The Good and Bad of Grounded Copper Pour

In PCB design, using grounded copper pour can be a double-edged sword. Learn how to make smart design decisions on balancing copper layers, preventing dead copper issues, and enhancing PCB performance.
Copper pour, also known as copper fill or copper polygons, refers to filling a region of the PCB with copper while automatically applying clearances. This feature is very convenient and has important electrical uses. Besides quickly creating large areas connected to a power rail, its most common usage is to fill the unused space in a PCB layer with ground.

It’s unclear whether the practice of filling unused areas of a layer with ground came before or after the tool that performs this function. Regardless, filling unused areas of a layer with ground has become a standard recommendation that is followed by many designers.

Should you use ground fill in your design? The famous answer to this question is: “it depends.” In this article, we’ll outline some good and bad uses of ground fill and explain how this PCB layout practice impacts electromagnetic compatibility (EMC).

Balancing Copper on All Layers

One reason to use copper fill is to balance the distribution of copper on all layers of the PCB. Copper conducts heat, and asymmetric copper distribution can impact heat distribution during the pressing of layers into the stackup. Using copper fill helps balance heat during this process. An example where copper fill can be beneficial for this purpose is shown in Figure 1.

Figure 1. This PCB uses copper pour on the back layer to provide a ground plane, but it is not present on the top layer. Copper can be balanced in the PCB by applying a copper pour region in the top layer as well.

This approach can be a double-edged sword, requiring sufficient clearances around pads and traces. While traces carrying power or DC signals aren’t typically problematic, impedance-controlled traces can be affected. Filling a layer with grounded copper pour can introduce random mutual capacitance between a trace and the nearby pour, which will alter the trace’s impedance (Figure 2).

Figure 2. Copper pour can cause undesirable changes in trace impedance.

To prevent this, set a larger clearance between those traces and the copper pour. Some impedance solver tools in PCB design software can calculate the correct trace width for a given clearance, but online calculators are less accurate. A simple, conservative rule is to apply a “3W” clearance—in other words, clearance equal to three times the width of the trace—between the trace edge and the copper pour.

Providing Ground in Two-Layer PCBs

In a two-layer PCB, most space is used for components and signals, making it impractical to route ground traces next to every signal. The most convenient solution is to fill your signal layers with grounded copper pour. This balances the copper distribution and provides ground on both layers for most signals. Then, the ground is connected between the two layers with strategically placed stitching vias.

This method works for low-density two-layer PCBs (Figure 3). Once both layers are filled with traces and components, there may not be much room left for grounded copper pour. Some designers might fill unused areas with copper pour, strategically place stitching vias, and then leave the design as is. However, this often results in excessive radiated emissions due to unintentional routing over ground splits and lack of ground for noise-producing components.

Figure 3. The component density in this two-layer PCB is right at the limit where a ground plane may be required.

The simplest solution here is not to rely on grounded copper pour and instead use a four-layer PCB. High-density routing and component arrangements benefit from a four-layer stackup with two internal ground planes (or one ground plane and one power plane), because this provides better signal integrity and lower radiated emissions.

Copper Antennas

Sometimes, copper fill leaves behind regions of copper that are almost completely enclosed and won’t be touched by your automated stitching via tool. See the image in Figure 4 below for an example. Though these leftover regions are electrically connected to ground, they act as antennas with EMI susceptibility. Larger regions have lower resonant frequencies, making them susceptible to EMI at lower frequencies. Relying on an automated stitching-via feature to connect ground everywhere can leave behind these copper antennas, and these are often missed by the design rule engine in your PCB software.

Figure 4. Regions of copper pour that are physically separated may act like antennas and become an EMI liability.

Another issue is dead copper, which is a copper region within a copper pour object that is not connected to the rest of the ground system. This floating copper also acts like an antenna. Fortunately, this problem can be resolved simply by placing one or two stitching vias in these small bits of copper.

Plane Capacitance

Finally, we come to a matter of power integrity, which is also related to radiated emissions. High-speed digital PCBs with large processors generally need large power-rail copper pours or an entire power plane layer. Such designs often begin with a six-layer stackup like the one shown in Figure 5.

Figure 5. Layer 4 is dedicated to power rails, and the use of ground on Layer 5 provides significant plane capacitance for these rails.

In this stackup, the ground plane on Layer 5 is adjacent to the power plane or rails on Layer 4, providing plane capacitance that aids power delivery in the 10 MHz to 100 MHz range.

Layer 3 is a signal layer, which may or may not require controlled impedance. If the trace density on Layer 3 is low, filling the layer with grounded copper pour provides more capacitance in the power distribution network. The increase in plane capacitance can be significant, sometimes even a factor of two to four with larger boards.

As long as sufficient clearance is applied, the pour won’t impact trace impedance. From an EMI perspective, this results in smaller transients on power rails, which in turn leads to lower high-frequency radiated emissions from the board edge.

Ground Pour Not a Cure-All for EMI Problems

In summary, using grounded copper pour everywhere is not a magic bullet for all your EMI problems. Sometimes, ground pour creates new problems or accidentally solves an existing one. Sometimes the board works well and passes EMC testing despite the presence of copper pour.

EMC tools like those offered by DENPAFLUX (previously Mitai) supply crucial insight and guidance to help you navigate the complex role of copper pour and overcome other EMC design challenges. Designers should carefully consider the use of grounded copper pour to target PCB features and electrical conditions that could lead to radiated emissions and potential EMC failure.

All images used courtesy of DENPAFLUX (previously Mitai)

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