Ground Fills

I received an e-mail from a reader of PCD Magazine. The paragraph of interest follows:

I would be very interested to learn more about the value and usefulness of a poured ground. I would describe this as a copper pour that is created by filling and connecting together open unused areas with copper. Does this system have usefulness? I have seen this used with double side boards not having the luxury of a dedicated layer. I have seen this as a cross hatching (for thermal relief) around the perimeter of the board. Perhaps a future article? If not would you have a resource to find out more about doing this? Your book (High-Speed Digital Design, pp. 197-199) relates a little to this, but not directly.

My reaction is that a poured ground probably can't hurt, but may not help much, either. Would you agree? Or are there some cross talk or EMI issues that I am missing?

Thanks for your interest in High-Speed Digital Design.

The "poured ground" (more commonly called a "ground fill") is a technique useful on two-layer boards for reducing crosstalk due to ELECTRIC FIELD coupling (that is, capacitive coupling). It is quite effective on high-impedance analog designs.

In high-speed digital designs, we have low-impedance circuits. Most noise problems in digital circuits are caused by MAGNETIC FIELD coupling (that is, inductive coupling). The ground fill does not help in these situations.

Furthermore, in the world of multi-layer boards with solid ground and power planes, the ground fill has no place. In the multi-layer environment it does not significantly reduce crosstalk and it has the distinct disadvantage of modulating the impedance of traces as they cross back and forth across filled regions.

I don't use it at all for multi-layer boards.

Best regards,
Dr. Howard Johnson