How much stitching do I need?

By Patrick Carrier

If signal integrity engineers had the power to make their jobs as easy as possible, every signal in an electronic device would have its own coaxial cable to connect driver to transmitter.  But then electronics would be the size of buildings again and certainly wouldn’t fit in your pocket.  So instead, we try to cram as much stuff as possible onto a little PCB and make it work at 100s and 1000s of MHz.   And, for the most part, we are successful, even when faced with cost reductions and crazy form factors that make our jobs even harder.

A consequence of that is trying to control all the coupling that occurs between signals, and to the outside world.  Something like a via transition makes that really hard to do.  Ideally, every signal via transition would be surrounded by as many vias as possible, all conected together to mimic the shield of a coaxial cable.  But that is pretty unrealistic, so we usually add a couple of transition vias around the signal via.  Some people only add one, and some don’t add any!!  How many do you need?  Well, you can answer that by running SI-PI co-simulations using Hyperlynx SI/PI.  It allows you to experiment with different configurations of stitching vias, or stitching capacitors (necessary if the via is transitioning between layers that reference different planes of different voltages).  Once you’ve determined what configuration works for your stackup and your signals, you can use the Vertical Reference Plane Change DRC in HyperLynx DRC to verify that it was done correctly, and that your board will be free from all the coupling that occurs from signals transitioning through vias.

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This article first appeared on the Siemens Digital Industries Software blog at