Bill Hargin

The impedance implications of the trapezoidal trace.

Until recently I thought those who believed in rectangular traces were about as common as those who believe in square waves and a flat earth. Recently, though, I came to realize it’s not as clear as I thought, not only for newbies but in general. Over the past 25 years, I’ve acquired a good number of books on PCB design and signal integrity, and you wouldn’t know from reading most of the industry literature that traces were anything but rectangular. Interesting, right?

If you’ve read previous “Material Matters” columns, you may recognize the following cross-section from our Z-solver software. Among other things, it shows that the base of a trace, facing the core dielectric, is wider than the side of the trace that faces the prepreg. As such, the trace trapezoids face both up and down in a multilayer stackup. There’s no relationship to the layer number or whether the trace is on the top or bottom half of the board. For this reason, some including me – but not everyone – avoid using terms like “top” or “bottom” with regard to trapezoidal traces.

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