Ask the Experts: Copper Pour on Unused Layer Sections?

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Q: Is it best practice to flood with copper (tying all areas to GND as needed) all unused portions of all layers?

Rick Hartley: Copper pours can be a double-edged sword. From a manufacturing point of view, copper pours in unused areas will help balance the construction of the PC board, helping to prevent bow and twist and ensuring even copper plating on the outer layer features (traces, pads, etc.). If the only concern is that of even plating on the outer surfaces, thief copper (in the form of copper dots) can be used instead of copper pours. That said, copper dots do not help with bow and twist caused by uneven copper on inner layers. Talk to your fabricator, who will explain why these statements are generally true and how to best handle your particular designs.

Copper pours can have great value to the circuit, but only when implemented correctly. Randomly placing ground pours on all signal layers generally offers no value to the performance of the circuit, nor do these pours help control EMI. Worse, randomly placed copper pours can cause problems if the segments are not properly attached to a plane with adequate vias to keep copper elements from resonating. Resonating copper elements can cause coupling of energy into other features on the PC board.

One technique that can be very helpful is to alternate layers of power and ground in the board stackup. For example, in a six-layer board, with a ground plane on layer 2 and a power plane on layer 5, placing power pours on layers 1 and 3 and placing ground pours on layers 4 and 6 will create a PC board of the following arrangement:

L1: Signal / Power
L2: Ground Plane
L3: Signal / Power
L4: Ground / Signal
L5: Power Plane
L6: Ground / Signal 

The above stackup, due to the added power pours on L1 and L3 and ground pours of L4 and L6, will improve power delivery to ICs on the board. This can and usually will improve both signal integrity and EMI. The EMI improvement is not caused by the shielding effect of the copper pours, but rather by the improved power delivery due to lower power bus impedance at high frequencies (the harmonics of the square waves).

To read this entire article, which appeared in the July 2021 issue of Design007 Magazine, click here.