20 MHz frequencies, using 1 oz copper, FR-4, 1.6mm thickness, and ENIG finish.
Once I pour the copper, it seems like the carefully designed ground traces become irrelevant. If I’m using ground pours, do I still need to adhere to the “no daisy-chaining of ground” rule?
Additionally, I tend to route most signals on the top layer, keeping the ground layer less crowded. Is this practice actually beneficial, or is it just as effective to have a balanced distribution of traces on both sides?
Lastly, would there be any advantage to pouring VCC instead of GND on the top layer, especially if that layer is already densely populated with signal traces?
Ideally, the return traces will no longer be visible, because they’ll be fully included in an unbroken ground pour.
Usually, the ground pour is not perfect; having the return traces there first can serve as a reminder that “when you do have to break up the pour, at least try not to do it here”.
Well, if you’re using thru-hole components on a two layer board then you want your ground pour on the bottom. And the currents will always take the path of least resistance, but your routing the ground is useful. I’ve often run ground traces to verify netlist DRC is ok, the place the plane or pour later when I’m happy with everything.
I also tend towards routing everything I can on the top layer, avoiding vias and using Microstrip. The idea of a “single point ground” doesn’t seem to be popular anymore, but you definitely want to avoid daisy-chaining ground. I guess you could think of the pour or a plane as your “star” point. If it’s absolutely necessary to make a voltage net pour on an external layer I think the bottom is better, mostly because I picture myself shorting Vcc to something every time I go to probe or if I drop a screw, etc. Of course soldermask helps a ton on that and you could conformal coat if you need to. The problem with laying out most boards is someone else makes up half the rules, assigning how many layers they want, where some stuff is placed, whether they can coat a board, etc. And that someone else is usually the customer…
Using a GND copper pour, especially on the bottom layer, is a widely accepted practice for two-layer PCB design. This approach minimizes interruptions in the ground return path, effectively mimicking a ground plane similar to what you’d find in a multi-layer board. Here’s how this aligns with the design rules you mentioned:
With a continuous ground pour beneath signal traces, the AC return currents naturally follow the path of least impedance, staying close to the signal trace. This satisfies the intent of maintaining a mirrored return path for high-frequency signals.
While star grounding is a traditional concept, in the context of ground pours, each sub-circuit can effectively use its local ground pour. These localized pours connect to a single point or are separated by strategic slits to manage noise coupling or interference.
A ground pour inherently minimizes the return path length by providing a low-impedance, widely available path for return currents. This aligns with the spirit of the rule, though it shifts the focus from specific routing to ensuring the ground pour remains unbroken.
The recommendation to match ground trace width to signal traces is not typically necessary with a ground pour, as the pour inherently offers much lower impedance compared to individual traces. In general, wider traces or pours for ground improve current handling and reduce noise susceptibility.
As for pouring VCC instead of GND on the top layer, this is usually avoided unless there’s a specific need. Ground pours help shield and reduce crosstalk between signals, especially in sensitive designs. Using VCC as a pour increases the risk of unintended shorts during probing or assembly, as noted by others.
Your current practice of routing signals predominantly on the top layer while maintaining a near-continuous ground pour on the bottom layer is beneficial. It strikes a balance between simplicity and effectiveness for the operating frequencies and design constraints you described.
When working with two-layer PCBs and ground pours, here’s how the design rules apply to your situation:
High-frequency return currents naturally follow the outgoing signal path to minimize loop area, which helps with signal integrity and EMI. This remains crucial even at lower frequencies with fast edge rates. A ground pour aids in achieving this by providing a low-impedance path.
Star grounding works well in simple or specific applications (e.g., a single mixed-signal IC), but in complex designs, it often conflicts with the need to minimize return path impedance. A ground pour effectively acts as a “distributed star,” connecting sub-circuits to a shared ground without creating loops or impedance issues.
The primary goal is to minimize return path impedance rather than just its length. For low frequencies, this means wide traces; for high frequencies, it’s about ensuring the return path is directly under the signal trace. Ground pours naturally fulfill this requirement if unbroken.
There’s no need to match ground trace width to signal traces. Wider ground connections or continuous pours are preferred, as they reduce impedance and improve performance.
Ground pours reduce the need for dedicated ground traces, but manually routed ground traces can serve as a safety net in areas where the pour might get fragmented. The pour itself typically addresses concerns about daisy-chaining by providing a low-impedance, widely distributed path.
Keeping most signals on the top layer while reserving the bottom for a continuous ground pour is a sound strategy. Routing on both layers can cut up the ground pour, compromising its effectiveness. However, if you must route on both layers, ensure the ground pour remains as continuous as possible to maintain its benefits.
Pouring VCC on a signal layer is generally not recommended. A ground pour offers shielding and reduces crosstalk, while a VCC pour may introduce risks like accidental shorts during probing. Stick to using ground pours unless your design has specific requirements.
By adhering to these principles, you can effectively balance practicality and performance in your PCB designs.