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The Do’s and Don’ts of Signal Routing for Controlled Impedance

In this column, we will once again be focusing on controlled impedance structures, both from the layout side and the simulation side. I will break them down into the sub-categories of the models they represent and the important points to remember when using the various models. I will also be asking questions such as, “Why would a fabricator ask for a larger impedance tolerance?” and “Where does the fabricator draw the line for controlling various structures?”

Later, I will break down my Top 10 do’s and don’ts of signal routing.

A Few Rules of Thumb

Let’s start with single-ended structures, both co-planar and those in “free space,” i.e., not coupled to any adjacent copper pour.

For external single-ended structures starting with quarter or half-ounce copper, the trace width is typically about twice the dielectric needed between the impedance signal and its reference plane.

Example: A 4.25 mil trace needs about a .0026”–.0028” dielectric to be a reference plane for 50 ohms on half-ounce starting copper (1.5 ounces after plating).

Keep the copper pour that resides on the impedance layer a minimum of 3x the chosen trace width for impedance; this ensures no unwanted co-planar coupling occurs. At larger trace widths upwards of .012” this distance can be as little as 2x the trace width.

To read this column by Mark Thompson which appeared in the May 2015 issue of The PCB Design Magazine, click here.